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Chemistry Paper 2 (Essay) , May/June 2008  
Questions:   1 2 3 4 5 6 7 8   Main
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Question 8

(a) (i) Define hard water.
(ii) Name two substances responsible for hardness in water.
(iii) Give two methods for the removal of hardness in water.   [6 marks]

(b) (i) What are the raw materials required for the manufacture of
tetraoxosulphate (VI) acid by the contact process?
(ii) Write an equation for the reaction that requires a catalyst in the contact process.
(iii) State the catalyst used in 8(b)(ii). [5 marks]

(c) (i) Give two uses of sodium tetraoxosulphate (VI)
Consider the reaction represented by the following equation:

                                                   exposure to air
                        Na2SO4 (aq)   IOH2O (s)   →  Na2SO4    H2O  + 9H2O (g)
(ii) What name is given to this type of reaction?
(iii) Calculate the solubility of Na2CO3 at 25oC, if 30.0cm3 of its saturated
solution at that temperature gave 1.80 g of the anhydrous salt.
                                       [ C  =  12, O  =  16, Na  =  23 ]  [7 marks]

(d) (i) Define the term activated complex.
(ii) State one reason why a collision may not produce a chemical reaction.
(iii) The formation of water gas is represented by the following equation.
                                   
      C(s)   +  H2 O(g)   →    CO (g)   H2 (g)     ΔH   =   +   131   kJmol-1

Draw an energy profile diagram for the reaction showing the

I.  activated complex

II.  enthalpy of reactants.  [7 marks]
_____________________________________________________________________________________________________
OBSERVATION

The question was attempted by most candidates and the performance was good.

In (a)(i), candidates correctly defined hard water as water which will not readily form lather with soap.

In (a)(ii) candidates correctly named two substances responsible for hardness in water from among the following:

  • calcium tetraoxosulphate (VI)
  • calcium hydrogentetraoxosulphate (IV)
  • magnesium tetraoxosulphate (VI)
  • magnesium hydrogentetraoxosulphate (IV)
  • In (a) (iii), candidates gave two methods for the removal of hardness in water from among the following:

  • heating the water to boiling/distillation
  • addition of calculated amount of calcium hydroxide [Ca(OH) 2]
  • addition of washing soda crystals to the water [Na2CO3.10H2O]
  • ion exchange resin/permutit method.
  • In (b)(i), candidates correctly gave the raw materials for the manufacture of   tetraoxosulphate (VI) acid by the contact process as sulphur and air/oxygen.

    In (b)(ii) and (iii), candidates correctly wrote an equation for the reaction that requires a catalyst in the contact process and stated the catalyst used.

    The equation is

                2SO2      +  O2   »  2SO3(g)

    and the catalyst employed is V2O5/ vanadium (V) oxide.

    In (c)(i) most candidates were able to correctly give two uses of sodium tetraoxosulphate (VI) from among the following:

  • water softener
  • in glass/paper manufacture
  • as analytical reagent
  • as purgative
  • in the manufacture of detergents
  • In (c)(ii) most candidates correctly gave the name of the type of reaction that was represented by the given equation.

    In (c)(iii) candidates correctly calculated the solubility of  Na2SO3 at 25°C.

    In (d)(i), candidates did not know that activated complex is the intermediate (complex) that reactants must attain in order to form a product or a temporary species formed by reactant molecules as a result collision before the products are formed, instead they gave the definition of activation energy.

    In (d)(ii), most candidates could not state one reason why collision may not produce a chemical reaction.  They did not know that if the energy of the colliding particles is less than the activation energy or the reactants are not properly aligned, chemical reaction may not occur inspite of collision.

    In (d)(iii), candidates drew the energy profile diagram for the endothermic reaction but most of them lost marks because they indicated the enthalpy of reaction instead of the enthalpy of reactants.  The correct energy profile diagram is as drawn below.         
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