Kenya Certificate Of Secondary Education (KCSE 2008) Chemistry Paper 3

1. You are provided with:

•     Solid A
•     2.0 M hydrochloric acid, solution B
•     0.1 M sodium hydroxide

You are required to determine the enthalpy change ΔH, for the reaction between solid A and one mole of hydrochloric acid.

Procedure A.

a. Using a burette, place 20.0 cm³ of 2.0 M  hydrochloric acid, solution B in a 100ml beaker. Measure the temperature of the solution after every half-minute and record the values in table 1. At exactly 2½     minutes, add all of solid A to the acid. Stir the mixture gently with the thermometer. Measure the temperature of the mixture after every half-minute and record the values in table 1. (Retain the               mixture for use in procedure B)

  

         i. Plot a graph of temperature (Y-axis) against time. (3 marks)
        ii. Using the graph, determine the change in temperature, ΔT. (1 mark)
       iii. Calculate the heat change for the reaction. (Assume that the specific heat capacity of the mixture is 4.2 Jg^-1K^-1 and the density of the mixture is 1g/cm³). (3 marks)

Procedure B

  b. Rinse the burette thoroughly and fill with sodium hydroxide. Transfer all the contents of the 100ml beaker used in procedure A into a 250 ml volumetric flask. Add distilled water to make up to the             mark. Label this solution C. Using a pipette and  a pipette filler, place 25.0 cm³ of solution C into a 250ml conical flask. Add two or three drops of phenolphthalein indicator and titrate against sodium         hydroxide. Record your results in table 2. Repeat titration two more times and complete table 2.

  

  Calculate the:

         i. Average volume of sodium hydroxide used. (3 marks)
        ii. The number of moles of:
               I. Sodium hydroxide used. (1 mark)
               II. Hydrochloric acid in 25cm³ of solution C. (1 mark)
              III. Hydrochloric acid in 250cm³ of solution C. (1 mark)
              IV.  Hydrochloric acid in 20.0 cm³ of solution B. (1 mark)
               V.   Hydrochloric acid that reacted with solid A. (1 mark)

  c. Calculate the enthalpy of reaction between solid A and one mole of hydrochloric acid.  (Show the sign of ΔH). (2 marks)

2. You are provided with solid D. Carry out the tests below. Write your observations and inferences in the spaces provided.

    a. Place all of solid D in a clean dry test-tube and heat it strongly until no further change occurs. Test any gases produced with both blue and red litmus papers. Allow the residue to cool and use it for             test (b).

       

    b. Add about 10cm³ of 2M of hydrochloric acid to the residue and shake for about three minutes. Keep the mixture for test (c).

       

    c.

        i.  Place about 1 cm³ of the mixture in a test-tube and add aqueous ammonia dropwise until in excess.

      ii. To the rest of the mixture, add all of solid E provided and shake the mixture well.

3. You are provided with solid F. Carry out the tests below. Write your observations and inferences in the spaces provided.
    a. Place about one third of solid F on a metallic spatula and burn it using a Bunsen burner.

    b. Place the remaining of solid F in a test-tube. Add about 6 cm³ of distilled water and shake the mixture well. (Retain the mixture for use in test (c).

    c.  

         i.  To about 2cm³ of the mixture, add small amount of solid sodium hydrogen carbonate.

         ii.  To about 1cm³ of the mixture, add 1 cm³ of acidified potassium dichromate (VI) and warm.

         iii.  To about 2cm³ of the mixture, add two drops of acidified potassium manganate (VII).