Pok Oi Hospital Tang Pui King Memorial College

Second Term Examination ( 1996-1997 )

F.6 Chemistry I

Class : ________________ Date : __________________

Name : ________________ Time allowed: 180 mins

Class number : ________________ ( P.1 - P.15 /38 )

Instructions to students:

1. The total score of this paper is 100 marks.

2. There are three sections in this paper, Section A, Section B and Section C.

3. All questions in this paper are COMPULSORY. Write your answers in the space provided in this

Question- Answer Book.

4. Section A carries 60 marks.

Section B carries 20 marks.

Section C carries 20 marks

5. A Periodic Table is printed on the last page of this Question- Answer Book.

________________________________________________________________________

Section A

1. (a) (I) Complete the following equations by filling in the three boxes.

(II) Give one use of Gamma radiation.

(2 marks)

(b) The diagram below shows the electronic energy levels for the hydrogen atom.

Draw on this diagram and clearly label

(I) the first three transitions corresponding to visible (Balmer) spectral lines,

(II) one transition corresponding to an absorption spectral line, and

(III) the transition corresponding to the ionization of the hydrogen atom.

(3 marks)

	S	Cl	K	Ca
Atomic radii / nm	0.104	0.099	0.203	0.170
Common ionic radii / nm	0.184	0.181	0.133	0.099

(I) Explain why the atomic radius of chlorine is smaller than that of sulphur but potassium is larger

than chlorine.

(II) Explain why the sizes of sulphide and chloride ions are larger than that of their parent atoms, but

potassium and calcium ions are not.

(7 marks)

(d) A hydrocarbon contains 85.7 % carbon and 14.3 % hydrogen by mass. The main

peaks in its mass spectrum are as follows:

(I) Calculate the empirical formula of the compound.

(II) Calculate the molecular formula of the compound.

(3 marks)

(e) Carbon - 14 dating is used to determine the age of articles of historical and

archaeological interest. The radioactive isotope carbon - 14 is formed by the

bombardment of neutrons on nitrogen. Carbon - 14, in turn, disintegrates by

emitting beta - particles, the half - life of which is found to be 5570 years.

(I) Explain the underlying principle of carbon - 14 dating.

(II) Write a nuclear equation for the formation of ¹⁴C in the upper atmosphere.

(III) If 30 counts per minute per gram of carbon is emitted by living organisms, while a fossilized bone

gives 8 counts per minute per gram of carbon, calculate the age of the fossil.

(5 marks)

2. (a) Under what conditions

(I) is the value of an equilibrium constant unaffected by changes of pressure at constant temperature,

and

(II) does an equilibrium constant, K_c, have no units?

(2 marks)

(b) The diagram below is the schematic descriptions of the reaction taking place at

temperature T and pressure P.

(I) Calculate the equilibrium constant K_c.

(II) What will increase the value of the equilibrium constant in (I)?

(3 marks)

(II) For the following equilibrium,

N₂O₄ (g) ==== 2NO₂ (g)

Given that the equilibrium constant is 2.5x10⁴ Nm^-2 at 1 atm and 25 °C.

Calculate the partial pressure of NO₂(g)

(5 marks)

(d) (I) Is it true that the pH of a neutral solution is always equal to 7? Explain briefly.

(II) Calculate the pH value of 0.01 M aqueous HA solution and the degree of

dissociation of an acid, the dissociation constant being 1.2x10^-5 mol dm^-3.

(5 marks)

(e) A buffer solution is prepared by adding 30 g of ethanoic acid (CH₃COOH) and

205 g of sodium ethanoate (CH₃COONa) to enough pure water to make 1.0 dm³ of

solution. (pK_a of ethanoic acid is 4.75)

(I) What is the definition of a buffer solution?

(II) What is the pH of the solution?

(III) Give an example of an alkaline buffer solution.

(5 marks)

3. (a) Calculate the oxidation numbers of the underlined elements in the following

compounds or ions. (No working step is required)

(I) O₂^- _______ (II) O₂^2- _______

(III) ClO₄^- _______ (IV) NH₄NO₃ _______

(V) NH₄NO₃ _______ (VI) S₂O₃^2- _______

(3 marks)

(b) The following questions refer to the standard hydrogen electrode.

(I) Give a fully labelled diagram of standard hydrogen electrode.

(II) What do you understand by the term standard conditions?

(III) Write down the cell diagram for the standard hydrogen electrode.

(5 marks)

(4 marks)

(d) At 25 °C, the vapour pressure of trichloromethane (CHCl₃)is 199 mmHg and that

of tetrachloromethane (CCl₄) is 115 mmHg. Assuming the mixture is an ideal

solution, determine

(I) the composition of the vapour formed from a mixture of 1 mole

trichloromethane and 1 mole tetrachloromethane, and

(II) the total vapour pressure of this mixture.

(4 marks)

(e) A mixture of three amino acids A, B, and C was separated by 2 - way paper

chromatography. Their R_f values are listed below.

Amino acid	R_fvalue in phenol	R_fvalue in butan-1-ol / ethanoic acid
A	0.30	0.25
B	0.45	0.40
C	0.65	0.70

(I) Make on the following figure the spots which correspond to A, B and C.

(II) State one use of paper chromatography.

(4 marks)

Section B

4. (a) The following diagrams show some of the basic apparatus for use in titration work.

(I) Why should a conical flask, not a beaker, be used to contain solution for

titration?

(II) Before titration, the conical flask should be rinsed with water, but not with the solution to be contained.

Furthermore, the apparatus is not necessary to be dried. Why?

(III) On the other hand, burette and pipette should be washed with the solution to be contained before use.

Why?

(IV) Before use of burette, a few drops of the contained solution should be run off from it before the first

reading is taken. Explain.

(V) What is the approximate volume of one drop of solution coming from burette?

(VI) After running off the solution from pipette, a drop of solution is usually left in its stem. Should it be

blown off?

(VII) Both pipette and burette should not be used to store a concentrated solution of sodium hydroxide.

Why?

(VIII) A piece of white paper is usually put on the retort stand in titration work. Why?

(9 marks)

(b) A mixture of hydrochloric acid and sulphuric acid was made up to 500 cm³. It was found that 25 cm³ of the mixture required 34 cm³ of 1 M sodium hydroxide for complete neutralization. On addition of excess silver nitrate solution to another portion of 25 cm³ of the mixture, 3 g of white precipitate of silver chloride was formed. What is the percentage composition by mass of the mixture?

(5 marks)

apparatus. The reported values are 78, 78.1, 78.2, 78.2, 78.2, 78.3 and 78.8 °C

respectively.

(I) What do you think might have been the cause for the discrepancies in boiling point readings?

(II) Which boiling point readings would you not use to calculate the mean boiling point of ethanol?

Explain your answers.

(III) Calculate the mean boiling point of ethanol.

(6 marks)

Section C

5. Write an essay on corrosion of iron.

(Your answers should include (I) socioeconomic implications; (II) the electrochemical process involved in rusting and (III) prevention of corrosion.)

(20 marks)

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Chemical knowledge:

Organization:

Presentation:

END OF PAPER

Pok Oi Hospital Tang Pui King Memorial College

Second Term Examination ( 1996-1997 )

F.6 Chemistry II

Class : ________________ Date : __________________

Name : ________________ Time allowed: 180 mins

Class number : ________________ ( P.1 - P.7 /38 )

Instructions to students:

1. The total score of this paper is 100 marks.

2. All questions in this paper are COMPULSORY. Write your answers in the answer book provided.

3. A Periodic Table is printed on the last page of this paper.

_________________________________________________________________________________

1. (a) The following data refer to the reaction of magnesium and hydrochloric acid.

Concentration of hydrochloric acid ( mol dm^-3 )	Time for the formation of 20 cm³ of hydrogen (s)
0.30	240
0.40	136
0.60	60
0.80	34
1.20	15

In each case the reaction was carried out at room temperature and pressure, a constant length of

magnesium ribbon was used, and there was a large excess of hydrochloric acid.

(I) Give a diagram of an apparatus which could have been used for these

experiments.

(II) Why was a large excess of hydrochloric acid used in each experiment?

(III) Why was a constant length of magnesium ribbon used in each experiment?

(IV) (1) Write an expression relating reaction time and the concentration of

hydrochloric acid.

(2) What is the order of reaction with respect to hydrochloric acid?

(V) Sketch and label the energy profile for the reaction between magnesium and hydrochloric acid.

(VI) Draw a labelled diagram to show the distribution of molecular energies in hydrogen at two temperatures T k and (T-t) k.

(13 marks)

(b) The rate law is often related to a constant times the product of concentrations of

all or some of the reactants, each raised to some power. For the reaction

2NO (g) + 2H₂ (g) ® N₂ (g) + 2H₂O (g)

The rate law is : Rate = k [ NO ]² [ H₂]

(I) Identify the rate constant in the rate law and indicate the overall reaction order.

(II) What is the unit of k if the unit of concentration is atmospheres?

(III) How does the initial rate change if the initial concentration of NO (g) is doubled and that of H₂ (g) remains constant?

(4 marks)

2. (a) The graphs below give the percentage of C in the equilibrium mixture at constant

pressure and temperature for the reversible reaction:

(I) Suggest how you would increase the yield of C.

(II) In practice, the conditions chosen for the synthesis of C are 750K and

300 atm. Justify this choice.

(III) A catalyst has been used for the above reaction . What would happen if the catalyst were poisoned?

Draw a new graph for the reaction at 900K to illustrate your answer.

(8 marks)

(b) In vapour phase, formic acid (CH₃COOH) exists as both dimers and monomers.

At 298 K and 1380 Nm^-2, the mole fraction of the dimers is 0.761 while at 348 K

and 2180 Nm^-2, the mole fraction of the dimers is reduced to 0.296.

(I) Write an expression for the dissociation of formic acid dimers.

(II) Calculate the degree of dissociation a , and the equilibrium constant, K_p at

298 K and 348 K.

(III) The dependence of K_p on temperature is given by the following relationship:

where DH is the enthalpy change for dissociation.

Calculate DH for the dimerization of formic acid and hence determine the energy of a hydrogen

bond.

(R = 8.314 JK^-1mol)

(9 marks)

3. (a) Give the definition of acid and base with respect to

(I) Bronsted and Lowry’s theory

(II) Arrhenius definition

(III) Lewis definition

Give an example for each of the above definitions.

(6 marks)

(b) The following curve was obtained when 0.01 M NH₃ (aq) were titrated against

0.01 M HCl (aq).

(I) Explain why

(1) there was a rapid change in pH value when the first 5 cm³ of HCl were added, and

(2) the pH value did not change much in the volume between 10 cm³ to 15 cm³.

(II) Explain why the mixture did not have a pH of 7 when 25 cm³ of HCl were added.

(III) Deduce the K_b value for NH₃.

(IV) Suggest an indicator for the titration and indicate the colour change at the end point.

(9 marks)

4. (a) (I) Write balanced half equations for the redox reaction of Crⁿ⁺ and MnO₄^- in

acidic solution. In terms of n, how many moles of electrons per mole of Crⁿ⁺,

are involved in the oxidation reaction?

Write the balanced overall chemical equation for the above reaction.

(II) An acidified solution containing 1.50x10^-3 mol of Crⁿ⁺ is titrated with

0.0250 M KMnO₄ solution. The equivalance point of the reaction is reached

after 48.00 cm³ 0f KMnO₄ has been added.

Calculate the value of n.

(9 marks)

(b) The reaction:

6Fe²⁺ (aq) + Cr₂O₇^2-(aq) + 14H⁺ (aq) ® 6Fe³⁺ (aq) + 2Cr³⁺(aq) + 7H₂O (l)

takes place in an electrochemical cell.

Given:

=+0.77V

= +1.33V

(I) Calculate the standard e.m.f. of the cell.

(II) Draw a labelled diagram to show how you would set up such a cell which could produce the

standard e.m.f. Your diagram must specify the compounds to be used in making up the required

solutions.

(III) Calculate the quantity of electricity in Faradays that has passed through the external circuit, if

2.00x10^-3 mol of Cr₂O₇^2- has reacted in this cell.

(8 marks)

5. (a) What is meant by an ideal solution when mixing two liquids together?

State any assumptions under which the concept of an ideal solution operates.

(3 marks)

(b) (I)(1) What is meant by the term mole fraction?

(2) State Raoult’s law.

(II) A mixture of ethanol and cyclohexane shows a positive deviation from

Raoult’s law. For this liquid mixture, sketch

(1) the vapour pressure - liquid composition curve.

(2) the boiling point - liquid composition curve.

(The boiling point of ethanol is 78 °C and that of cyclohexane is 81.5 °C)

(III) Explain why the temperature of the liquids decreases when ethanol and

cyclohexane are mixed together.

(7 marks)

(c) The following figure is a boiling point - liquid / vapour composition diagram (not to scale) for the ethanol - water system at 1 atm.

(I) State the meaning of the points labelled A and B, the curves C and D, and the physical states of the

system in the regions labelled E, F and G.

(II) An ethanol - water mixture of composition H is fractionally distilled. Describe briefly with the aid of

the diagram, what happens to both vapour and liquid phase as fractional distillation proceeds.

(7 marks)

6. (a) Consider the following phase diagram of a compound X.

(I) State the conditions under which

(1) the solid sublimes, and

(2) compound X exists as a liquid.

(II) Describe what happens when the pressure is increased from point 1 to 3.

(III) Describe the system at B.

(IV) Describe the system beyond the point C.

(5 marks)

(b) The following figures describe the pressure - temperature relationship of carbon

dioxide and water. They have two significantly different features.

Point out these differences and use them to explain the characteristic properties of :

(I) Liquid water, in that it is denser than ice.

(II) Solid carbon dioxide, in that it sublimes rather than melts.

(6 marks)

(c) 2.0 g of an organic compound X in 25 cm³ aqueous solution is extracted by diethyl ether. Calculate the amount extracted if using

(I) 25 cm³ of diethyl ether in one portion, and

(II) two successive 12.5 cm³ portions of diethyl ether.

(Partition coefficient of X between diethyl ether and water is 3.0)

Comment on the results obtained.

(6 marks)

END OF PAPER