Question 4
QUESTION 4
(a) Using appropriate equations, show the chemical changes that occur when a piece of sodium metal is exposed to the atmosphere for a long period of time. [6 marks]
(b)The sketch below shows the main steps involved in the Contact Process. Use the sketch to answer the questions that follow.
.
(i) State one main source of sulpur (IV) oxide used in this process;
(ii) Name one catalyst used in this process;
(iii) Identify each of the following substances:
I. Q;
II. R;
III T.
(iv) Write the equation for the reaction in the absorption tower.
[7 marks]
(c) (i) Describe briefly the laboratory preparation of oxygen from hydrogen peroxide.
(ii) Write the equation for the reaction in 4(c)(i).
[6 marks]
(d) (i) Write equations to illustrate the amphoteric nature of aluminium oxide.
(ii) Write an equation to show how chlorine reacts with concentrated NaOH.
[6 marks]
Observation
Majority of the candidates avoided this question, and those that responded to it performed averagely.
In part (a), majority of the candidates could not use appropriate equations to show the chemical changes that occur when a piece of sodium metal is exposed to the atmosphere for a long period of time.
In part (b), majority of the candidates could not state one main source of sulphur (IV) oxide used in contact process. Also, they could not name one catalyst used in this process.
In part (c), majority of the candidates that responded to this question were able to describe the laboratory preparation of oxygen from hydrogen peroxide.
In part (d), majority of the candidates could not write equations to illustrate the amphoteric nature of aluminium oxide. In addition to this, they could not write an equation to show how chlorine reacts with concentrated NaOH.
The expected answers include:
(a) 4Na(s) + O2(g) → 2Na2O(s)
- Na2O(s) + H2O(l) → 2NaOH(aq)
- 2NaOH(aq) + CO2(s) → Na2CO3(aq) + H2O(l)
- 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
(b) (i)burning of sulphur / sulphide ore
- heating or roasting of sulphide ores in air
(ii) vanadium (V) oxide
- platinum
(iii) Q - SO3 / Sulphur (VI) oxide
II. R - Conc. H2SO4 / Concentrated tetraoxosulphate (VI) acid
III. T - Oleum (H2S2O7)
(iv) H2SO4(l) + SO3(g) → H2S2O7(l)
(c) (i) Place some MnO2 in a flat bottomed flask
- add H2O2 to MnO2 through thistle / dropping funnel
- effervescence occurs/colourless, odourless gas is produced
- gas O2 is collected over water
(ii) 2H2O2MnO22H2O + O2
(d) (i) As a base
Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O
OR
Al2O3(s) + 6H+(aq) → 2Al3+(aq) + 3H2O(l)
As an acid
Al2O3(s) + 2NaOH(aq) → 2NaAlO2(aq) + H2O
OR
Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4
OR
Al2O3(s) + 2OH-(aq) + 3H2O(l) → 2[Al(OH)4]-