Pok Oi Hospital Tang Pui King Memorial College

Second Term Test ( 1996-1997 )

F.6 Chemistry I

Class : ________________ Date : __________________

Name : ________________ Time allowed: 60 mins

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

Instructions to students:

1. The total score of this paper is 40 marks.

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

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

Question- Answer Book.

4. Section A carries 20 marks.

. Section B carries 20 marks.

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

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Section A

1. The pH meter should be calibrated before use. What is the procedure?

( 2 marks )

2. Consider the following reversible reactions:

N₂O₄ (g) === 2NO₂ (g)

H₂ (g) + I₂ (g) === 2HI (g)

What effect would each of the following changes have on the above equilibria,

assuming ideal gas behaviour?

(a) Increasing the total pressure by adding helium gas (volume and temperature of the reaction are held

constant ).

(b) Increasing the total pressure by decreasing the volume (at constant temperature ).

( 4 marks )

3. The iodination pf propanone is catalysed by hydrogen ions. The overall equation is:

CH₃COCH₃ (aq) + I₂ (aq) ® CH₃COCH₂I (aq) + HI (aq)

Using four mixtures B, C, D and E, the progress of the reaction was followed by

colorimetric measurement. The results are tabulated below.

*Mixture*	*Composition by volume of mixture / cm³*
	propanone	water	1.00M HCl	0.05M I₂ in KI	Initial rate /mol dm^-3s^-1
B	10.0	60.0	10.0	20.0	4.96 x 10^-6
C	10.0	50.0	10.0	30.0	5.04 x 10^-6
D	5.0	65.0	10.0	20.0	2.45 x 10^-6
E	10.0	65.0	5.0	20.0	2.47 x 10^-6

(a) Determine the effects of the changes in concentration of each of the reactants

( iodine and propanone ) and the catalyst ( hydrochloric acid ) on the reaction

rate. Write the rate expression for the reaction.

(b) For mixture B, calculate the rate constant for the reaction at the temperature of

the experiment.

( Density of CH₃COCH₃ = 0.789 gcm^-3)

( 4 marks )

4. The diagram below gives the Maxwell- Boltzmann energy distribution of a system of

molecules at two temperatures:

What do the shaded areas of the curves represent and why are they different at

different temperatures?

( 2 marks )

5. Thiosulphate ions decompose in dilute hydrochloric acid according to the equation

S₂O₃^2-(aq) + 2H⁺(aq) ® H₂O (l) + SO₂ (g) + S (s)

The kinetics of the above reaction can be studied by varying

(I) The thiosulphate ion concentration,

(II) the hydrogen ion concentration, and

(III) the temperature.

(a) State what measurements or observations should be made in order to determine the initial rate in different runs of the reaction.

( 3 marks )

(b) List the necessary chemical substances and apparatus required to carry out the

above experiment.

( 3 marks )

6. What is the difference between “equivalent point” and “end point”?

( 2 marks )

Section B

1. Discuss the chemical bonding present in the following substances:

(a) caesium chloride,

(b) diamond, and

In your essay, describe the crystalline structures of the given substances. Also, state

ONE important physical property for each one and explain how the properties

are related to the types of bonding.

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Chemical knowledge:

Organization:

Presentation:

END OF PAPER

Pok Oi Hospital Tang Pui King Memorial College

Second Term Test ( 1996-1997 )

F.6 Chemistry II

Class : ________________ Date : __________________

Name : ________________ Time allowed: 90 mins

Class number : ________________ ( P.1 - P. 5/38 )

Instructions to students:

1. The total score of this paper is 60 marks.

2. All questions in this paper are COMPULSORY. Write your answers in the foolscap papers provided.

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

_________________________________________________________________________________

1. (a) Write the Arrhenius equation and use this equation to describe the effect of

following change to the rate of reaction:

(I) increasing the temperature, and

(II) adding with catalyst.

( 3 marks )

(b) Using the Arrhenius equation, show how you would determine the activation

energy of the following reaction from a set of experimental measurement of rate

constant k at various temperature. ( R = 8.314 JK^-1 mol^-1 )

NO₂ (g) + CO (g) ® NO (g) + CO₂ (g)

*T / K*	600	650	700	750	800
*k / mol^-1dm³ s^-1*	0.028	0.22	1.3	6.0	23

( 7 marks )

step 1: X ----k₁----> Y

step 2: Y ----k₂----> Z

The energy profile for this conversion is illustrated in the following diagram:

(I) Explain which step is more likely to be the rate-determining step in this

reaction.

(II) What is the DH value for the conversion of X to Z?

(III) Compare k₂ with the rate constant for the conversion of Y to X.

( 3 marks )

2. (a) Apple juice has a pH of 3.5.

(I) (1) Define pH.

(2) Calculate the molar concentration of hydrogen ions in apple juice.

Apple juice can be titrated with standard alkali. A 25.0 cm³ sample of apple juice

was exactly neutralised by 27.5 cm³ of 0.10 mol dm^-3sodium hydroxide using

phenolphthalein as indicator.

(II) Assuming that apple juice contains a single acid which is monobasic, calculate

the molar concentration of the acid in the juice.

(III) (1) How can you explain the difference between the two results you have

obtained in (a) (I) (2) and (II)?

(2) What constant can be determined from these two results?

(3) Calculate a numerical value of this constant.

(IV) Suggest an reason why phenolphthalein is a suitable indicator.

( 9 marks)

(b) The following solutions can act as a buffer:

(i) a solution with [ CH₃COOH ] = 0.100M and

[ CH₃COO^- ] = 0.075M

(ii) a solution with [ CH₃COOH ] = 1.000M and

[ CH₃COO^- ] = 0.750M

(I) Calculate the approximate pH value of solutions (i) and (ii).

[ Ka = 1.85 x 10^-5mol dm^-3for CH₃COOH ]

(II) Which of the above solutions is the better buffer? Explain your answer.

(III) Can a solution of ethanoic acid alone act as a buffer? Explain your answer.

( 5 marks )

during the titration of aqueous sodium hydroxide with benzoic acid.

(I) Why is it important to use an a.c. source rather than a d.c. source?

(II) The solution is stirred continuously during the titration. Explain.

(III) A curve of conductivity against the volume of sodium hydroxide added is

drawn.

Show on this graph how the best estimate of the end-point may be obtained.

(IV) The titration is continued well beyond the end-point. Why?

(V) Explain why the conductivity increases after the end-point.

( 5 marks )

3. (a) State three characteristics of a chemical equilibrium.

( 3 marks )

(b) A system is in equilibrium at 520^oC when the partial pressures of HI, H₂ and I₂ are

0.80 atm, 0.10 atm and 0.10 atm respectively.

2HI (g) === H₂ (g) + I₂ (g)

(I) Determine the equilibrium constant K_p of the system.

(II) Suppose HI is added to raise its partial pressure temporarily to 1.00 atm,

determine the partial pressure of each gas when the equilibrium is restored.

( 4 marks )

2SO₂ (g) + O₂ (g) === 2SO₃ (g)

(I) A 1 dm³ reaction vessel contained 0.11 mole of SO₂, 0.12 mole of SO₃ and

0.05 mole of O₂ at a fixed temperature.

(1) Write an equilibrium expression for the reaction.

(2) Calculate the equilibrium constant, K_c, for this closed system.

(II) Another reaction vessel of 1 dm³ contains 64g of SO₂ at the same

temperature. Determine the mass of O₂ which must be added to the reaction

vessel such that half of the SO₂ will be oxidized to SO₃ at equilibrium.

( 7 marks )

4. (a) (I) Briefly describe the bonding and intermolecular forces in ice and in

silicon(IV) oxide solid.

(II) Give the type of interactions that must be overcome to melt these solids.

( 4 marks )

(b) (I) The bond lengths ( in nanometres ) and also the dipole moment ( in Debye

units ) of the gaseous hydrogen halides are shown below.

	HF	*HCl*	*HBr*	HI
*Bond length*	0.092	0.127	0.141	0.161
*Dipole moment*	1.91	1.05	0.80	0.42

(1) What is meant by “dipole moment”?

(2) Comment briefly on the reasons for the decrease in dipole moment from

HF to HI.

(II) (1) State whether the following molecules would have a dipole moment or not,

giving reasons.

*Molecule*	*Dipole moment( yes or no )*	*Reason*
*CH₃Cl*
*CCl₄*
*BCl₃*
*NH₃*

(2) CO₂ has no dipole moment, while SO₂ has quite a large one. What

differences in structure does this suggest?

the fact that the former has hydrogen bonds but the latter has not.

(I) Explain what is meant by “hydrogen bonds”.

(II) List three differences between water and hydrogen sulphide which can be

ascribed the hydrogen bonding in the former.

( 10 marks )

END OF PAPER