Pok Oi Hospital Tang Pui King Memorial College

First Term Examination ( 1996-1997 )

F.6    Chemistry

 

Class                      :               ________________                           Date        :               __________________

Name                      :               ________________                           Time allowed: 180 mins

Class Number       :               ________________                           ( P.1 - P.8/38 )

 

Instructions to students:

The total score of this paper is 100 marks.

There are two sections in this paper.

Answer all questions.

Answer Section A in this question paper.

Answer Section B in the answer book provided.

 

Section A (30%)

 1. (a) Show how you would convert the molar gas constant from

         R = 82.05 cm3 atm K-1 mol-1  to R = 8.314 J K-1mol-1.

         Given : 1 atm = 101325 Nm-2

 

 

 

 

 

 

                                                                                                           (1 mark)

 

    (b) Complete the following table on the classification of crystalline solids:

        

 

 

Ionic

 

Covalent

 

Molecular

 

Metallic

 

Units constituting the crystal lattice

 

+ve and -ve ions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Binding force holding the constituent units

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Example (one each)

 

Sodium chloride

 

 

 

 

 

 

 

 

 

 

 

                                                                                                          (5 marks)

 

    (c) Fine jets of water and tetrachloromethane are separately run from a burette into a

         beaker, as shown in the diagram below:

     

What would you expect to observe when a

                                                    positively charged glass rod is brought near

                                                                   to each liquid jet in turn?                 

                                                                   Water: _____________________________

                                                                   ___________________________________

 

                                                                   Tetrachloromethane: __________________

                                                                   ___________________________________

 

                                                                                                        

                        (2 marks)

 

                  

 

     (d) Give reasons for your predictions in (c).              

 

 

                                                                                                               (2 marks)

 

2. (a) The bond enthalpy of the C¾H bond in methane is +414 kJ mol-1.

         (I)  Calculate the enthalpy change when methane is totally dissociated into its

           constituent gaseous atoms; give the equation to which your calculation refers.

 

 

 

  

        (II) The enthalpy change for the production of CH3(g) and H(g) from methane is

           +435 kJ mol-1. Explain why it is different to +414 kJ mol-1.

 

                                                                                                                (3 marks)

 

    (b) (I)  Define the term standard enthalpy change of formation.

 

         (II) Given:

                          DHf of ethane C2H4 (g) = + 52 KJ mol-1

                          DHf of benzene C6H6 (l) = +83 KJ mol-1     

 

 

               Calculate the standard enthalpy change for the reaction:

                            3C2H4(g) ® C6H6(l) + 3H2(g)

 

　

                                                                                                             (3.marks)

 

    (c) Graphite and diamond are both pure carbon. Why does graphite conduct electricity, whereas diamond does not?

 

 

    (3 marks)

 

    (d) Aluminium chloride vapour exists as a dimer at 623K. Draw the structure of the

        aluminium chloride dimer and comment briefly on its bonding.

 

 

 

 

                                                                                                        (2.marks)

 

3. (a) The plot in the diagram below shows the first ionization energies for the elements

      Ne, Na, Mg and Al, relative to one another. On the same diagram, plot the relative

       positions of their second ionization energies.

 

                                                                                                          (2 marks)

 

     (b) The figure below shows the unit cell structure of a crystal formed by the cation

         Am+ and Bn-.

  

          (I)  What are the co- ordination numbers of the Am+ and Bn-. ions?

 

 

         (II) Work out the unit cell contents and hence deduce the empirical formula of

               this compound.

 

 

                                                                                                     (4 marks)

 

    (c) (I)  Neither of the following reagents are suitable as primary standards in acid- base titrations. Suggest a      reason in each case.

            (1) concentrated nitric acid

 

            (2) sodium hydroxide pellets

 

       (II) Suggest a suitable primary standard in acid- base titration.

        (3 marks)

 

 

End of Section A

 

 

Section B (70%)

 

1. M+ (g) + Cl-(g) ® M+Cl-(s)                              DH298   = -697.8 kJ mol-1

    (M is a metal)

    (a) What is the term given to the enthalpy change associated with this reaction? With reference to the equation, explain the meaning of this term.

                                   (2 marks)

    (b) The experimental value of the above enthalpy change ( - 697.8 kJ mol-1) is determined indirected using an enthalpy level diagram. Draw and label the enthalpy level diagram used.

　

　

                          (3 marks)

    (c) Using the principles of electrostatics, the theoretical value of DH298   for M+Cl-(s)

         is -697.8 kJ mol-1.

         The corresponding values for silver chloride are also known. The two sets of values are given in the following table:

 

Compound

 

 

DH298   / kJ mol-1

 

 

 

Theoretical

 

Experimental

 

M+Cl-(s)

 

-697.8

 

-697.8

 

Ag+Cl-(s)

 

-768.6

 

-916.3

 

 

         (I)    What basic assumptions are made in determining the theoretical value?

 

         (II)   Why is there no difference between the theoretical and experimental values in the case of M+Cl-(s) and a large difference in the case of Ag+Cl-(s)?

 

         (III)  To which group in the Periodic Table do you think does metal M belong?

                 Explain.

　

      (5 marks)

 

2. Determine the enthalpy of formation of magnesium carbonate at room temperature

    by using the results from the two simple experiments below.

        

    Experiment A

 

    The temperature of a stirred solution of 50.00 cm3 of 2.0 mol dm-3  hydrochloric

    acid in a polythene beaker was noted at 0.5 minute intervals. At exactly 2.5

    minutes, 0.100g of magnesium ribbon was added and the temperature readings were

    continued.

    The results were as follows:

                          

Time/min

 

temperature/oC

 

Time/min

 

Temperature/oC

 

0

27.6

3.5

35.7

0.5

27.6

4

35.5

1

27.6

4.5

35.3

1.5

27.6

5

35.1

2

27.6

5.5

34.9

2.5

Mg added

6

34.7

3

35.9

 

 

 

     (a) Plot a graph to show the change in temperature with time. 

                 (2 marks)

    (b) Write an equation for the reaction in Experiment A and calculate its enthalpy

         change.    

         You may assume that the density of the solution is 1.0 g cm-3  and that the solution has the same specific

        heat capacity as water, i.e. 4.2 J g-1K-1.

　

      (4 marks)

 

    Experiment B

 

    This was similar to Experiment A except that magnesium carbonate was used in the

    place of the magnesium ribbon.

    The enthalpy of reaction between magnesium carbonate and hydrochloric acid in

    Experiment B was found to be -43.5 kJ mol-1.

    You are provided with further information as follows:

 

                     H2(g) + 1/2 O2(g) ® H2O(l)                  DH = -285.8 kJ mol-1

                     C(s) + O2(g) ® CO2(g)                       DH = -393.5 kJ mol-1

                                           

    (c) Determine the enthalpy of formation of magnesium carbonate, using the above

         information and the result in (b).

　

            (5 marks)

    (d) Point out two probable errors which might lead to the results being imprecise, and suggest how these   

         errors could be reduced.

　

         (4 marks)

 

3. (a) Compare the shape and bond angles of the following molecules:

                        BF3, H2O, NH3, CCl4

         on the basis of hybridisation and electron- pair repulsion theory.

    (8 marks)

    (b) Discuss whether the following statements are true or not, giving reasons for your

         answers.

         (I)    Covalent molecules are generally more volatile than ionic compounds because covalent bonds are much weaker than ionic bonds.

         (II)   Magnesium combines with chlorine to form the compound MgCl2 and not MgCl because the lattice

                energy of MgCl2 has a much more negative value than that of MgCl.

         (III)  Because the ions Na+, Mg2+  and Al3+  have the same electronic

               configuration, they should also have the same ionic radii.

         (IV) ‘Co- ordinate bond’ is only a different name for ‘covalent bond’.

       (6 marks)

 

4. (a) Outline the different typical close- packing of atoms in the space lattices of metals.

     (4 marks)

    (b) The diagram below represents a simple model of part of a metallic structure.

 

          (I)    Which electrons make up the ‘sea’ or ‘cement’ of electrons?

         (II)   Why are these electrons referred to as ‘mobile’ or ‘delocalized’?

          (III)  How does a metal conduct electricity?

         (IV) In what two ways is metallic bonding similar to both ionic and covalent

                 bonding?

  (4 marks)

 

    (c) (I)    Define the term electron affinity using chlorine to illustrate your answer.

 

         (II)   The electron affinity of chlorine is -364 kJ mol-1. Would the value of electron affinities of bromine

                 and iodine be greater or less than -364 kJ mol-1?

    (3 marks)

        

5. Write an essay on the emission spectrum of atomic hydrogen. The essay should include the origin of emission spectrum and explain what types of information can be obtained from a study of spectrum.

                                                                       

 (20 marks)

 

 End of Section B 

 

END OF PAPER