POK OI HOSPITAL TANG PUI KING MEMORIAL COLLEGE

FIRST TERM EXAMINATION ( 1997-1998 )

S6  CHEMISTRY

Class                :           ________________                Date     :           __________________

Name               :           ________________                Time allowed: 180 minutes

Class Number  :           ________________                ( P.1 - P.6 /38 )

Instructions to students:

1. Answer ALL questions.

2. Write your answers on the answer book provided.

3. The total score is 100 marks.

 1. (a) Sketch the curve showing the variation of the first ionisation energy with atomic

        number, for the elements Na to Cl.

                                                                                                        ( 3 marks )

    (b) 30 cm³ of a gaseous compound G, on complete decomposition, gave 60 cm³ of a

         mixture of nitrogen and hydrogen. The mixture was exploded with 30 cm³ of

         oxygen. After cooling, the volume of residual gas was 22.5 cm³. ( Nitrogen has no reaction with oxygen )

         (i)    Find the composition of the mixture.

         (ii)   Determine the molecular formula of compound G. ( N_xH_y )

                ( All volumes were measured at room temperature and pressure. )

                                                                                                                (5 marks )

    (c) Chlorine displaces iodine from iodides. The iodine formed can be determined by

         titration with a standard thiosulphate solution. Chlorate(I) solutions are often

         used as a source of chlorine as they liberate chlorine readily on reaction with

         acid: 

ClO^- (aq) + 2H⁺ (aq) + Cl^- (aq) ® Cl₂ (aq) + H₂O (l)

         The amount of chlorine available in a domestic bleach which contains sodium

         chlorate(I) can be found by allowing the bleach to react with an iodide solution

         to form iodine, and then titrating with thiosulphate solution: 

ClO^- (aq) + 2H⁺ (aq) + 2I^- (aq) ® I₂ (aq) + Cl^- (aq) + H₂O (l)

              A domestic bleach in solution is diluted by pipetting 10.00 cm³ and making this volume up to 250.00 cm³. A 25.00 cm³ portion of the solution is added to an excess of potassium iodide and ethanoic acid and titrated against sodium thiosulphate solution of concentration 0.0950 mol dm^-3, using starch as an indicator. The volume required is 21.3 cm³. Calculate the percentage of available of available chlorine in the bleach.

         ( Given: density of domestic bleach = 1.0 gcm^-3 )                                    (8 marks )

2. (a) (i)    Define the standard enthalpy of combustion of a substance.

          (ii)   Given the following thermochemical data:

                                    Compound                                                     H_f(298K)/kJ mol^-1

                        Stearic acid, C₁₈H₃₆O₂ (s)                                                         -896

                        Glucose, C₆H₁₂O₆ (s)                                                               -1274

                        Carbon dioxide (g)                                                                   -394

                        Water (l)                                                                                  -286

                (1)    Calculate the enthalpies of combustion of glucose and stearic acid ( in animal fats ).

                (2)   Calculate the heat energy available from the complete combustion of 1g of stearic acid and 1g of

                        glucose.                                                                                    ( 8 marks )

    (b) Two experiments were carried out to determine the enthalpy change ( DH₃ ) for the following reaction:

                                                 A (s) + xH₂O (l) ® A×xH₂O (s)               reaction (1)

         where A(s) represents a salt and A×xH₂O (s) represents the hydrated salt.

         The results of these two experiments and some relevant data are given below:

         Experiment A

         When 8.0g of an anhydrous salt A were added to 100 cm³ of water in an insulated polystyrene cup, there

        was a rise in temperature of the solution by 8.0 °C.

         Experiment B

         When 13.0g of an hydrated salt A×xH₂O (s) were added to 100 cm³ of water in an insulated polystyrene

           cup, there was a drop in temperature of the solution by 1.0 °C.

         (i)    Calculate the enthalpy change

                 (1) DH₁ for dissolving one mole of anhydrous salt A (s) in water in

                      experiment A, and

                 (2) DH₂ for dissolving one mole of hydrated salt A×xH₂O (s) in water in

                     experiment B.

          (ii)   Draw an enthalpy cycle linking the equation for the reactions in

               experiments A and B. Calculate the enthalpy change for reaction (1), DH₃.

                  ( Molar heat capacity of water = 75.6 JK^-1 mol^-1;

                   Molar mass of the anhydrous salt = 160g;

                   Molar mass of the hydrated salt = 250g )

          (iii)  List two sources of error in the experiments.                        ( 8 marks )

3. (a) Describe the bonding and structure found in both sodium chloride and caesium

      chloride crystals respectively. Briefly explain for any differences.       ( 6 marks )

     (b) (i)    How can ionic radii be measured?

          (ii)   Describe two factors which govern the size of an ion.

          (iii)  Refer to the following figure.

       Compare the sizes of ions with their  parent atoms. Explain the variation briefly.

                                                                                                                ( 10 marks )

4. (a) Catalytic reforming of hexane to cyclohexane and hydrogen is part of the process

      for the industrial production of petrol:

       The following bond enthalpies are given:

                   ( C-C )            347 kJmol ^-1

                  ( C-H )            413 kJmol ^-1

                  ( H-H )            436 kJmol ^-1

        Determine the enthalpy change for the catalytic reforming of hexane to cyclohexane and hydrogen.

                                                                                                                        ( 3 marks )

    (b) The figure below can be used to represent the unit cell of diamond:

        (i)    How many carbon atoms are in the unit cell of diamond?

        (ii)   What is the hybridization of carbon in diamond?

        (iii)  The figure below is the structure of graphite:

                 (1) State a physical property which is common to these allotropic forms of carbon, and explain why this is so.

                 (2) Give two contrasting properties of diamond and graphite.                ( 7 marks )

    (c) Account for the bond angles of the following species:

         (i)   

         (ii)

                                                                                                                                     ( 6 marks)

5. (a) Lithium and sodium both have body centred cubic structure, beryllium and

      magnesium have hexagonal close packed structure.

          (i)    Draw the structure of unit cell of sodium.

          (ii)   What are the coordination number of sodium and magnesium respectively?

          (iii)  Which type of packing does belong to magnesium?

         (iv)  For beryllium and magnesium, which melting point is higher? Explain your answer.

          (v)   The structure of aluminium is different to magnesium but both are close

                packing structure.

                (1) Write the name of this structure.

                (2) Draw the structure of the unit cell of Al.                        ( 8 marks )

    (b) (i)    Define the term electron affinity using chlorine to illustrate your answer.

        (ii) The electron affinity of chlorine is -364 kJmol ^-1. Would the value of electron affinities of bromine and

              iodine be greater or less than -364 kJmol ^-1?

        (iii)  Comment on the fact that the first electron affinity of oxygen is -142 kJmol ^-1,  but the second electron

                affinity of oxygen is +791 kJmol ^-1_.

         (iv)  Both electron affinity and electronegativity refer to the attraction by an atom for electrons. However,

                what is the basic difference between these two terms?                   ( 8 marks)

6. Explain how the value of lattice energy of sodium chloride may be derived from Born-Haber cycle; comment

    any differences between experimentally derived and theoretical values.

     You may use the following data:

    Show how lattice energies may be used to obtain values of hydration energy.   ( 20 marks )

    [ Marks will be allocated approximately as follows:

         chemical knowledge                                                50 %

         organization                                                            30 %

          presentation ( include proper use of English           20 %

       Equations, suitable diagrams and examples are expected where appropriate. ]

END OF PAPER