Question 4
(a) (i)Draw and label a diagram for the laboratory preparation of a dry sample of sulphur (IV) oxide.
(ii) Write a balanced chemical equation for the reaction in 4 (a) (i).
(iii) State the precaution that must be taken in the preparation of the gas
stated in 4 (a) (i).
(iv) Give a reason why precaution stated in 4 (a) (iii) must be taken. [12 marks]
(b) (i) Sate Dalton’s law of partial pressures.
(ii)The volume of a sample of methane collected over water at a temperature of 120C and a pressure of 700 mmHg was 30cm3. Calculate the volume of the dry gas at s.t.p. [Saturated vapour pressure of water at 120 C is mmHg] [7 marks]
(c) (i) Write an equation for the reaction between chlorine and water.
(ii) Why does litmus paper turn red when put in resulting solution in 4(c)(i)? [3 marks]
(d) (i) State the trend in the boiling points of chlorine, bromine and iodine.
(ii) Explain briefly why water has a higher boiling point than ammonia. [3 marks]
Observation
The performance of majority of the candidates in this question was poor.
In part (a), a large number of the candidates could not draw a workable diagram for the preparation of SO2. Majority of the candidates could not write a balanced chemical equation for the reaction.
In part (b), majority of the candidates could not satisfactorily state the Dalton’s law of partial pressures due to poor communication skills. They also performed poorly in the application of the law to solve the calculation that followed.
In part (c), few candidates responded to this question and majority of them could not correctly write an equation for the reaction between chlorine and water.
In part (d), majority of the candidates could not state the reason why water has a higher boiling point than ammonia.
The expected answers include:
(ii) Na2SO3(aq) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g)
OR
K2SO3(aq) + 2HCl(aq) --> 2KCl(aq) + H2O(l) + SO2(g)
Na2SO3(aq)+ H2SO4(aq) --> Na2SO4 + H2O(l) + SO2(g)
Cu(s) + 2H2SO4(aq) --> CuSO4 + 2H2O(l) + SO2(l)
MgSO3(aq) + H2SO4(aq) --> MgSO4 + H2O(l) + SO2
(iii) Should be prepared in a fume chamber / cupboard
(iv) because the gas is poisonous
(b) (i) It states that for a mixture of gases which do not react, the total pressure is equal to the sum of the partial pressures of the individual gases.
= 26.09 cm3
= 26.10 cm3
(c)i) Cl2 + H2O → HCl+HClO
(ii) H+ ions are released / HCl produced is acidic/ HClO produced is acidic /solution is acidic
(d) (i) boiling point of iodine is greater than that of bromine which is greater than
that of chlorine / boiling point of chlorine is less than that of bromine which is less than that of iodine / boiling point increases from chlorine to iodine
(ii) Water has stronger hydrogen bonds than ammonia because oxygen atom has two lone pairs of electrons and nitrogen atom has one pair of electron /oxygen is more electronegative than nitrogen.