POK OI HOSPITAL TANG PUI KING MEMORIAL COLLEGE

MOCK EXAMINATION ( 1997-1998 )

S7  CHEMISTRY I

Class                :           7 S                      Date  :           __________________

Name               :           ________________                Time allowed: 180 minutes

Class Number  :           ________________                ( P.1 - P.17 /38 )

Instructions to students:

1. There are THREE sections in this paper, Section A, Section B and Section C.

2. Section A carries 60 marks,Section B carries 20 marks and Section C carries 20 marks. 

  The total score is 100 marks.

3. All questions in Sections A and B are COMPULSORY. Write your answers in the

  spaces provided in this Question/ Answer Book.

4. Answer ONE question in Section C. Answers are to be written in this Question/ Answer Book.

5. Some useful constants and a Periodic Table are respectively printed on the last page of the question paper.

________________________________________________________________________

Section A

Answer ALL questions in this Section. Write your answers in the spaces provided.

1 (a) Codeine, C₁₈H₂₁NO₃, a weak base, is a drug derived from the opium poppy. It depresses the central nervous system to produce an analgesic action. A 25.0 cm³ sample of a 0.01M solution of codeine was titrated against 0.01M HCl. The pH of the titration was followed by a pH meter. The following results were obtained:

         (i)    Plot the pH values in the above table and sketch a titration curve for the titration.

                                                   volume of 0.0100M HCL added (ml)

          (ii)   Indicate on your titration curve

                 (1) the position of maximum buffering ( use the letter B )

                 (2) the position of the equivalence point ( use the letter E )

          (iii)                Explain why the pH values only decreased slightly when HCl was added to the codeine during the titration.

           (iv)  Indicator pKa values are given below:

                 Using the above indicator pKa values, choose the most suitable for this titration and explain your answer.

( 4 marks )

(b) For each of the following molecules, draw a three-dimensional structure and fill in the electronic configuration of the excited state of the central element. State the hybridization used for the central element.

(i)    BrF5

(ii)   CO2

 

( 3 marks )

    (c) Maxwell and Boltzmann calculated the fraction of the total number of molecules with energy x, and plotted this against the value of molecular energy x, as in the diagram below

(i)                  Sketch on the above diagram, the shape of the energy distribution which

you would expect at a lower temperature.

(ii)                Predict the area under the curve ( whether greater, equal or smaller ) if  measurements were made at a lower temperature.

       (iii)  Explain, with reference to the above sketch, why the rate of reaction increases as the temperature is increased.

( 3 marks )

2. (a) “ Covalent radius of hydrogen molecule is 0.036 nm and its Van der Waals’ radius is 0.121 nm.”

With the help of a diagram, show that what is meant by “covalent radius” and “Van der Waals’ radius”.

( 2 marks )

   (b) (i)    State Ideal Gas Law.

     (ii)   What assumptions are made in order to achieve Ideal Gas Law?

    (iii)  Account for deviations from ideal gas behaviour.

( 5 marks )

    (c) Calculate the K_a value of 0.01 M ethanoic acid at pH = 3.40.

( 3 marks )

3. (a) Give an reagent or product for each of the following conversions:

          (i)    S₂O₃^2- (aq) + ______________ ® S₄O₆^2-(aq)

         (ii)   VO₂⁺ (aq) + _______________ ® V²⁺(aq)

         (iii)  MnO₄^- (aq) + Mn²⁺(aq) + H₂O (l) ® ________________

( 3 marks )

    (b) Explain why NCl₃ and PCl₃ react differently with water.

         Write the formulae of the compounds formed when nitrogen(III) chloride and

         phosphorus(III) chloride are reacted separately with water.

( 3 marks )

    (c) (i)    Nickel forms a complex ion [Ni(NH₃)₆] ²⁺ . Show in a diagram the probable shape of this ion.

(ii)                Describe the bonding between the metal and the ligands in this complex

ion.

(iii)               Nickel forms a complex with the hexadentate ligand, EDTA, which has

the formula:

             (1) What is meant by hexadentate?

             (2) On the formula above indicate by asterisks (*) the points of bonding with the metal.

( 4 marks )

4. (a) (i)    Distinguish between the terms redox reaction and disproportionation.

(ii)                Show, with the help of a balanced equation, that the reaction of bromine

with hot sodium hydroxide is a disproportionation.

(4 marks )

       (b) Explain the following:

           (i)    Copper(I) compounds are unstable in aqueous solutions.

(ii)                Aluminium chloride is predominantly covalent whereas aluminium

fluoride is predominantly ionic.

( 3 marks )

    (c) How do you account for the fact that

(i)                  In solution, Cu⁺ ions are colourless but Fe²⁺ and Mn²⁺ ions are

coloured?

(ii)                Fe²⁺ ions are readily oxidized to Fe³⁺  ions, but Mn²⁺ ions are not

readily oxidised to Mn³⁺ ions?

( 3 marks )

5. (a) The compound X has the structure:

C₂H₅C(CH₃) = CHCH₂CH₂CH₂COOH

        (i)    Name X, as shown above, using IUPAC nomenclature.

        (ii)   X exists in two isomeric forms.

             (1)   Draw a structure for each of these isomers in such a way that the difference between them is clearly shown.

                (2)   What measurable property would be used to show the difference between these isomers?

( 3 marks )

    (b) Complete the following equations:

( 5 marks )

    (c) Explain why ethanoic acid is a stronger acid than ethanol.

 ( 2 marks )

 6. (a) Define the term “Biochemical oxygen demand ( BOD ).

( 2 marks )

    (b) (i) Indicate the major source(s) of high carbon monoxide concentration in air and discuss what might be done to lower carbon monoxide concentration.

(ii)                Why is continuous exposure to low concentration of carbon monoxide in the air a health hazards?

( 4 marks )

     (c) The graph on the right shows the concentration of smog components as a function of time of day.

(i)                  Explain why the hydrocarbon levels’ decrease as aldehyde levels increase.

(ii)                The concentration of NO₂ in an urban atmosphere increases to a

maximum rather early in the day, then decreases to a lower, steady value. What is the reason for this behaviour?

( 4 marks )

END OF SECTION A

Section B

Answer ALL questions in this Section. Write your answers in the spaces provided.

7. (a) You are given the following three compounds:

(i)                  Outline a schematic to separate a mixture of compounds A, B and C using a separating funnel and common laboratory reagents.

(ii)                Give one potentially dangerous hazard for this separation process, and give the necessary precaution to avoid accident.

( 4 marks)

    (b) A sample of magnesium sulphate is contaminated with about 1 % of barium sulphate.

(i)                  Outline the practical procedure you would use to remove the barium

sulphate and to obtain dry crystals of hydrated magnesium sulphate.

              (ii)   How would you obtain the flame colour of barium from your sample of barium sulphate?

(iii)                What explanation can you give for the origin of the flame colour of

barium?

                                                                                                              ( 6 marks )

8. The following is an account of the laboratory preparation of phenylamine from nitrobenzene. 5 g of nitrobenzene and 15 g of metallic tin are placed in a flask fitted with a vertical water-cooled condenser. 25 cm³ of concentrated hydrochloric acid are added slowly with stirring while heating the flask on a water bath. Heating is continued until no more oily drops of nitrobenzene are visible and the contents of the flask are made alkaline by the addition of an excess of concentrated aqueous sodium hydroxide. The mixture in the flask is then steam distilled until the distillate is clear. This distillate is extracted with two separate 10 cm³ portions of ether and sodium hydroxide pellets are added to the ether extract. This is then decanted into a distillation flask and the ether removed. When no more ether remains, the liquid is distilled using an air condenser and the distillate boiling between 180^o and 185^o collected.

(i)                  Why is the concentrated hydrochloric acid added slowly and with

stirring?

             (ii)   What is the significance of the phrase ‘until no more oily drops of nitrobenzene are visible?

             (iii)  Why is the reaction mixture made alkaline?

(iv)               Why is the ether extraction carried out with two separate portions of

ether?

        (v)   Why is this mixture decanted rather than filtered?

        (vi)  By what procedure should the ether be removed?

        (vii) Why is an air condenser and not a water-cooled ( Liebig ) condenser used in the final distillation?

        (viii) Why is the initial reaction carried out in a flask “fitted with a vertical water-cooled condenser”?

            (ix)              If the above procedure yield 3g of phenylamine, what was the

percentage yield of the reaction?

( 10 marks )

END OF SECTION B

                 Section C

Answer ONE question only and write your answers in the spaces provided.

Marks will be allocated approximately as follows:

chemical knowledge                            50%

organization                                   30%

presentation ( including proper use of English )       20%

Equations, suitable diagrams and examples are expected whereas appropriate.

The examiners are looking for the ability to analyse, to evaluate and to express ideas

clearly.

9. Give an account of the similarity and dissimilarity between aldehyde and ketone.

  In your answer, reaction type and reactivity should be included.

10. Contact process is an important industrial method to manufacture sulphuric acid in many countries. Comment on the conditions used in this process.

_______________________________________________________________________

                                         Chemical knowledge :

                                                                                                                   Organization :

                                                                                                                    Presentation :

    END OF PAPER

POK OI HOSPITAL TANG PUI KING MEMORIAL COLLEGE

MOCK EXAMINATION ( 1997-1998 )

S7  CHEMISTRY II

Class                :           7 S                     Date   :           __________________

Name               :           ________________                Time allowed: 180 minutes

Class Number  :           ________________                ( P.1 - P.10 /38 )

Instructions to students: 

1. There are TWO sections in this paper, Section A and Section B. 

2. Section A and Section B each carries 51 marks. The total score is 102 marks. 

3. There are FOUR questions in each section. Answer THREE questions from each

  section. 

4. All questions carry equal marks. 

5. Write your answers in the answer book provided. 

6. Some useful constants and a Periodic Table are respectively printed on the last page of the question paper.

________________________________________________________________________ 

Section A 

Answer any THREE questions in this Section. 

1. (a) When one mole of graphite is burned completely in oxygen to produce carbon

     dioxide, 394 kJ of heat is evolved whereas, when diamond is similarly

     burned, 396 kJ is evolved.

(i)         Write thermochemical equations summarising this information.

(ii)        Draw an energy level diagram comparing the combustion of diamond and     

graphite.

(iii)  What is the enthalpy change for the conversion of graphite into diamond?

( 5 marks )

 (b)       The following is a Born-Haber cycle for the formation of magnesium chloride. The value of DH^o for the changes B to G are given below.

                         ΔH˚ /kJ mol^-1                                        ΔH˚ /kJ mol^-1   

                               B          150                           E            244

                               C         738                           F            -728

                               D        1450                          G           -2493

(i)         A is the enthalpy of formation of magnesium chloride. Give the name of each of the following energy changes:

B, E, F and G

             (ii)        Calculate the enthalpy of formation of magnesium chloride.

(iii)  (1) Using a similar Born-Haber cycle to the above, it is possible to calculate the enthalpies of formation of MgCl and MgCl₃. In order to do so, an estimated value for one energy change in the cycle must be used. Which energy change is this?

(2) The value obtained for the standard enthalpies of formation of MgCl and MgCl₃ are, approximately -110 kJ mol^-1 and +4000 kJ mol^-1 respectively. Use your value for DH_f  ( MgCl₂ ) obtained above to deduce the relative energetic stabilities of the three possible magnesium chlorides with respect to the elements.

                            (3) The enthalpy change for the reactant

2MgCl (s) ® MgCl₂ (s) + Mg (s)

  is found by calculation to be about -420 kJ mol^-1. Use this information to deduce the relative stabilities of MgCl and MgCl₂ with respect to each other.

      (iv)  Given the following standard enthalpies of hydration

Mg²⁺ (g) + aq ® Mg²⁺ (aq)             DH^o = -1920 kJ mol^-1

Cl^- (g) + aq ® Cl^- (aq)                 DH^o = -364 kJ mol^-1

and using the value of DH^o for reaction G already given, calculate the standard enthalpy of solution of magnesium chloride by a Born-Haber cycle.     

( 13 marks )

2 (a) Arrange the following substances in order of decreasing boiling points:

 C₂H₅Br, CH₄ and C₂H₆

    Explain your order by comparing the relative magnitudes and nature of the intermolecular forces.

( 4 marks )

(b) (i)    Copy the axes below on your answer book, sketch the variation in boiling point of the hydrides of the Group VI elements.

        (ii)   How do you account for this variation?

   ( 4 marks )

     (c)  (i)    What is meant by the triple point of carbon dioxide?

(ii)                The triple point of carbon dioxide is -56°C and 515 kPa pressure. Solid

carbon dioxide expands on melting.

 Construct a sketch of the phase diagram of carbon dioxide. Label the phases and mark the triple point.

(iii)               Describe two features which make the phase diagram significantly

different from that of water.

( 9 marks )  

3. (a) The reaction between hydrochloric acid and sodium thiosulphate can be used to

     demonstrate the effect of varying the concentrations of the reactants on the rate of

     reaction.

Na₂S₂O₃ (aq) + 2HCl (aq) ® H₂O (l) + SO₂ (g) + 2NaCl (aq) + S (s)

   50 cm³ of 0.2 M sodium thiosulphate solution are put into a 100 cm³ conical flask. 5 cm³ of 2M HCl are added to the flask and timing the reaction is started by using a stop watch. The flask is swirled around once or twice and is then placed over a piece of white paper marked with a cross. The mark is observed from the top and the time is recorded when the sulphur produced just causes the cross to disappear. The experiment is repeated by using 40, 30, 20 and 10 cm³ of the sodium thiosulphate solution, but in each case, sufficient distilled water is added to make the total volume 50 cm³ before the acid is added. The volume of acid is kept constant each time. The following graphs represent the results of the experiment above.

(i)                  What do you notice about the total volume of sodium thiosulphate  

solution ( concentrated solution plus water ) used in each part of the experiment?

Explain. 

      (ii)   Describe the curve in graph A and explain why it is this shape.

      (iii)  Describe the curve in graph B and explain why it is this shape.

( 6 marks )

    (b) (i)    Sketch the unit cell of the caesium chloride structure, using marks of different colour or shape to represent the centres of ions.

       (ii)   Sketch the sodium chloride structure in the same way.

       (iii)  Write down the coordination numbers for each ion in the two structures.

       (iv)  Explain why sodium chloride could not adopt the caesium chloride structure.

 ( 5 marks )

    (c) Most radioactive nuclei emit one of  three different types of radiation. Give the names of these types and describe their principal properties.

( 6 marks )

4. (a) (i)    Describe, with essential experimental detail, how you would determine the equilibrium constant K_c for the reaction 

CH₃COOH (l) + C₂H₅OH (l) ==== CH₃COOC₂H₅ (l) + H₂O (l) 

         (ii)   When a mixture of 1.00 mol of ethanoic acid and 1.00 mol of ethanol is allowed to reach equilibrium at room temperature, the equilibrium mixture contains 0.67 mol of ethyl ethanoate. Calculate the equilibrium constant K_c at room temperature.

         (iii)  Predict the qualitative effect on the amount of ethyl ethanoate in the equilibrium mixture if 2.00 mol of ethanol and 1.00 mol of ethanoic were used initially.

( 9 marks ) 

    (b) Some of the difficulties in using hydrogen as an electrode are that

         - hydrogen is a gas at room temperature,

         - hydrogen does not conduct electricity,

         - the half-cell reaction is slow to reach equilibrium.

         How are these difficulties overcome?

( 2 marks )

    (c) Given the standard electrode potentials at 298K for the following electrodes:

             Cu²⁺ (aq) / Cu (s)                           +0.34V

             Ag⁺ (aq) / Ag (s)                            +0.80V

             Zn²⁺ (aq) / Zn (s)                            -0.76V

        (i)    Draw cell diagrams for

                (1) a copper-silver cell and

                (2) a copper-zinc cell

                Give the positive and negative poles for each cell, using IUPAC convention.

        (ii)   Give the overall cell reactions for the cells in (i) and calculate their standard e.m.f.s at 298K.

( 6 marks )

END OF SECTION A 

 Section B

Answer any THREE questions in this Section. 

5. (a) What is meant by the term optical isomerism? Draw the structural formula of the alkane with the lowest molecular mass that can exhibit optical isomerism.

( 3 marks )

  (b) Catalytic cracking is a process whereby long-chain alkanes are broken down into mixtures of molecules with shorter chains.

         (i)    Why is this process important industrially?

         (ii)   What conditions are used to carry out the reaction?

         (iii)  When the alkane C₁₄H₃₀ was subjected to catalytic cracking, four products were identified. The mass spectrum of the mixture showed molecular ion peaks at m/e values of 28, 42, 44 and 56, with relative abundances of 2:1:1:1, respectively. Suggest structural formulae for the four products, predict which one does not decolourise aqueous bromine, and write a balanced equation for their formation from C₁₄H₃₀.

( 4 marks )

    (c) The characteristic odour of rotting fish is due to the presence of simple amines like propylamine.

         (i)    Suggest reagents and conditions for each stage in the conversion of bromoethane into propylamine by the following route: 

CH₃CH₂Br ® CH₃CH₂CN ® CH₃CH₂CH₂NH₂ 

         (ii)   Suggest, with an explanation, whether propylamine is more or less basic than ammonia.

         (iii)  When propylamine reacts with a dibasic ( diprotic ) organic acid, a neutral salt is formed which has the following composition by mass: 

C : 46.1% ; H : 9.6%; O : 30.8%; N : 13.5% 

           Calculate the empirical formula of the salt, and suggest its molecular formula and a structure for the organic acid.

( 10 marks )

6. (a) Using equations, show the reactions you would employ for the following conversion in the laboratory. Give the reagent(s) for each step and the structures of all intermediate compounds.

( 9 marks )

    (b) Outline chemical tests which would allow you to distinguish between the compounds in the following pairs. Describe, with the aids of equations, what you would observe in each case.

       ( 4 marks )

    (c) Attempts have been made to develop compounds which have low ozone depletion potential ( ODP ) to replace CFCs as refrigerants, aerosol propellants and solvents.

Give four possible alternatives for chlorofluorocarbons. Describe briefly how they work.

( 4 marks )  

7. (a) A coal-fired power station ( which generates electricity ) is fitted with a Flue Gas Desulphurisation ( FGD ) plant, which removes some of the sulphur dioxide from waste gases.

In the FGD plant, the waste gases are treated with powdered limestone, CaCO₃, producing calcium sulphite, CaSO₃. This is oxidised by air to form solid calcium sulphate, CaSO₄. The diagram below shows the amounts of substances used, and produced, by such a coal-fired power station with an FGD plant in one year.

         (i)    (1) What process provides the energy used in the power station?

               (2) Which gas, not listed in the diagram, is the chief component of the flue gases?

               (3) Explain why oxides of nitrogen ( NO₂ ) are present in the flue gases.

         (ii)   Write a balanced equation in each case to show how 

                 (1) limestone reacts with sulphur dioxide,

                 (2) air oxidises calcium sulphite. 

         (iii)  (1) Use the equation in (b) (ii) (1) to determine the maximum mass of sulphur dioxide which could be removed by 3x10⁵ t of limestone in the FGD plant.

  (2) Use the equations in (b) to determine the maximum mass of calcium sulphate which would be produced from the 3x10⁵ t  of limestone.              

          (iv)  The FGD plant removes 90% of the sulphur dioxide from the waste gases.

Calculate the mass of sulphur dioxide which is released into the atmosphere each year by this power station when 5x10⁵ t coal are burnt.

          (v)   Which other substance, as well as calcium sulphate, will be formed by the limestone acting on the flue gases?

          (vi)  Suggest two possible disadvantages of the use of an FGD plant.

( 11 marks ) 

    (b)   (i)    Give the ways to solve the plastic waste dispersal.

         (ii)   Give the importance and problems of recycling plastics.

( 6 marks )

8. (a) This part concerns the Group II elements : beryllium, magnesium, calcium,

         strontium and barium. 

         (i)    (1) Why are these elements called s-block elements?

                 (2) Give the electronic configuration of a calcium ion. 

         (ii)   Explain why 

                 (1) the chlorides of the elements become more ionic with increasing atomic number,

                 (2) beryllium chloride is much more covalent than the other chlorides. 

         (iii)  Explain why the solubility of the sulphates decreases with increasing atomic number of the metal.

( 6 marks ) 

    (b) (i)    Why are the elements of Group IA of the periodic table, ( Li to Fr ) known as the alkali metals?

      (ii)   Give two reactions of these elements or their compounds in which all members of the Group behave similarly.

      (iii)  Give two properties of lithium or its compounds which show a difference in chemical properties between lithium and the other elements of the Group.

      (iv)  If electropositivity were defined in terms of ionisation energy, lithium would be the most electropositive alkali metal. If it is defined in terms of reduction potential, ( E^o ), lithium would be the least electropositive element in the group. Account for this fact. Which interpretation best fits the chemistry of lithium?

( 7 marks ) 

    (c) Explain the following: 

         (i)    Unlike the other halogens, fluorine exhibits only one oxidation state in its compounds. 

         (ii)   The boiling points of the hydrogen halides vary down the halogen group. 

         (iii)  The acid strength of hydrogen fluoride is different from other hydrogen halides.

                                                                        ( 4 marks )

 END OF PAPER