- Definition
- Displacement and REDOX Reactions
- Displacement Reactions among Metals
- Oxidation and Reduction in Terms of Electron Loss and Gain
- Summary: Strength of Reducing/Oxidizing Agent of metals
- Summary on Displacement Reactions for metals
- Oxidation Numbers/Oxidation State
- Redox Reactions Involving Halide Ions and Halogens
- Further Examples of Redox Reactions
- Summary: Order of Oxidizing Power for Halogens
- Terms Used in Describing Oxidation-reduction
- The Tendency of Metals to Form Ions
- Cell Diagram for a Voltaic Cell
- Voltaic Cells
- Electrolysis
Definition
- Electrochemistry is the chemistry of electrochemical reactions; which deal with the relationship between electrical energy and chemical reactions.
- Electrochemical reactions involve transfer of electrons and are essentially REDOX reactions.
Displacement and REDOX Reactions
Experiment 1:- Displacement Reactions among Metals
Procedure
- 5 cm3 of 1M CuSO4(aq) is put in a test-tube and its temperature recorded.
- In the solution, a spatula end-full of iron fillings is added.
- Any observations and temperature change are determined and recorded.
- The procedure is repeated with fresh samples of CuSO4 with Zn, Mg, and Cu powders. - The procedure is repeated with 1M magnesium sulphate solution instead of CuSO4(aq).
Observations:
Metal solid | Copper (II) sulphate | Magnesium sulphate |
Iron fillings | - A red brown solid (Cu) is formed - The blue colour of the solution (Cu2+) fades then changes to green (Fe2+) |
- No reaction |
Zinc powder | - A red brown solid, copper metal is deposited. - The blue colour of the solution (Cu2+ ) fades then turns colourless; |
- No observable reaction (change) |
Copper powder | - No reaction | - No reaction |
Explanations
- Reactions between metals and ions of another metal involve transfer of electrons from the metal to the other metal ion in solution.
Examples:
- Fe(s) and CuSO4(aq)
Copper being lower in the electrochemical series accepts electrons easier (than Fe) to form copper atoms (brown solid);
Half equations
Fe(s) → Fe2+(aq) + 2e-
Cu2+(aq) + 2e- → Cu(s)
Overall reaction
Cu2+(aq) + 2e- + Fe(s) → Fe2+(aq) + 2e- + Cu(s)
Then;
Cu2+(aq) + Fe(s) → Fe2+(aq) + Cu(s)
(Blue) (Green) (Brown solid)
Oxidation and Reduction in Terms of Electron Loss and Gain
- The loss of electrons is oxidation and the species that gains electrons (causes electron loss); Cu2+ in this case is called oxidizing agent ; and is itself reduced.
- Reduction: refers to gain of electrons, and the species that donates electrons (iron solid in this case) is called a reducing agent and is itself oxidized.
- Displacement reactions generally involve reduction and oxidation simultaneously and are thus termed Redox Reactions.
Further examples
Zinc solid and CuSO4(aq)
(i) Zn(s) → Zn2+(aq) + 2e- (Oxidation)
(ii) Cu2+(aq) + 2e- → Cu(s) (Reduction)
Then:
Magnesium solid and copper (II) sulphate
(i). Mg(s) → Mg2+(aq) + 2e- (Oxidation)
(ii). Cu2+(aq) + 2e- → Cu(s) (Reduction)
Then:
Silver nitrate and copper solid.
(i). Cu(s) → Cu2+(aq) + 2e- (Oxidation)
(ii). 2Ag+(aq) + 2e- → 2Ag(s) (Reduction)
Then:
Note: - Amount of heat evolved in these redox reactions depends on the position of the metal in the activity series relative to the metal ion in solution.
- The closer the metals are in the activity series; the less readily displacement occurs and the lower the heat evolution during the displacement.
E.g.: Heat evolved Mg//Cu2+ is higher than that evolved between Fe//Cu2+.
Conclusion:
- Metals displace from solutions, those metals lower than themselves in the activity series.
Note: - The more the reactive a metal is; the stronger a reducing agent it is and the weaker an oxidizing agent it is.
Example: - Potassium is stronger reducing agent; but weaker oxidizing agent than silver, gold etc.
Summary: Strength of Reducing/Oxidizing Agent
Summary on Displacement Reactions
Metal ion (in solution) |
Mg | Al | Zn | Fe | Pb | Cu |
K+ | X | X | X | X | X | X |
Na+ | X | X | X | X | X | X |
Ca2+ | X | X | X | X | X | X |
Mg2+ | X | X | X | X | X | X |
Al3+ | √ | X | X | X | X | X |
Zn2+ | √ | √ | X | X | X | X |
Fe2+ | √ | √ | √ | X | X | X |
Pb2+ | √ | √ | √ | √ | X | X |
Cu2+ | √ | √ | √ | √ | √ | X |
Ag2+ | √ | √ | √ | √ | √ | √ |
Key:
- A cross (x) indicates no reaction hence no redox reaction occurs.
- A tick (√) indicates redox reaction occurs.
Oxidation Numbers/Oxidation State
- Is the apparent charge that atoms have in molecules or ions.
- For monoatomic ions, the oxidation number (state) is the magnitude and sign of charge;
Example:
Oxidation no of Aluminium in Al3+ is +3;
Importance of Oxidation Numbers:
- Helps in keeping track of electron movement in redox reactions; hence determination of the reduced and oxidized species.
Oxidation and Reduction in Terms of Oxidation Numbers
- Oxidation
Is an increase in oxidation number. - Reduction
Refers to a decrease in oxidation number
Rules in assigning oxidation numbers
- Oxidation number of an uncombined element is zero (0)
- The charge on a monoatomic ion is equivalent to the oxidation number of that element;
- The oxidation number of hydrogen in all compounds is +1 except in metal hydrides where its 1;
- The oxidation number of oxygen in all compounds is 2 except in peroxides where it is 1 and 0F2 where it is +2.
- In complex ions the overall charge is equal to the sum of the oxidation states of the constituent elements.
- In compounds, the sum of oxidation numbers of all constituent atoms is equal to zero.
Worked examples
- Calculate the oxidation number of nitrogen in:
- NO3-
Solution:
N + (-2 x 3) = -1
N = -1 + 6
= +5
Note: thus nitric acid with a nitrate ion (NO3- ) is called nitric (V) acid since the oxidation number of nitrogen in it is +5; - NO2;
Solution:
N + (-2 x 2) = 0
N = 0 + 4
= +4
Note: Thus the gas NO2 is referred to as nitrogen (IV) oxide because the oxidation umber of nitrogen in it is +4 - NO2-;
Solution:
N + (-2 x 2) = -1
N = -1 + 4
= +3
Note: thus nitrous acid containing nitrite ion is called nitrous (III) acid since the oxidation number of nitrogen in it is +3. - AgNO3;
Solution:
1 + N + (-2 x 3) = 0
1 + N + (-6) = 0
N = 0 - 1 + 6;
N = +5
- NO3-
- Determine the oxidation number of manganese in each of the following, and hence give the systematic names of the compounds.
- MnSO4
Solution:
Mn + 6 + (-2 x 4) = 0
Mn = 0 - 6 + 8;
Mn = +2
Systematic name: Manganese (II) sulphate; - Mn2O3;
Solution:
2Mn + (3 x -2) = 0;
2Mn = 0 + 6
Mn = ½ x 6;
Mn = +3;
Systematic name: Manganese (III) oxide; - KMnO4
Solution:
1 + Mn + (-2 x 4) = 0
Mn = 0 - 1 + 8;
Mn = +7;
Systematic name: Potassium manganate (VII) oxide - MnO3-;
Solution:
Mn + (-2 x 3) = -1
Mn = -1 + 6;
Mn = +5
Systematic name: Manganese (V) ion;
- MnSO4
Experiment: Redox Reactions Involving Halide Ions and Halogens
Procedure:
- 2 cm3 of chlorine gas are bubbled into each of the following solutions: - KI, KCl, KBr, and KF.
- The observations are made and recorded.
- The procedure is repeated using fluorine, bromine and iodine in place of chlorine.
Precaution:
- Chlorine and bromine are poisonous.
Observations
Halogen |
Potassium fluoride | Potassium chloride | Potassium Bromide | Potassium Iodide |
Fluorine (F2) | No visible, change | Green yellow gas is evolved | Colourless solution changes to red brown | Colourless solution turns black |
Chlorine (Cl2) | No visible colour change | No visible colour change | Colourless solution turns to red brown | Colourless solution turns black |
Bromine (Br2) | No visible colour change | No visible colour change | No visible colour change | Colourless solution turns black |
Iodine (I2) | No visible colour change | No visible colour change | No visible colour change | Colourless solution turns black |
Note: Colours of halogens in tetra chloromethane:-
Halogen | Colour in tetrachloromethane |
Fluorine | |
Chlorine | Yellow |
Bromine | Red - brown |
Iodine | Purple |
Explanations
- Fluorine displaces all the other halogens; Cl2 , Br2 and I2 because it has a greater tendency to accept electrons than all the rest.
- Chlorine displaces both Bromine and Iodine from their halide solutions
- Cl2 takes electrons from the bromide and iodide ions i.e. oxidizes them, to form bromine and iodine respectively.
Equations:- Chlorine and potassium bromide:
Cl2(g) + 2KBr(aq) → 2KCl(aq) + Br2(l)
Ionically:
Cl2(g) + 2Br-(aq) → 2Cl-(aq) + Br2(l)
Green-yellow Red brown
Redox equation: - Chlorine and Potasium iodide:
Cl2(g) + 2KI(aq) → 2KCl(aq) + I2(l)
Ionically:
Cl2(g) + 2I-(aq) → 2Cl-(aq) + I2(l)
Green-yellow Black
Half cell reactions:
Oxidation: 2I-(aq) → I2(aq) + 2e-
Reduction: Cl2(g) + 2e- → 2Cl-(aq)
Redox equation:
- Chlorine and potassium bromide:
- Bromine takes electrons form iodide ions but not from fluorine and chlorine.
- Iodine is formed i.e. due to oxidation of iodide ions by the Bromine.
Equations
Bromine and Potasium iodide:
Br2(g) + 2KI(aq) → 2KBr(aq) + I2(l)
Ionically:
Br2(g) + 2I-(aq) → 2Br-(aq) + I2(l)
Red brown Black
Half cell reactions:
Oxidation: 2I-(aq) → I2(aq) + 2e-
Reduction: Br2(g) + 2e- → 2Br-(aq)
Redox equation: - Note: oxidation number of chlorine decreases from 0 to -1 hence reduction ; while oxidation number of iodine increases from -1 to 0; hence oxidation;
Conclusion:
- The stronger the tendency of an element to accept electrons, the stronger is its oxidizing power.
- Fluorine is the strongest oxidizing agent of the 4 halogens considered.
Summary: Order of Oxidizing Power for Halogens
Further Examples of Redox Reactions
- Action of acid on metals
Mg(s) + 2HCl(aq) → 2MgCl(aq) + H2(g)
Ionically:
Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g)
Half cell reactions:
Oxidation: Mg(s) → Mg2+(aq) + 2e-
Reduction: 2H+(g) + 2e- → H2(g)
Redox equation:
Note: oxidation number of hydrogen decreases from 1 to 0 hence reduction ; while oxidation number of magnesium increases from 0 to 2; hence oxidation ; - Reaction of active metals with water
Example
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Ionically:
2Na(s) + 2H2O(l) → 2Na+(aq) + 2OH-(aq) + H2(g)
Half cell reactions:
Oxidation: 2Na(s) → 2Na+(aq) + 2e-
Reduction: 2H2O(g) + 2e- → 2OH-(aq) + H2(g)
Redox equation:
Note: oxidation number of water decreases from 0 to -1(total) hence reduction ; while oxidation number of sodium increases from 0 to 1; hence oxidation ; - Reaction of heated Iron with dry chlorine
2Fe(s) + 3Cl2(g) → 2FeCl3(s)
Ionically (assumed):
2Fe(s) + 3Cl2(aq) → 2Fe3+(aq) + 6Cl-(g)
Half cell reactions:
Oxidation: 2Fe(s) → 2Fe3+(aq) + 6e-
Reduction: 3Cl2(g) + 6e- → H2(g)
Redox equation:
Note: oxidation number of chlorine decreases from 0 to -1 hence reduction; while oxidation number of iron increases from 0 to 3; hence oxidation; - Reaction between Bromine and Iron (II) ions
Ionically:
Br2(l) + 2Fe2+(aq) → 2Fe3+(aq) + 2Br-(aq)
Half cell reactions:
Oxidation: Br2(l) → 2Br-(aq) + 2e-
Reduction: 2Fe2+(g) + 2e- → 2Fe3+(aq)
Redox equation:
Note: oxidation number of bromine decreases from 0 to -1 hence reduction ; while oxidation number of Fe2+ increases from 2 to 3 (in Fe3+); hence oxidation ; - Oxidation by potassium Manganate (VII) (KMnO4 )
Procedure:- Purple Potassium manganate (VII) is added into a solution containing iron (II) ions in a test tube.
- A few drops of concentrated sulphuric (VI) acid are added.
Observations- The purple solution (containing Manganate (VII) ions) turns to colourless (manganate (II) ions) i.e. the purple solution is decolourised;
Explanation- The Manganate (VII) ions which give the solution a purple colour are reduced to Manganese (II) ions which appear colourless. This is a redox reaction.
Equations
Ionically:
MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)
Purple Colourless
Half cell reactions:
Oxidation : 5Fe2+(s) → 5Fe3+(aq) + 5e- ; (since number of electrons is 5)
Reduction: MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l)
Redox equation
Note:- oxidation number of Manganate ions in KMnO 4 decreases from 7 to 2 (in Mn2+) hence reduction ; while oxidation number of iron increases from 2 (in Fe2+) to 3 (in Fe3+); hence oxidation
- The presence of Fe 3+ at the end of the reaction can be detected by adding sodium hydroxide solution to form a red brown precipitate of Fe(OH)3 ;
- Action of potassium dichromate (VI) on iron (II) ions (Fe2+):
Procedure:- A solution containing iron (II) ions is added into a solution of oxidized potassium dichromate (VI).
Observations- The orange solution of potassium dichromate turns green.
Explanations- The iron (II) ions are oxidized to iron (III) ions
- The chromium (VI) ions (orange) are reduced to chromium (III) ions
- This is thus a REDOX reaction.
Equations
Ionically:
Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq) → 2Cr3+(aq) + 6Fe3+(aq) + 7H2O(l)
Orange Green
Half cell reactions:
Oxidation: 6Fe2+(s) → 6Fe3+(aq) + 6e-; (since number of electrons is 6)
Reduction: Cr2O72-(aq) + 14H+(aq) + 5e- → 2Cr3+(aq) + 7H2O(l) + 6Fe3+(aq)
Redox equation:
Note:- The oxidation number of dichromate ions in K2Cr2O7 decreases from 6 to 3 (in Cr3+) hence reduction; while oxidation number of iron increases from 2 (in Fe2+) to 3 (in Fe3+); hence oxidation ;
- The presence of Fe3+ at the end of the reaction can be detected by adding sodium hydroxide solution to form a red brown precipitate of Fe(OH) 3 ;
- Action of acidified potassium permanganate on Hydrogen Peroxide.
- The overall reaction is a Redox reaction
Redox equation
2MnO4-(aq) + 5H2O(l) + 6H+(aq) → 2Mn2+(aq) + 5H2O(l) + 5O2(g)
Purple Colourless
Half cell reactions:
Oxidation: 5H2O2(aq) → 10H+(aq) + 5O2(g) + 10e-;
Reduction: 2MnO4-(aq) + 16H+(aq) + 10e- → 2Mn2+(aq) + 8H2O(l)
Redox equation:
Note :- The oxidation number of manganate ions in KMnO4 decreases from 7 to 2 (in Mn2+) hence reduction ; while oxidation number of hydrogen increases from -1 (in H2O2) to 1 (in H+); hence oxidation ;
- Thus the acidified potassium manganate (VII) oxidizes hydrogen peroxide to water and hydrogen;
- The overall reaction is a Redox reaction
- H2O2 oxidizes Iron (II) salts to Iron (III) salts in acidic medium
Oxidation:
2Fe2+(aq) →2Fe3+(aq) + 2e-
Reduction:
2H+(aq) + H2O2(aq) + 2e- → 2H2O(l)
Overall redox:
2Fe2+(aq) + H2O2(aq) + 2H+(aq) → 2Fe3+(aq) + 2H2O(l)
Terms Used in Describing Oxidation-reduction
Term | Electron change | Oxidation number change |
Oxidation Reduction Oxidising agent Reducing agent Substances oxidized Substances reduced |
Loss of electrons Gain of electrons Receives electons Loses electrons Loses electrons Gains electrons |
Increases Decreases Decreases Increases Increases Decreases |
The Tendency of Metals to Form Ions
- When a metal is placed in an aqueous solution of its ions, some of the metal dissolves;
Equation:
M(s) → Mn+(aq) + ne- - Dissolution of the metal causes electron build up on its surface; making it negatively charged, while the surrounding solution becomes positively charged.
Diagram: Dissolution of metal and electron build up. - The positive charge of the solution increases and some of the cations start recombining with the electrons on the metal surface to form atoms.
Equation:
Mn+(aq) + ne- → M(s) - Consequently, an electric potential difference is created between the metal rod and the positively charged ions in solution.
- This arrangement of a metal rod (electrodes) dipped in a solution of its ions constitutes a half cell.
Note: - The tendency of a metal to ionize when in contact with the ions differs form one metal to another.
- This difference can be measured by connecting two different Half cells to make a full cell .
- The electrodes of the 2 half cells are connected by a metallic conductor; while the electrolytes (solutions) of the half cells are connected through a salt bridge .
Diagram: Connection of two half cells to a full cell.
Experiment: - To Measure the Relative Tendency of Metals to Ionize
Procedure:
- A Zinc rod is placed into 50 cm3 of 1M zinc sulphate in a beaker
- Into another beaker containing 50 cm3 of 1M CuSO4(aq), a copper rod is dipped
- The two solutions are connected using a salt bridge.
- The two metal rods are connected through a connecting wire connected to a voltmeter
- The experiment is repeated using the following half-cells instead of the Zinc-half cells:-
- Mg rod dipped in 1M MgSO4(aq)
- Lead dipped in 1M lead Nitrate
- Copper dipped in 1M CuSO4(aq)
Apparatus
Observation
- The zinc rod in the zinc-zinc ions half cell dissolves;
- The blue colour of the copper (II) Sulphate solution fades/ decrease;
- Red-brown deposits of copper appear on the copper rod in the copper-copper ions half-cell.
- A voltage of 1.10 V is registered in the voltmeter.
Equations/reactions, at each half cell
- In zinc-zinc ions half cell
Zn(s) → Zn2+(aq) + 2e- - In the copper-copper ion half cell
Cu2+(aq) + 2e- → Cu(s);
Explanations
- Zinc rod has a higher tendency to ionize than the copper rod, when the metal rods are placed in solutions of their ions.
- Thus the zinc rod has a higher accumulation of electrons than the copper rod.
- This makes it more negative compared to the relatively more positive copper rod, which has a lower accumulation of electrons.
- On connecting the 2 half cells; electrons will flow form the zinc rod to the copper rod through the external wire.
- The copper rod gains the electrons lost by the Zinc rod.
Roles of the slat bridge- Note: It forms a link between the 2 half cells, thereby completing the circuit
- It completes the circuit by:
- Allowing its ions to carry charge from one half - cell to the other.
- Maintaining the balance of charge in the two half-cells; by providing the ions which replace those used up at the electrodes.
- The overall reaction in the cells is a Redox reaction
Half cell reactions:
Zn(s) → Zn2+(aq) + 2e- (oxidation)
Cu2+(aq) + 2e- → Cu(s); (reduction)
Overall redox equation - The voltage of 1.10 V registered in the voltmeter is a measure of the difference between the electrode potential (Eθ ) of Zinc and Copper electrodes, i.e. the potential difference/the Electromotive force.
- Thus: An electrochemical/ voltaic cell;
- Is the combination of two half – cells to give a full cell capable of generating an electric current from a redox reaction.
Cell Diagram for a Voltaic Cell
Rules/Conventions for Cell Representation
- A vertical continuous line (/); represents the metal-metal ion or metal ion-metal interphase.
- Vertical broken line (|); between the 2 half-cells; represents the salt bridge.
Note : The salt bridge may also be represented by two unbroken parallel lines (//).
Example: - cell diagram for the above cell;
Alternatively:
Zn(s)/Zn2+(aq)//Cu2+(aq)/Cu(s);
Electrode potential Eθ , values of other metal – metal ions relative to the Cu/Cu2+ half cell
Metal/metal ion half cell | Electrode potential Eθ relative to Cu2+/Cu half cell |
Mg(s)/Mg2+(aq) | +2.04 |
Zn(s)/Zn2(aq) | +1.10 |
Pb(s)/Pb2+(aq) | +0.78 |
Cu(s)/Cu2+(aq) | +0.00 |
Ag(s)/Ag+(aq) | -0.46 |
Positive and negative E values
- Positive E values -
- If the Eθ value for a metal/metal ion is positive then the metal undergoes oxidation (loses electrons) while the reference electrode undergoes reduction (accepts electrons)
Example : - The Eθ value for Zn(s)/Zn2+(aq) relative Cu2+(aq)/Cu(s) is positive because the zinc metal is oxidized to zinc ions while the copper ions are reduced to copper metal.
- If the Eθ value for a metal/metal ion is positive then the metal undergoes oxidation (loses electrons) while the reference electrode undergoes reduction (accepts electrons)
- Negative E values :
- Implies that the reference half cell undergoes oxidation (donates electrons) while the other metal ions in the other half cell undergoes reduction (accepts electrons)
Example : - The E value for Ag(s)/Ag+(aq) is negative because Cu is more reactive than silver and gives out electrons (oxidation); while the less reactive Ag has its ions accepting electrons (reduction) to form Ag solid.
- Implies that the reference half cell undergoes oxidation (donates electrons) while the other metal ions in the other half cell undergoes reduction (accepts electrons)
- The 0 (Zero) E value:
- Always indicate the reference electrode / half-cell; in which case there would be no potential difference (with itself)
Example: - The 2 half cells of Cu(s)/Cu2+(aq) or Cu2+(aq)/Cu(s) have no potential difference in between them hence a zero (0) E value.
Note:- Any other element could be chosen as the reference electrode in place of copper Cu and difference electrode potentials values would be obtained for the same elements.
- The electrode potential of a single element is usually determined by measuring the difference between the electrode potential of the element and a chosen standard electrode.
- This gives the standard electrode potential (E0 ) of the element.
- Always indicate the reference electrode / half-cell; in which case there would be no potential difference (with itself)
The standard Electrode Potential (E0 )
Definition
- Is the potential difference for a cell comprising of a particular element in contact with 1 molar solution of its ions and the standard hydrogen electrode.
- It is denoted with the symbol E0 .
Importance
- It is useful in comparing the oxidizing and reducing powers of various substances.
The standard Hydrogen Electrode
- Is the hydrogen half-cell, which has been conventionally chosen as the standard reference electrode.
- It has an electrode potential of zero at:-
- A temperature of 25oC
- A hydrogen pressure of 1 atmosphere
- A concentration of 1M hydrogen ions
- Note: The ions in the other half-cell must also be at a concentration of 1 molar.
Components of the Hydrogen Half Cell
- Consist of an inert platinum electrode immersed in a 1M solution of Hydrogen ions
- Hydrogen gas at 1 atmosphere is bubbled into the platinum electrode.
- The hydrogen is adsorbed into the platinum surface and an equilibrium (state of balance) is established between the adsorbed layer of molecular hydrogen and hydrogen ions in the solution.
Equation:
½H2(g) → H+(aq) + e-
Platinised platinum- Is platinum loosely coated with finely-divided platinum.
- This enables it to retain comparatively large quantity of hydrogen due to its porous state.
- Platinised platinum also serves as a route by which electrons leave or enter the electrode.
- The hydrogen electrode is represented as: H2(g)/H+(aq); 1M
Diagram: The standard hydrogen electrode: - The electrode potential of any metal is taken as the difference in potential between the metal electrode and the standard hydrogen electrode.
Negative and positive electrode potentials
- Negative electrode potential
- If the metal electrode has a higher/greater tendency to loose electrons than the hydrogen electrode; then the electrode is negative with respect to hydrogen electrode; and its electrode potential is negative.
Examples: Zinc, Magnesium etc.
- If the metal electrode has a higher/greater tendency to loose electrons than the hydrogen electrode; then the electrode is negative with respect to hydrogen electrode; and its electrode potential is negative.
- Positive electrode potential
- If the tendency of an electrode to loose electrode is lower than the hydrogen electrode, then the electrode is positive with respect to the hydrogen electrode; and its potential is positive.
Examples: copper, silver etc
- If the tendency of an electrode to loose electrode is lower than the hydrogen electrode, then the electrode is positive with respect to the hydrogen electrode; and its potential is positive.
Reduction Potentials
- Is a standard electrode potential measured when the electrode in question is gaining electrons.
- The lower the tendency of an electrode to accept/gain electrons; the lower (more negative) the reduction potential and vise versa.
Examples:
K+(aq) + e- → K(s); E0 = -2.92V
F2(g) + e- → 2F-(aq) ; E0 = +12.87V
Mg(s) + 2e- → Mg2+(aq); Eθ = -2.71 V - Thus potassium ions with E0 = -2.92V have a lesser tendency to gain electrons than magnesium ions.
- Thus Potassium is the weakest oxidizing agent; but the strongest reducing agent, since it has the greatest tendency to donate electrons.
Note: - Oxidation potentials will be the potentials of electrodes measured when they are losing electrons hence undergoing oxidation.
Standard electrode potentials (reduction potentials) of some elements
Reduction equation | Eθ volts | |
↑Increasing strength of oxidising agent ↓Decreasing strength of oxidising agent |
F2(g) + 2e- → 2F-(aq) | +2.87 |
Cl2(g) + 2e- → 2Cl-(aq) | +2.87 | |
Br2(g) + 2e- → 2Br-(aq) | +1.36 | |
Ag+(aq) + 2e- → Ag(s) | +0.80 | |
I2(g) + 2e- → 2I-(aq) | +0.54 | |
Cu2+(aq) + 2e- →Cu(s) | +0.34 | |
2H+(aq) + 2e- →H2(g) | +0.00 | |
Pb2+(aq) +2e- →Pb(s) | -0.13 | |
Fe2+(aq) + 2e- → Fe(s) | -0.44 | |
Zn2+(g) + 2e- → Zn(s) | -0.76 | |
Al3+(aq) + 3e- → Al(s) | -1.66 | |
Mg2+(aq) + 2e- → Mg(s) | -2.71 |
Note:
- The standard electrode potentials in the above table are reduction potentials.
- The greater the tendency to undergo reduction, the higher (more positive) the Eθ value.
- The reverse reaction (oxidation ) would have a potential value equal in magnitude but opposite in sign to the reduction potential.
Example :
Zinc
Reduction potential
Zn2+(aq) + 2e- → Zn(s); E0 = -0.76V;
Oxidation potential
Zn(s) → Zn2+(aq) + 2e-; Eθ = + 0.76V;
Uses of E0 values
- Comparing the reducing powers and oxidizing powers of various substances;
- Predicting whether or NOT a stated REDOX reaction will take place.
The E0 value for a REDOX reaction
- Is usually calculated as the sum of the E 0 value for the half cells involved.
Note: - If the sum is positive then the reaction can occur simultaneously;
- If the value of the sum is negative the reaction cannot occur;
Sample calculations
- Cu2+ can oxidize Zinc but Zn2+ cannot oxidize Cu;
- Cu2+(aq)/Cu(s)//Zn(s)/Zn2+(aq)
Cu2+(aq) + 2e- → Cu(s); E0 = + 0.34 V
Zn(s) → 2e- + Zn2+(aq); E0 = + 0.76 V
Cu 2+(aq) + Zn(s) → Cu(s) + Zn(aq) ; E0 = - 1.10 V
The overall reaction is positive, hence zinc can be oxidized by copper (II) ions, and hence reaction occurs; - Zn2+(aq)/Zn(s)//Cu(s)/Cu2+(aq)
Zn2+(aq) + 2e-→ Zn(s); E0 = - 0.76 V
Cu(s) → 2e- + Cu2+(aq); E0 = - 0.34 V
Zn2+(aq) + Cu(s) → Zn(s) + Cu2+(aq) ; E0 = - 1.10 V
The overall E0 is negative; thus Zn2+ cannot oxidize Cu to Cu2+ (Cu cannot reduce Zn2+ to Zn); and hence the reaction cannot occur.
- Cu2+(aq)/Cu(s)//Zn(s)/Zn2+(aq)
- Can chlorine displace bromine form bromide solution?
Cell diagram : Cl2(g)/Cl-(aq)//2Br-(aq)/Br2(g)
Cl2(g) + 2e- → 2Cl-(aq); E0 = + 1.36 V
2Br-(aq) → 2e- + Br2(g) ; E0 = - 1.09 V
Cl2(g) + 2Br-(aq) → 2Cl-(aq) + Br2(g) ; E0 = +0.27 V
The overall E0 for the reaction is positive, so chlorine can displace bromine from a bromide solution.
Worked examples
- The diagram below represents part of the apparatus to be used for the determination of the standard electrode potential of Aluminum, Eθ Al3+(aq)/Al(s)
- Name the solutions which could be placed in beakers A and B; specifying their concentrations.
Answer:- In beaker A; 1M HCl(aq) i.e. any solution with 1M hydrogen ions
- In beaker B; 1M Al (NO3)(aq); i.e. any aqueous solution with 1M Al3+.
- One essential part of the cell has been omitted. Name the missing part and give its functions.
Answer:-
Missing part : salt bridge
Function: completes the circuit by;- Allowing its ions to carry charge form one half cell to another.
- Providing ions which repose those used up of the electrodes, hence maintaining a balance of charge in the 2 half cells.
- The voltmeter reading was found to be 1.66 V.
- Give the standard electrode potential for the aluminum electrode.
Solution: it is -1.66 since the Hydrogen-hydrogen ions half-cell = 0.00V. - Show the direction of flow of electrons in the circuit;
Solution: From Al3+/Al half cell to the H2/2H+(aq) half cell
- Give the standard electrode potential for the aluminum electrode.
- Give the half-cell equations and the overall cell equation
Solution:
3H+(aq) + 3e- → 1½H2(g); E0 = + 0.00V;
Al(s) → 3e- + Al3+(aq); E0 = + 1.66V;
Overall:
Al(s) + 3H+(aq) →Al3+(aq) + 1½H2(g); E0 = +1.66 V
- Name the solutions which could be placed in beakers A and B; specifying their concentrations.
- You are given the following half-equations;
Mg2+(aq) + 2e- →Mg(s); Eθ = - 2.71 V
Zn2+(g) + 2e-→ Zn(s) ; Eθ = - 0.76 V- Obtain an equation for the cell reaction.
Mg(s) → 2e- + Mg2+(aq); E0 = +2.71V
Zn2+(aq) + 2e- → Zn(s) ; E0 = - 0.76V
Mg(s) + Zn2+(aq) →Mg2+(aq) + Zn(s); E0 = +1.95V
Thus equation:
Mg(s) + Zn2+(aq) → Mg2+(aq) + Zn(s); E0 = +1.95V - Calculate the E0 value for the cell.
Mg(s) → 2e- + Mg2+(aq); E0 = +2.71V
Zn2+(aq) + 2e- → Zn(s); E0 = - 0.76V
Mg(s) + Zn2+(aq) → Mg2+(aq) + Zn(s); E0 = +1.95V - Give the oxidizing species and the reducing species
- Reducing species
Magnesium i.e. The species that undergoes oxidation since its oxidation number increases (from 0 to 2); as it reduces the other;
- Oxidizing species
Zinc/Zinc ions; - the species that undergoes reduction; since its oxidation number decreases (from 2 to 0) as it oxidizes the other species (Mg).
- Obtain an equation for the cell reaction.
- Given the following half-equations
I2(g) + 2e- → 2I-(aq); Eθ = + 0.54V
Br2(g) + 2e- → 2Br-(s); Eθ = +1.09 V- Obtain an equation for the all reaction
2I-(aq) → 2e- + I2(g); E0 = -0.54V
Br2(g) + 2e- → 2Br-(s); E0 = +1.09V
Br2(g) + 2I-(aq) → 2Br-(aq) + I2(g); E0 = +0.55V - Calculate the E0 value for the cell
2I-(aq) → 2e- + I2(g); E0 = - 0.54V
Br2(g) + 2e- → 2Br-(s); E0 = +1.09V
Br2(g) + 2I-(aq) → 2Br-(aq) + I2(g); E0 = +0.55V - Give the oxidizing species;
Oxidizing species : Bromine; Br2(aq)
- Obtain an equation for the all reaction
- Consider the following list of electrodes and electrode potential values.
Electron reaction Eθ volts A2+/A +0.34 B2+/B -0.71 C2+/C -0.76 D+/D +0.80 E2+/E -2.87 F2+/F -2.92 - Which of the ions is the strongest oxidizer?
D+; because it is most readily reduced/have the highest tendency to accept electrons as evidenced by its highest positive Eθ value when the ions change to element (D+/D-) - Which of the ions is the strongest reducer?
- Is least readily reduced hence lowest E0 value (-2.92 V); accepts electrons least readily i.e. shows the lowest E0 when its ions gain electrons/ are reduced (F+/F, = -2.92 V)
- Which of the ions is the strongest oxidizer?
- The following is a list of reduction standard electrode potentials.
Metal Eθ volts (standard electrode potential) Magnesium -2.36 Zinc -0.76 Iron -0.44 Hydrogen 0.00 Copper +0.34 Silver +0.79 - Which two metals, if used together in a cell would produce the largest e.m.f?
Magnesium-silver cell;
Mg2+(aq) + 2e- → Mg(s); Eθ = - 2.36 V
2Ag+(g) + 2e- → 2Ag(s); Eθ = - 0.79 V
Thus;
Mg(s) → 2e- + Mg2+(aq); E0 = +2.36V
2Ag+(aq) + 2e- → 2Ag(s); E0 = - 0.79V
Mg(s) + 2Ag+(aq) → Mg2+(aq) + 2Ag(s); E0 = +3.15V - What would be the voltage produced by:-
- Zinc-copper cell
Cu2+(aq) + 2e- → Cu(s); Eθ = + 0.34 V
Zn2+(g) + 2e- → Zn(s) ; E θ = - 0.76 V
Thus; Zn(s)/Zn2+(aq)//Cu2+(aq)/Cu(s);
Zn(s) → 2e- + Zn2+(aq); E0 = + 0.76V
Cu2+(aq) + 2e- → Cu(s); E0 = + 0.34V
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s); E0 = +1.10V - Copper-silver cell
Cu2+(aq) + 2e- → Cu(s); Eθ = + 0.34 V
Ag+(g) + 2e- → Ag(s); Eθ = + 0.79 V
Thus; Cu(s)/Cu2+(aq)//2Ag+(aq)/2Ag(s);
Cu(s) → 2e- + Cu2+(aq); E0 = - 0.34V;
2Ag+(aq) + 2e- → 2Ag(s); E0 = + 0.79V;
Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s); E0 = + 0.45V
- Zinc-copper cell
- Explain the meaning of the positive and negative signs;
Positive signs
- The metal in question has a lower tendency to loose electrons than hydrogen hence more relatively positive to hydrogen;
- They are stronger oxidizing agents but weaker reducing agents but weaker reducing agents than hydrogen;
Negative signs
- The particular metal has a higher tendency to loose electrons than hydrogen; hence relatively more negative than hydrogen.
- They are weaker oxidizing agents but stronger reducing agents than Hydrogen.
- Which two metals, if used together in a cell would produce the largest e.m.f?
- The following are some half-cell electrode potentials of some elements.
Reaction- Select two half cells which when oxidized give the burst E value; and fill the cell representation.
Solution: The silver copper cell; i.e. Cell representation - Calculate the E0 value
- Give the strongest reducing agent and strongest oxidizing agent.
Strongest reducing agent
Strongest oxidizer - silver - has highest reduction potential
- Select two half cells which when oxidized give the burst E value; and fill the cell representation.
- Study the table below and answer the questions that follow.
- Which two metals would form a metallic couple with the highest EMS.
- Calculate the e.m.f. of the cell that would be produced by (i) above.
- Write down the cell representation for the cell above.
- Which metal is the strongest reducing agent in the above list
Metal A - have the lowest reduction potential.
-
- The table below gives reduction potentials obtained when the half-cells for each of the metals represented by letters J, K, L, M and N where connected to a copper half- cell as the reference electrode.
- What is metal L likely to be? Give a reason
Copper It has an E0 value/ reduction potential of 0.00, with copper as the reference electrode - Which of the metals cannot be displaced from the solution of its salt by any other metal in the table? Give a reason.
Metal J:Has the lowest reduction potential; meaning it least readily accepts electrons (most readily donates electrons) than any other metal. - Metal K and M were connected to form a cell as shown in the diagram below;
- Indicate on the diagram, the direction of flow of electrons. Explain.
from K to M.
K is a stronger reducing agent than M, as evidenced by its lower reduction potential.
It thus loses electrons faster becoming more Negative than M; hence electrons move from K through external wires to M. - Write the equations for the half-cell reactions that occur at:-
Metal K electrode:-
Metal M electrode - If the slat bridge is filled with saturated sodium Nitrate solution, explain how it helps to complete the circuit.
Answer
It allows its ions (Na+(aq) , and NO3-(aq)) to carry charge from one half cell to another. Providing ions which replace those used up at the electrodes.
- Indicate on the diagram, the direction of flow of electrons. Explain.
- What is metal L likely to be? Give a reason
- The table below gives reduction potentials obtained when the half-cells for each of the metals represented by letters J, K, L, M and N where connected to a copper half- cell as the reference electrode.
Voltaic Cells
- Are also called electrochemical cells.
- Are cells in which electrical energy is generated from chemical reactions.
Types of Electrochemical Cells
- Primary cells -
- Electrochemical cells which are not rechargeable
- secondary cells-
- Voltaic/electrochemical cells which are rechargeable.
Primary cells/dry cells
- Are of various types and an example is the Le clanche dry cell.
Diagram: The Le clanche dry cell
Structure - Consist of a Zinc can with carbon rod at the centre.
- The central graphite/carbon rod is surrounded by powdered Manganese (IV) oxide and carbon; which are inturn surrounded by a paste of NH4Cl (s) and Zinc Chloride.
- The protruding portion of the carbon rod is covered with a brass cap; and the zinc can covered with a sealing material.
Chemical reaction - The Zinc can is the negative terminal; while the carbon/graphite rod is the positive terminal;
- a the Zinc can/negative terminal
- positive terminal /brass cap
(NH3(g) + 2e- → 2NH3(g) + H2(g)
- Note:These gases (NH3(aq) and H2(g)) are NOT used immediately but are used in more complex reactions.
- The ammonia gas forms a complex with the zinc chloride in the paste.
- The hydrogen gas is oxidised to water by the Manganese (IV) oxide.
Functions of the various components.- Brass cap -
- Functions as the positive terminal where the reduction reaction occurs.
- Zinc can
- Is the Negative terminal; where the oxidation reaction occurs.
- Carbon rod
- It serves as the positive electrode.
- Acts as the connecting wire between the positive and negative terminal through it electron flow form the Zinc can to the brass cap.
- Manganese (IV) Oxide
- To oxidise the Hydrogen gas produced at the anode/positive terminal/brass cap to water.
- A single dry cell can produce a potential of 1.5 V
- Brass cap -
- Note:Dry ammonium chloride does not conduct electric current. This explains why a paste, which is a conductor, is used.
- The dry cells cannot provide a continuous supply of electricity for an undulate period of time.
Reason -The reactants (electrolytes) are used up and cannot be replaced.
Secondary cells
- Are voltaic/electrochemical cells that are rechargeable.
- A common example is the lead acid accumulator.
Diagram: The lead acid accumulator.
Structure: - The positive plate is a lead grill filled with lead (IV) oxide; while the negative plate consists of a similar lead grill filled with spongy lead.
- The grills are immersed in sulphuric acid; which serves as the electrolyte.
Reaction- During discharge/when in use
- At the negative terminal/lead - The lead dissolves forming lead (II) ions
- At the positive terminal (lead (IV) oxide; lead (IV) oxide reacts with the Hydrogen ions (H+) in sulphuric acid; also forming lead (II) ions.
- Then; the lead (II) ions formed at both electrodes react instantly with the Sulphate ions to form lead (II) Sulphate.
- The insoluble lead Sulphate adheres to the electrodes.
Overall reaction - Note: The Lead Sulphate should NOT be left for too long to accumulate on the electrodes
Reason: The fine PbSO4(s) will charge to coarse non reversible and inactive form and the accumulator will become less efficient. - During use/discharge; the lead and the lead (IV) oxide are depleted, and the concentration of sulphric acid declines.
- During recharging
- Is usually done by applying a suitable voltage to the terminals of the cell.
- At the negative terminal - the lead ions accept electrons to form lead solid.
Overall reaction - This process restores the original reactants.
- During discharge/when in use
Electrolysis
Definition of Electrolysis
- Is the decomposition of molten or aqueous solutions by passage of electric current through it.
Terminologies Used in Electrolysis
- Electrolyte
- Is a solution which allows electric current to pass through while it gets decomposed.
- Electric current transfer in electrolyte occur through ions.
- The electrolyte can be aqueous solutions or molten solutions
- Electrolyte with may ions are called strong electrolyte; while those with few ions are called weak electrolytes.
Examples
- Electrodes
- Are the solid conductors, usually roots, which usually complete the circuit between electrolytes and cell/battery
- Are of two types
- Anode - The electrode connected to the positive terminal f a battery/cell
- Cathode -Electrode connected to the negative terminal of the battery/cell.
- Note: Graphite rods are commonly preferred as electrodes in most access.
Reasons:- They are inert/unreactive
- Are cheap
- Platinum is also relatively inert; but not less preferred to Graphite because its expensive.
Preferential discharge of ions
- The products of electrolysis of any given electrolyte depend on the ions present in an electrolyte.
- Commonly most molten electrolytes have only two ions; a cation and an anion and are termed Binary electrolytes.
- As the electrolyte decomposes, ions collect/move to the opposite poles.
- Negatively charged ions move to the Anode; the positive electrode, while the positively charged ions move to the cathode, the negative electrode;
- Regardless of how many ions move to an electrode; only one can be discharged ot give a product.
- Both cations and anions have a preferential discharge series.
- Discharge for cations
- Cations are discharged by reduction (accepting electrons) to form their respective products.
- The ease of reduction of cations depends on their position of electrochemical series.
- Thus Ag+ is most readily discharged as it s the weakest reducing agent.
- Discharge for anions
- Anions are discharged by oxidation (electron loss) to form their respective products.
- Discharge of anions is viz.
- Discharge for cations
Electrolysis of Various Substances
- Electrolysis of Dilute Sulphuric Acid
Apparatus.
Procedure
Anelectric current is passed through the dilute sulphuric acid.
Observation
At the Anode- A colourless gas; collects
- The gas collected relights a glowing splint; and its volume is half the volume of the gas at cathode. The gas is oxygen.
At the cathode- A colourless gas collects
- The collected gas burns with a pop-sound; and its volume is double the volume of gas at the anode.
- The gas is Hydrogen gas.
Explanations- Ions present in the electrolyte
- Hydrogen ions and Sulphate ions form sulphuric acid.
- Hydrogen ions and hydroxide ions from water.
At the Anode (Positive electrode) - The negatively charged Sulphate ions and hydroxide ions migrate to the anode.
Reason: OH-(aq) ions have a greater tendency to loose electrons than the SO42-(aq) ions
Anode equation
4OH-(aq) → O2(g) + 2H2O(l) + 4e-At the cathode (negative electrodes)
- The positively charged hydrogen ions migrate to the cathode
Equation
2H+(aq) + 2e- → H2(g)
Note:
- The volume of oxygen produced at the anode is half the volume of hydrogen produced at the cathode.
Reason:The 4 electrons lost by the hydroxide ions to form 1 mole (1 volume) of oxygen molecules are gained by the four hydrogen ions which form 2 molecules (2 volumes) of hydrogen molecules. - During the electrolysis, the concentration of the electrolyte (H2SO4(aq), increases
Reasons:The volumes of hydrogen and oxygen gas liberated are in the same ratio as they are combined in water.
Thus the amount of water in the electrolyte progressively decrease; hence the increased electrolyte concentration.
- The volume of oxygen produced at the anode is half the volume of hydrogen produced at the cathode.
Conclusion
- Electrolysis of dilute sulphuric acid is thus the electrolysis of water.
Note: The Hoffmans voltmeter can be used instead of the circuit above. Viz.
- Electrolysis of dilute sodium chloride
Apparatus
Procedure- An electric current is passed through dilute sodium chloride solution; with carbon rods as the electrodes.
- Gases evolved of each electrode are collected and tested.
Observations
At the anode:- A colourless gas is collected
- The gas relights a glowing splint, and its volume is half the volume of the gas collected at the cathode.
- The gas is Oxygen, O2
At the cathode- A colourless gas collects
- The gas burns with a pop sound; and its volume is twice the volume of the gas collected at the anode.
- The gas is hydrogen gas;
Explanations- The ions present in the electrolyte are:-
- Na+(aq) and Cl- from sodium chloride
- H+(aq) and OH-(aq) from water.
At the Anode- Cl- and OH- migrate to the anode
- OH- are preferentially discharged coz they have greater tendency to lose electrons than the chloride ions; - the OH-(aq) lose electrons to form water and O2(g) at anode.
Anode equation
4OH-(aq) → 2H2O(l) + O2(g) + 4e-
At the Cathode
- The positively charged Na+(aq), and H+(g) migrate to the cathode
- the H+(aq) are preferentially discharged.
Reason:They H+(aq) have a greater tendency to gain electrons than Na+(aq) ions. The H+(aq) gain electrons to form Hydrogen atoms (H) which then form molecules of hydrogen which bubble off at the electrode.
Cathode equation
2H+(aq) + 2e- → H2(g)
Conclusion - Ratio of the volumes of H2(g) and O2(g) evolved at cathode and anode is 2:1 respectively.
- Electrolysis f dilute NaCl is thus the electrolyze of water since only water is decomposed.
- Electrolysis of Brine/concentrate sodium chloride.
Apparatus
Procedure- An electric current is passed though concentrated sodium chloride/brinc
Observation
At the Anode- A greenish yellow gas is evolved.
- The gas has a pungent irritating smell; and its volume is equal to the volume of the gas evolved at he cathode.
- The gas is chlorine Cl2(g)
At the Cathode- A colourless gas is liberated
- The gas burns with a pop sound; and its volume is equal to volume of gas evolved at the anode.
- The gas is Hydrogen gas; H2(g)
Explanations- The ions present in the electrolyte are:-
- Na+(aq) and Cl- from sodium chloride
- H+(aq) and OH-(aq) from water
At the anode- Cl-(aq), and OH-(aq), migrate to the anode
- The chloride ions are preferentially discharged.
Reason; -OH-(aq) have higher tendency to lose electrons than Cl- ions. - However coz of the higher concentration Cl-(aq), relative to OH-(aq), the Cl-(aq), are preferentially discharged hence the evolution of Chlorine gas.
At the Cathode
- Na+(aq) and H+(aq) migrate to the cathode.
- H+ with a higher tendency to gain electrons are preferentially discharged; hence the evolution of hydrogen gas at the cathode.
NB:- The pH of the electrolyte becomes alkaline/increases with time.
Reason: The removal of H+(aq) which come form water leaves excess hydroxide ions (OH-(aq), hence the alkalinity. - Evolution of chlorine gas at anode soon stops after sometime and is replaced by O2(g)
Reason: Evolution of Cl2(g) decrease/lowers the concentration of Cl-(aq) in the electrolyte. - As soon as the Cl-(aq) concentration becomes equal to that of OH-(aq)
- The pH of the electrolyte becomes alkaline/increases with time.
The Mercury Cathode Cell - Is an electrolytic arrangement commonly used for the large scale manufacture of chlorine and sodium hydroxide.
Apparatus - Electrolyte in the mercury cell is Brine (concentrated NaCl)
- Anode is carbon or titanium
- Cathode is a moving mercury film.
Reactions
At the Anode - Both Chloride and Hydroxide ions are attracted
- Due to their high concentrations the chloride ions are preferentially discharged.
- The Cl-(aq) lose electrons to form Chlorine gas. (greenish yellow)
Equation
2Cl-(aq) → Cl2(g) + 2e-
Cathode (moving mercury - The Na+(aq) and H+(aq) are attracted
- The discharge of H+(aq) is more difficult than expected
- Hydrogen has a high over voltage at the moving mercury electrode and so sodium is discharged.
Equation
Na+(aq) + e- → Na(s) - The discharged sodium atoms combine with mercury to form sodium amalgam
Equation
Na(s) + Hg(l) → NaHg(l) - The sodium amalgam reacts with water to form sodium hydroxide, hydrogen and mercury.
Equation
2NaHg(l) + 2H2O(l) → 2NaOH(aq) + H2(g) + 2Hg(l) - Hydrogen is pumped out while the Mercury is recycled.
- The resultant NaOH is of very high party.
Limitations/disadvantages of the Mercury Cathode cell. - Its expensive due to the high cost of mercury.
- Pollution form Mercury; i.e. Mercury is poisonous and must be removed from the effluent.
- Electrolysis of Copper (II) Sulphate solution.
NOTE: The products of electrolysis of copper (II) Sulphate solution depends on the nature of the elctrodes used.
Apparatus
Ions present in the elctrolyte:- From copper (II) Sulphate
- From water
- During electrolysis
- Using carbon/platinum electrodes
Observations
At the Anode:- A colourless gas is liberated
- The gas relights a glowing splint; hence its oxygen.
At the cathode- A reddish brown coating (of Cu solid) is deposited.
In the Electrolyte- The blue colour of the solution (CuSO4)(aq)/ becomes pale and finally colourless after a long time.
Reason : The blue colour is due to Cu2+. As the Cu2+(aq) are continuously being discharged at the cathode; the concentration of CU2+ decreases i.e. decrease in the concentration of Cu2+(aq) in the solution - The electrolyte become acidic/pH decreases (declines)
Reason:Accumulation of H+(aq) in the solution since only OH- (from water) are being discharged (at the anode).
Explanation
At the anode- The SO42-(aq) and OH–(aq) migrate to the anode.
- The hydroxide ions have a higher tendency to lose electrons than the SO42-(aq)
- They (OH-) easily loose electrons to form the neutral and unstable hydroxide radical (OH-)
- The hydroxide radical (OH) decomposes to form water and Oxygen.
Anode equation
4OH-(aq) → O2(g) + H2O(l) + 4e-
At the cathode
- Copper ions and H+(aq) migrate to the cathode
- Cu2+(aq) have a greater tendency to accept electrons than H+(aq)
- The Cu2+(aq) are thus reduced to form copper metal which is deposited as a red-brown coating on the cathode.
Cathode equation.
Cu2+(aq) + 2e- → Cu(s)
- Using copper rods electrodes
Observations
At the anode- Mass of the anode (Copper anode) decreases
At the Cathode- reddish brown deposit
- cathode increases in mass
Electrolyte- no apparent change
Note: The gain in mass of the cathode is equal to the loss in mass of the anode.
Explanations
At the anode- The SO42-(aq) and OH-(aq) are attracted to the anode.
- However, none of them is discharged;
- Instead; the copper anode itself gradually dissolves; hence the loss in mass of the anode;
Reason: it s easier to remove electrons form the copper anode itself than format the hydroxide ions
Anode equation
Cu(s) → Cu2+(aq) + 2e-
At the cathode
- H+(aq) migrate to the cathode
- The Cu2+(aq) are preferentially discharged; because they have a greater tendency to accept electrons
- The copper cathode is thus coated with a reddish brown deposit of copper metal hence increase in mass.
Cathode equation
Cu2+(aq) + 2e- → Cu(s)
Factors Affecting Electrolysis and Electrolytic Products
- Electrochemical series
- Electrolytic products at the anode and cathode during electrolysis depends on its position in the Electrochemical series.
- Cations:The higher the cation in the electrochemical series; the lower the tendency of discharge at the cathode.
Reason:Most electropositive cations require more energy in order to be reduced and therefore are more difficult to reduce.
Anions: Discharge is through oxidation and is as follows.
- Concentration of electrolytes
- A cation or anion whose concentration is higher is preferentially discharge if the ions are close in the electrochemical series.
Example : dilute and concentrated NaCl
Product at the anode .
- A cation or anion whose concentration is higher is preferentially discharge if the ions are close in the electrochemical series.
- The electrodes used:Products obtained at electrodes depend on the types of electrodes used
Examples:in the electrolysis of CuSO4(aq) using carbon and copper rods separately.
Applications of Electrolysis
- Extraction of reactive metals
- Reactive metals/elements like sodium, magnesium, aluminum are extracted form their compounds by electrolysis.
Example :Sodium is extracted from molten sodium chloride using carbon electrodes.
- Reactive metals/elements like sodium, magnesium, aluminum are extracted form their compounds by electrolysis.
- Purification of metals
- It can be used in refining impure metals
Examples: Refining copper- The impure copper is made of the anode.
- Their strips of copper are used as the cathode
- Copper (II) Sulphate are used as the electrolyte.
- During the electrolysis the anode dissolves and pure copper is deposited on the cathode.
- The impurities (including valuable amounts of silver and gold) from the crude copper collect as a sludge become the anode.
- It can be used in refining impure metals
- Electroplating
- Is the process of coating one metal with another, using electrolysis so as to reduce corrosion or to improve its appearance.
- During electrolysis:
- the item to be electroplated is made the cathode
- the metal to be used in electroplating is used as the anode
- the electrolyte is made from a solution containing the ions of the metal to be sued in electroplating.
- Examples
- Gold-plated watches; silver plated utensils
- Steel utensils marked EPNS. I.e. Electroplated Nickel Silver.
- Anodizing Aluminum
- Is the reinforcement of the oxide coating on Aluminum utensils/articles
- Is done by electrolysis of dilute sulphuric acid using Aluminum articles as anode.
- Importance: Prevention arrosion of Aluminum articles
- Manufacture of sodium hydroxide, chlorine and hydrogen
- Sodium hydroxide is prepared by the electrolysis of brine, for which 3 methods are available
- The method depends on the type of electrolytic cell.
- These cells are
- The mercury cell
- The diaphragm cell
- The membrane cell
- The mercury cell
Components- The electrolyte is concentrated sodium chloride
- The anodes are made of graphite or titanium, which are placed above the cathode.
- The cathode consists of mercury, which flows along the bottom of the cell.
Chemical reactions
Anode- Both chloride and hydroxide ions are attracted.
- Chloride ions are preferentially discharged due to their high concentration
- The chloride ions undergo oxidation to form green yellow chlorine gas.
Equation
At the cathode (flowing mercury)- Na+(aq) migrate to the cathode
- Sodium ions are preferentially discharged.
- They undergo reduction to form sodium solid.
Equation
Na+(aq) + e- → Na(s) - the discharged sodium atoms combine with mercury to form sodium amalgam
Equation
Na(s) + Hg(l) → NaHg(l) - The sodium amalgam is then passed into another reactor containing water.
- The amalgam reacts with water forming hydrogen and sodium hydroxide.
Equation
2NaHg(l) + 2H2O(l) → 2NaOH(aq) + H2(g) + 2Hg(l)
Main product:- Sodium hydroxide
By products- Sodium and chlorine.
Advantages of mercury cathode cell- The resultant sodium hydroxide is very pure; as it has no contamination from sodium chloride.
- It is highly concentrated; i.e. about 50%.
Disadvantages
- Some of the mercury said its way into the environment leading to mercury pollution; a common case of brain damage in humans.
- At the operating temperatures (70o C - 80o C), mercury vapours escape into the atmosphere and cause irritation and destruction of lungs tissues.
- Its operation requires highly skilled man power.
- Diaphragm cell
Components- An asbestos diaphragm; to separate the electrolytic cell into two compartments; thus preventing mixing of H2 and Cl2 molecules
- The anode compartment contains a graphite rod/Titanium.
- The cathode compartment contains a stainless steel cathode.
Diagram: The diaphragm cell
Chemical reactions- The asbestos diaphragm is permeable only to ions, but not to the hydrogen or chlorine molecules.
- It thus prevents H2(g) and Cl(g) form mixing and reacting to yield HCl(g)
- It also separates NaOH and Cl2 which would otherwise react.
At the anode- Chloride ions undergo oxidation to form chlorine gas.
Equation
2Cl-(aq) → Cl2(g) + 2e-
At the cathode- H+ and Na+(aq) migrate to the cathode compartment.
- The H+ are preferentially discharged.
- They (H+(aq)) undergo reduction to form hydrogen gas.
Equation
2H2O(l) + 2e- → H2(g) + 2OH-(aq)
The discharge of H+ causes more water molecules to dissociate, thus increasing the concentration of OH- in the solution. Therefore, the Na+ and OH- ions also react in the cathode compartment to form sodium hydroxide.
Equation
2H2O(l) + 2Cl-(aq) + 2Na+(aq) → 2Na+(aq) + 2OH-(aq) + H2(g) + Cl2(g)
Advantage- Does not result into pollution
Disadvantages- The resultant NaOH is dilute (12% NaOH)
- It is also not pure due to contamination with NaCl (12% NaOH + 15% NaCl by mass.
- Note: -The concentration of the NaOH can be increased by evaporating excess water, during which NaCl with a lower solubility crystallizes out first, leaving NaOH at a
higher concentration.
- The solid NaCl (Crystals) are then filtered off.
- This is a case of fractional crystallization . - The membrane cell
- Is divided into 2 compartments by a membrane
- Most commonly used type of Membrane is the cation exchange membrane.
- This membrane type allows only cations to pass through it.
Components - A cation exchange that divides the cell into 2 compartments; an anode and a cathode compartments.
- Both electrodes are made of graphite
- The electrolyte in the anode compartment is purified brine
- The electrolyte in the cathode compartment is pure water.
Diagram
Chemical reactions
The anode - Chloride ions undergo oxidation to form green yellow chlorine gas.
Equation
2Cl-(aq) → Cl2(g) + 2e-
The cathode - As current passes through the cell H+ and Na+ pass across the membrane to the cathode
- H+ are preferentially discharged.
- They undergo reduction to liberate hydrogen gas.
- Continuos discharge of H+ leaves the OH- at a higher concentration
- The OH- react with Na+ to form sodium hydroxide
Equation
2H2O(l) + 2e- → H2(g) + 2OH-(aq
Advantages- Resultant sodium hydroxide is very pure, since it has no contamination from NaCl.
- The sodium hydroxide has a relatively high concentration; at about 30 35% NaOH by mass.
Uses of sodium hydroxide, chlorine, and hydrogen
- Sodium Hydroxide
- React with chlorine to form sodium chlorate sodium hypochlorite or I, NaOCl. This is a powerful oxidising agent which is used for sterilization and bleaching in textiles, paper and textile industries.
i.e. 2NaOH(aq) + Cl2(g) → NaOCl(aq) + NaCl(g) + H2O(l) - Manufacture of sodas, detergents and cosmetics.
- Neutralization of acidic solutions in the laboratories.
- React with chlorine to form sodium chlorate sodium hypochlorite or I, NaOCl. This is a powerful oxidising agent which is used for sterilization and bleaching in textiles, paper and textile industries.
- Hydrogen
- For hydrogenation in the manufacture of margarine
- Manufacture of ammonia
- Production of hydrochloric acid
- Chlorine
- Formation of sodium chlorate I; for bleaching in pulp, textile and paper industries.
- Sewage and water treatment
- Manufacture of polymers such as polyvinyl chloride.
- Manufacture of pesticides.
Quantitative Aspects of Electrolysis
Basic Terminologies and Concepts
- Ampere
- Is the standard unit used to measure an electric current; the flow of electrons
- Is usually abbreviated as amps.
- Coulomb
- Is the quantity of electricity, when a current of 1 ampere flows for one second. I.e. 1 Coulomb = 1 Ampere x 1 second
- Generally:
Quantity of Electricity = current x Time in seconds
A = It; Where
Q = Quantity of electricity in coulombs
I = Current in Amperes
T = time in seconds
- Faraday
- Is the quantity of electricity produced by one mole of electrons; and is usually a constant equivalent to 96487 (approx. 96500) coulombs
Faradays laws of Electrolysis.
First law;
- The mass of substance liberated during electrolysis is directly proportional to the quantity of electricity passed.
Worked examples
- A current of 2.0 Amperes was passed through dilute potassium sulphate solution for two minutes using inert electrodes.
- Write the equation for the reaction at anode.
- Work out the mass of the product formed at the cathode. (H = 1.0, Faraday = 96,000 C)
Solution; quantity of electricity = current x time
= 2 x 2x 60
= 240 coulombs
cathode reaction
1 mole of electrons = 96000 C
4 moles of e- = 4 x 96000 C
= 384,000 C
- What mass of copper would be coated on the cathode from a solution of copper (II) Sulphate by a current of 1 amp flowing for 30 minutes. (Cu = 63.5; Faraday constant = 96487 Cuo/-)
solution
Cathode reaction
Cu2+(aq) + 2e- → Cu(s)- 1 mole of Cu requires 2 moles of electrons
- Quantity of electricity passed; = 1 x 30 x 60 coulombs; = 1800C
- 1 mole of electrons carriers a charge of 96487 coulombs = 192974 coulombs
- 192.974 coulombs deposit 63.5 g of Cu.
- Thus 1800 C will deposit 63.5 x 1800/192974 = 0.592 grams
- An element x has relative atomic mass of 88g. when a current of 0.5 amperes was passed through a solution of x chloride for 32 minutes, 10 seconds; 0.44 g of x was deposited at the cathode. (1 faraday = 96500 C) Calculate the charge on the ion of x.
- In the electrolysis of dil CuSO4 solution, a steady current of 0.20 Amperes was passed for 20 minutes. (1 Faraday = 96, 500 C Mol-, Cu = 64)
Calculate- The number of Coulombs of electricity used
- The mass of the substance formed at the cathode.
2 moles of electrons liberate 1 mole of Cu. i.e. Cu 2+ + 2e- → Cu(s)
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