Chemistry Paper 2 Questions and Answers - KCSE 2020 past papers

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Instructions to candidates

Answer all the questions
Non-programmable silent electronic calculators and KNEC mathematical tables may be used.
All working must be clearly shown where necessary.
Candidates should answer the questions in English.

Crude oil is a mixture of hydrocarbons which are separated by fractional distillation. One of the components obtained contains an alkane A, with eleven carbon atoms.
1. Write the molecular formula of A (1 mark)
2. Pentane can be obtained from compound A as shown
  A → Pentane + B
  1. Give the name of this conversion process. (1 mark)
  2. State the conditions used in this process. (1 mark)
  3. Give the name of compound B. (1 mark)
3. Draw and name two isomers of pentane (4 marks)
  Isomer 1 Structure Name
  Isomer 2 Structure Name
4. Incomplete combustion of pentane may result in air pollution. Write an equation to illustrate this combustion. (1 mark)
5. The main component in natural gas is methane. Describe how methane in natural gas is formed. (2 marks)
6. In the laboratory, methane can be prepared from salts of alkanoic acids. Describe how methane is prepared from sodium ethanoate. (2 marks)
1. 1. State what is meant by the term 'dynamic equilibrium. (1 mark)
  2. Dichromate(VI) ions are orange in colour while chromate(VI) ions are yellow. (i1) Consider the following equilibrium.
    Cr₂O₇²⁻(aq) + 2OH⁻(aq) 2CrO₄^2−(aq)+ H₂O(l)
    State and explain the observation that will be made if sulphuric(VI) acid is added to the mixture. (2 marks)
2. One of the reactions in the manufacture of nitric(V) acid involves catalytic oxidation of ammonia as shown in the equation.
  4NH₃(g)+ 5O₂(g) 4NO(g) + 6H₂O(g); ΔH=−909 kJmol⁻¹The reaction is carried out at a pressure of 10 atmospheres and a temperature of 900°C
  1. Other than nitric(V) acid, name another product that is formed. (1 mark)
  2. State and explain the effect on the position of equilibrium if the reaction is carried out.
    1. at 10 atmospheres pressure and 450°C; (2 marks) .
    2. at 900 °C and 20 atmospheres pressure; (2 marks)
    3. in the absence of a catalyst. (1 mark)
3. State and explain the effect on the rate of the reaction if the reaction is carried out at 10 atmospheres and 450°C. (2 marks)
4. A factory uses 100 kg of ammonia each day to produce 160 kg of nitrogen(II) oxide. Calculate the percentage yield of nitrogen(II) oxide. (3 marks)
1. One of the ores of iron is haematite, Fe₂O₃. Give the name and formula of two other ores of iron. (2 marks)
  Name Formula
2. In a certain factory, iron is extracted from the haematite ore using the blast furnace as shown in Figure 1. The other raw materials are coke, limestone and air. The melting and boiling points of iron are 1535°C and 3000 °C, respectively.
  
  Figure 1
  1. State how the temperature in region I compares with that in region II. Give a reason. (1 mark)
  2. The main reducing agent in the furnace is carbon(II) oxide formed by the reaction
    CO₂(g) + C(s) →2C0g)
    Write two equations to show how carbon(IV) oxide is formed in the furnace. (2 marks)
  3. Suggest a value for the temperature in region III. Give a reason. (2 marks)
  4. Name the main component in the slag. (1 mark)
  5. State one role that slag plays in the blast furnace. (1 mark)
  6. The iron produced in the blast furnace is brittle due to presence of impurities
    1. Name the main impurity in this iron (1 mark)
    2. State one use of this iron (1 mark)
  7. Recycling is one method used to reduce production costs. State and explain the by products that can be recycled in this factory. (2 marks)
Table 1 shows the elements in period 3 of the periodic table. Study it and answer the questions that follow.
Table 1

Element Na Mg Al Si P S Cl Ar
1. Write the formulae of two oxides, for each of the following
  1. sodium: Oxide I... Oxide II... (1 mark)
  2. chlorine Oxide I ... Oxide II....(1 mark)
2. The products of the reaction between phosphorus and chlorine depend on the conditions used. Write the equation for the reaction when chiorine reacts with excess phosphorus. (1 mark)
3. Identify the element with the highest electrical conductivity. Give a reason. (2 marks)
4. Describe an experiment that can be used to illustrate the variations in reaction of sodium, magnesium and aluminium with water. (3 marks)
5. State and explain the differences in the melting points of:
  1. Chlorine and argon. (2 marks)
  2. magnesium oxide and silicon(IV) oxide. (2 marks)

Table 2 gives standard reduction potentials for some half cells.
Table 2

Half cell	Half cell equation	E⁰/V
I	Fe³⁺(aq) + e → Fe²⁺(aq)	+0.77
II	K⁺(aq) +e → K(s)	−2.92
III	Ag⁺(aq) + e → Ag(s)	+0.80
IV	Pb²⁺(aq) +2e → Pb(s)	−0.13
V	l₂(aq) + 2e → 2l⁻(aq)	+0.54

State the standard conditions of an electrochemical cell. (2 marks)
An electrochemical cell was constructed using half-cells III and IV.
1. Complete Figure 2 by labelling the parts of the cells indicated as A₁- A₄(2 marks)
  
  Figure 2
  A₁A₂
  A₃
  A₄
2. Write an equation for the cell reaction and calculate the e.m.f. of the cell.
  Equation (1 mark)
  e.m.f. (1 mark))
3. The salt bridge helps in completing the circuit. Explain why a saturated solution of potassium chloride is not suitable for use in the salt bridge in this electrochemical cell. (1 mark)
State why it is not possible to construct a similar electrochemical cell using half-cells II and III. (1 mark)
State and explain the observations made when aqueous potassium iodide is added to aqueous iron(III) sulphate. (2 marks)
Acidified potassium dichromate(VI) and acidified potassium manganate(VII) may be used in determining concentration of Fe2⁺ ions in a sample. If acidified potassium dichromate(VI) is used, an indicator is added to determine the end point but for acidified potassium manganate(VII), no indicator is added.
1. Explain why it is not necessary to use an indicator when acidified potassium manganate(VII) is used. (1 mark)
2. An alloy containing iron was dissolved in an acid and the total volume made up to 250cm³. 25.0 cm³ of this solution required 18.0 cm³ of 0.15 M acidified potassium dichromate(VI) to react completely. The equation for the reaction is:
  Cr₂O₇²⁻ (aq) + 6Fe²⁺(aq)+ 14H⁺(aq) →2Cr³⁺(aq)+ 6Fe³⁺(aq) +7H₂O(l)
  Calculate the mass of iron in the alloy (Fe = 56.0). (3 marks)

1. water containing hydrogen carbonate, HCO₃⁻ and calcium Ca²⁺ ions, is said to be hard water
  1. Describe one way in which HCO₃⁻ ions get into river water (1 mark)
  2. Explain the disadvantage of using this type of water in boilers. (2 marks)
2. Analysis of a river water sample showed the presence of the following ions Ca²⁺, Na⁺, Cl⁻ , NO₃⁻.
  1. Name the type of water hardness present in the sample. (1 mark)
  2. Describe one precipitation method that can be used to soften the water. (2 marks)
  3. The water sample was passed through a resin as shown in Figure 3.
    1. Write an equation for a reaction that took place in the column. (1 mark
    2. Complete treatment of the water sample required passing it through another resin. Give the formula of this resin. (1 mark)
    3. Explain why a river water sample that has been treated using resins may still require boiling to make it safe for drinking. (2 marks)
3. Compound C was used to prepare a potassium soap.
  1. Give the formula of the potassium soap obtained. (1 mark)
  2. State one difference in the properties of potassium and sodium soaps. (1 mark)
4. A soapless detergent has the formula
  CH₃(CH₂)₁₀CH₂OSO₃Na
  With reference to this formula, identify the hydrophobic and the hydrophilic parts of the detergent. (1 mark)
  Hydrophobic (1 mark)
  Hydrophilic (1 mark)

Marking scheme

Crude oil is a mixture of hydrocarbons which are separated by fractional distillation. One of the components obtained contains an alkane A, with eleven carbon atoms.
1. Write the molecular formula of A (1 mark)
  - C₁₁ H₂₄ ✓1
    Accept open structural formula.
    Acc. CH₃(CH₂)₉CH₂
2. Pentane can be obtained from compound A as shown
  A → Pentane + B
  1. Give the name of this conversion process. (1 mark)
    - Cracking ✓1
      Acc. Catalytic/ Thermal cracking
  2. State the conditions used in this process. (1 mark)
    - High temperatures ✓½/Acc. 400-700°C/Heat
    - High pressure✓½ Acc. 70 atmospheres /Rej. Pressure alone.
    - Catalyst✓½ Acc. Zeolite catalyst/Silica catalyst/Alumina catalyst
  3. Give the name of compound B. (1 mark)
    - Hexene ✓1 Acc. C₆ H₁₂Acc.Hex-1-ene/Hex-2-ene/Hex-3-ene.
3. Draw and name two isomers of pentane (4 marks)
  Isomer 1 Structure Name
  CH₃CH₂CH₂CH₂CH₃   Pentane ✓1
  CH₃(CH₂)₃CH_{3    ✓1}
  
  Rej. molecular formula C₅H₁₂ (Name correct, wrong structure =0)
  
  Isomer 2 Structure Name
  2-methylbutane✓1
    ✓1
  2,2-dimethylpropane ✓1
  ✓1
  Acc. condensed/open structure
4. Incomplete combustion of pentane may result in air pollution. Write an equation to illustrate this combustion. (1 mark)
  - C₅H₁₂(g) + 3O₂(g) → 5C(s) + 6H₂O(l) ✓1
    OR
  - 2C₅H₁₂(g) + 11O₂(g) → 10CO(g) + 12H₂O(l)
    (Either one ingnore states
5. The main component in natural gas is methane. Describe how methane in natural gas is formed. (2 marks)
  - Decomposition/breakdown/decay of organic matter ✓1 in the absence of oxygen ✓1
6. In the laboratory, methane can be prepared from salts of alkanoic acids. Describe how methane is prepared from sodium ethanoate. (2 marks)
  - Heat✓½ a mixture of sodium ethanoate and sodalime✓1
  - Collect the gas over water ✓½ /use syringe/upward delivery/downward displacement of air.
    (condition of heat must)
    Accept correct diagram
  - Mixture of sodalime and ethanoate ✓1
  - Arrow for heat ✓½
  - Method of collection✓½
    Accept equation CaO
    Sodium ethanoate + sodalime Methane + Na₂CO ✓2
    Heat
    then mention method of collection
1. 1. State what is meant by the term 'dynamic equilibrium. (1 mark)
    - State of reaction where the rate ✓1 of forward and backward reaction are the same but in opposite direction.
  2. Dichromate(VI) ions are orange in colour while chromate(VI) ions are yellow. (i1) Consider the following equilibrium.
    Cr₂O₇²⁻(aq) + 2OH⁻(aq) 2CrO₄^2−(aq)+ H₂O(l)
    State and explain the observation that will be made if sulphuric(VI) acid is added to the mixture. (2 marks)
    - Intensity of orange colour increases
    - Addition of H⁺ removes OH⁻ hence reverse/backward reaction is favoured✓1 / equilibrium shifts to the left.
      Accept concentration of OH⁻ reduces.
2. One of the reactions in the manufacture of nitric(V) acid involves catalytic oxidation of ammonia as shown in the equation.
  4NH₃(g)+ 5O₂(g) 4NO(g) + 6H₂O(g); ΔH=−909 kJmol⁻¹The reaction is carried out at a pressure of 10 atmospheres and a temperature of 900°C
  1. Other than nitric(V) acid, name another product that is formed. (1 mark)
    - Nitrons acid/HNO₂✓1
      Nitric (III) Acid
  2. State and explain the effect on the position of equilibrium if the reaction is carried out.
    1. at 10 atmospheres pressure and 450°C; (2 marks)
      - Equilibrium shifts to the right✓1, forward reaction is favoured since it is exothermic✓1
    2. at 900 °C and 20 atmospheres pressure; (2 marks)
      - Equilibrium shifts to the left✓1
      - Backward reaction is favoured/favours ✓1 the direction with fewer molecules/due decrease in number of mole/molecules
    3. in the absence of a catalyst. (1 mark)
      - No effect
3. State and explain the effect on the rate of the reaction if the reaction is carried out at 10 atmospheres and 450°C. (2 marks)
  - Rate of reaction decreases/reduces/slowers✓1.
  - decrease in temperature leads to reduction/decrease in kinetic energy✓½ of the molecules hence less effective✓½ collisions.
4. A factory uses 100 kg of ammonia each day to produce 160 kg of nitrogen(II) oxide. Calculate the percentage yield of nitrogen(II) oxide. (3 marks)
  Moles of NH₃ =100,000 ✓½ =5882.35
  Moles of NO = mole ratio 1:1
  Molar mass NH₃ =17 ✓½ Molar mass N=30✓½
  Mass of NO = 5882.35 × 30✓½ =176470.58
  %yield of No = 160000 × 100 ✓½ OR 160 × 17 × 100
  176470 30 × 100
  =90.667% ✓½ =90.67%
1. One of the ores of iron is haematite, Fe₂O₃. Give the name and formula of two other ores of iron. (2 marks)
  Name Formula
  1. Siderite ✓½ FeCO₃✓½
  2. Magnetite✓½ Fe3O₄✓½
    Iron pyrite✓½ FeS₂✓½
2. In a certain factory, iron is extracted from the haematite ore using the blast furnace as shown in Figure 1. The other raw materials are coke, limestone and air. The melting and boiling points of iron are 1535°C and 3000 °C, respectively.
  
  Figure 1
  1. State how the temperature in region I compares with that in region II. Give a reason. (1 mark)
    - Temp. in region I is lower than that in II ✓½
    - Raw materials are not pre-heated ✓½
      Region II is nearer to hot air blast that pre-heats
      Hot air raises in the furnace it becomes cooler
  2. The main reducing agent in the furnace is carbon(II) oxide formed by the reaction
    CO₂(g) + C(s) →2C0g)
    Write two equations to show how carbon(IV) oxide is formed in the furnace. (2 marks)
    1. C(s) + O₂(g) → CO₂(g) ✓1
      Heat
    2. CaCO₃(g) CaO(s) +CO₂(g) ✓1
      (condition)
      Fe₂O₂(s) + 3CO(g) → 2Fe(l) + 3CO₂(g)
  3. Suggest a value for the temperature in region III. Give a reason. (2 marks)
    - 1535°C - 3000°C ✓1 Any value.
    - Helps maintain Iron molten state. ✓1
  4. Name the main component in the slag. (1 mark)
    - Calcium Silicate/ CaSiO₃
  5. State one role that slag plays in the blast furnace. (1 mark)
    - Prevents oxidation of iron by hot air .
  6. The iron produced in the blast furnace is brittle due to presence of impurities
    1. Name the main impurity in this iron (1 mark)
      - Carbon
    2. State one use of this iron (1 mark)
      - Making manhole covers
      - Making scissors
      - Bunsen burner bases
      - Making iron pipes
      - Electric poles
      - Fire grills
      - Manufacture of steel
      - Manufacture of building blocks
      - Electric fence arch
  7. Recycling is one method used to reduce production costs. State and explain the by products that can be recycled in this factory. (2 marks)
    - Waste gases✓1
      - used to preheat the air blasts ✓1
    - CO₂(g)✓1
      - used to preheat the air blasts. ✓1
      - CO₂is converted to CO. Used as a reducing agent
    - CO(g)✓1
      - used as a reducing agent ✓1
      - used to preheat air blasts.
Table 1 shows the elements in period 3 of the periodic table. Study it and answer the questions that follow.
Table 1

Element Na Mg Al Si P S Cl Ar
1. Write the formulae of two oxides, for each of the following
  1. sodium: Oxide I Na₂O Oxide II Na₂O₂ (1 mark)
  2. chlorine Oxide I Cl₂O Oxide II Cl₂O₇(1 mark)
    ClO, ClO₂, Cl₂O₂, Cl₂O₅, Cl₂O₆, Cl₂O, ClO₃
2. The products of the reaction between phosphorus and chlorine depend on the conditions used. Write the equation for the reaction when chlorine reacts with excess phosphorus. (1 mark)
  - 2P(s) + 3Cl₂(g) → 2PCl₃(l) ✓1
    P₄(s) + 6Cl₂(g) → 4PCl₃(g) ✓1
    P₁₀(s) + 15Cl₂(g) → 10PCl₃(g) ✓1
3. Identify the element with the highest electrical conductivity. Give a reason. (2 marks)
  - Al (Reject Al³⁺).
  - Highest number of delocalised electrons/highest number of valence electrons per atom
  - has 3 delocalised electrons per atom.
4. Describe an experiment that can be used to illustrate the variations in reaction of sodium, magnesium and aluminium with water. (3 marks)
  - Sodium reacts vigorously✓1 with water/hissing sound is produced
  - Magnesium reacts slowly✓1with water/produces few bubbles on the surface.
  - Aluminium does not react ✓1with water/produces no bubbles.
    (Description is not a marking point)
5. State and explain the differences in the melting points of:
  1. Chlorine and argon. (2 marks)
    Melting point of chlorine is greater/higher ✓1than that of argon. Cl₂(g) is diatomic✓½ while Ar(g) is monoatomic hence Cl₂(g) has stronger✓½ vanderwaals. Forces of attraction/molecules of Cl₂(g) are larger/bigger than Ar(g) molecules hence intermolecules forces in Cl₂ are stronger than Ar.
  2. magnesium oxide and silicon(IV) oxide. (2 marks)
    - Magnesium oxide has a higher melting than Silicon (IV) Oxide.✓2

Table 2 gives standard reduction potentials for some half cells.
Table 2

Half cell	Half cell equation	E⁰/V
I	Fe³⁺(aq) + e → Fe²⁺(aq)	+0.77
II	K⁺(aq) +e → K(s)	−2.92
III	Ag⁺(aq) + e → Ag(s)	+0.80
IV	Pb²⁺(aq) +2e → Pb(s)	−0.13
V	l₂(aq) + 2e → 2l⁻(aq)	+0.54

State the standard conditions of an electrochemical cell. (2 marks)
- 1M solution
- 1 atmospheric pressure.
- Temperature of 25°C or 278K
  3 correct 2mks
  2 correct 1 mk
  1 correct ½ mk
An electrochemical cell was constructed using half-cells III and IV.
1. Complete Figure 2 by labelling the parts of the cells indicated as A₁- A₄(2 marks)
  
  Figure 2
  Soluble salt of Pb
  A₁ Pb/Lead electrode ✓½A₂ Pb²⁺/ Lead(II)Nitrate solution/1M Pb²⁺✓½
  Soluble salt of Ag
  A₃ Ag/ Silver electrode ✓½
  A₄ Ag⁺/ Silver Nitrate/1M Ag⁺ ✓½
2. Write an equation for the cell reaction and calculate the e.m.f. of the cell.
  - Equation (1 mark)
    Pb(s) + 2Ag⁺ (aq) → Pb²⁺ (aq) + 2Ag(s) ✓1
  - e.m.f. (1 mark)
    +0.8−−0.13 ✓½
    +0.93V✓½
3. The salt bridge helps in completing the circuit. Explain why a saturated solution of potassium chloride is not suitable for use in the salt bridge in this electrochemical cell. (1 mark)
  - Formation of insoluble PbCl₂✓½ That reduces the concentration of ions✓½ in electrolyte
    OR
  - Formation of AgCl that reduces the effectiveness of the cell
State why it is not possible to construct a similar electrochemical cell using half-cells II and III. (1 mark)
- K reacts explosively with water.✓1
  Acc. E.M.F of cell is very high which can explode with the cell.
State and explain the observations made when aqueous potassium iodide is added to aqueous iron(III) sulphate. (2 marks)
- (Yellow) Brown solutions✓½ turns to green. Fe³⁺ ions✓½ are reduced to Fe²⁺
- Grey/black precipitate✓½ is formed. Iodide ions✓½ are oxidised to iodine
  Acc.Equation
- Fe³⁺(aq) + 2I⁻(aq) → Fe²⁺(aq) + I₂(s) is +0.23V
  Brown/yellow✓½solubles Green✓½ Grey/black✓½ Explanation✓½
Acidified potassium dichromate(VI) and acidified potassium manganate(VII) may be used in determining concentration of Fe2⁺ ions in a sample. If acidified potassium dichromate(VI) is used, an indicator is added to determine the end point but for acidified potassium manganate(VII), no indicator is added.
1. Explain why it is not necessary to use an indicator when acidified potassium manganate(VII) is used. (1 mark)
  - H⁺/KMnO₄, acts as its own indicator changing from purple to colourless✓1 (decolourised)
2. An alloy containing iron was dissolved in an acid and the total volume made up to 250cm³. 25.0 cm³ of this solution required 18.0 cm³ of 0.15 M acidified potassium dichromate(VI) to react completely. The equation for the reaction is:
  Cr₂O₇²⁻ (aq) + 6Fe²⁺(aq)+ 14H⁺(aq) →2Cr³⁺(aq)+ 6Fe³⁺(aq) +7H₂O(l)
  Calculate the mass of iron in the alloy (Fe = 56.0). (3 marks)
  Moles of Cr₂O₇²⁻=18 × 0.15 ✓½
  1000 =0.0027
  Moles of Fe²⁺ in 25.0cm³ = 6 × 0.0027 = 0.0162✓½
  Moles of Fe²⁺ in 250cm³ = 0.0162 × 10✓½ = 0.162✓½
  0.648M in 250cm³
  M=0.0162 × 1000
  25
  =0.648M
  Mass of Iron=0.162 × 56✓½
  =9.072g✓½
  Condensed working
  18 × 0.15 × 6 × 250 × 56 = 9.072g
  1000×25
  (All marks awarded)

1. water containing hydrogen carbonate, HCO₃⁻ and calcium Ca²⁺ ions, is said to be hard water
  1. Describe one way in which HCO₃⁻ ions get into river water (1 mark)
    - CO₂ dissolves in rain water to form Carbonic acid ✓½
    - Carbonic acid reacts with carbonate rock/Ca and Mg forming hydrogen carbonates of Ca and Mg that gets into rivers. ✓½
  2. Explain the disadvantage of using this type of water in boilers. (2 marks)
    - The CaHCO₃ and MgCO₃ decomposes forming CaCO₃/scales/fur ✓1 lining the boilers and causing poor thermal conductivity/reducing efficiency.
2. Analysis of a river water sample showed the presence of the following ions Ca²⁺, Na⁺, Cl⁻ , NO₃⁻.
  1. Name the type of water hardness present in the sample. (1 mark)
    - Permanent
  2. Describe one precipitation method that can be used to soften the water. (2 marks)
    - Addition of washing soda/NaCO₃✓1
    - Ca²⁺and Mg²⁺ precipitates as carbonates which can be filtered off✓1
  3. The water sample was passed through a resin as shown in Figure 3.
    1. Write an equation for a reaction that took place in the column. (1 mark)
      Resin
      RNH₃⁺OH⁻(s) + NO₃⁻(aq) or Cl⁻→ RNH₃⁺NO⁻(s)(Cl⁻if its over used) + OH(aq)
      (show changes and balance/ Ignore states)
    2. Complete treatment of the water sample required passing it through another resin. Give the formula of this resin. (1 mark)
      - RSO₃⁻H⁺ OR RCOO⁻H⁺ (Ignore changes)
    3. Explain why a river water sample that has been treated using resins may still require boiling to make it safe for drinking. (2 marks)
      - Resin does not kill bacteria/pathogen✓1
      - Boiling kills✓1
3. Compound C was used to prepare a potassium soap.
  1. Give the formula of the potassium soap obtained. (1 mark)
    - C₁₅H₃₁COOK/ CH₃(CH₂₎₁₄ COOK/ CH₃(CH₂₎₁₅ CH₂ COOK
  2. State one difference in the properties of potassium and sodium soaps. (1 mark)
    - Potassium soaps lather more readily in water than Na soaps.
    - Potassium soaps have lower/less melting points.
    - Potassium salts are more soluble in water.
    - Potassium soaps are soft/mild while sodium soaps are hard.
4. A soapless detergent has the formula
  CH₃(CH₂)₁₀CH₂OSO₃Na
  With reference to this formula, identify the hydrophobic and the hydrophilic parts of the detergent. (1 mark)
  - Hydrophobic (1 mark)
    - CH₃(CH₂)₁₀CH₂⁻ OR (CH₃)₃(CH₂)₁₁⁻
      Reject CH₃(CH₃)₁₀
  - Hydrophilic (1 mark)
    - OSO₃⁻Na⁺ OR OSO₃Na
      Reject CH₂OSO₃⁻Na⁺