This question was not so popular with the candidates and the performance was fair.
In part (a), candidates correctly stated that when gases react, they do so in volumes which are simple ratios to one another and to the volume of the products if gaseous, provided that the temperature and pressure remain constant was the statement of Gay Lussac’s Law.
In part (b), candidates wrote balanced equation for the reaction of carbon (II) oxide and oxygen to form carbon (IV) oxide as follows:
2CO(g), + O2(g) → 2CO2(g) .
They also tackled the calculation arising therefrom correctly thus:
- from the given equation,
2 volumes of CO(g) and 1 volume of O2(g) → 2 volumes CO2 2
ratio of reacting volumes
40cm3 + 20cm3 → 40cm3
40cm3 of CO requires 20cm3 of oxygen to form 40cm3 of CO2 .
∴ 40cm3 of CO2 is produced
- excess reactant is oxygen
volume is 30cm3 – 20cm3 = 10cm3
- total volume of gaseous mixture
10cm3 O2 + 40cm3CO2 = 50cm3
Candidates correctly answered (c), but could not explain why colourless concentrated trioxonitrate (V) acid turns yellow nor dilute trioxonitrate (V) acid does not liberate hydrogen when it reacts with magnesium in (d).
Also in (e), candidates failed to correctly write a chemical equation for the thermal decomposition of the nitrates given.
The expected responses were
(c) (i) SiO2
(v) CO and NO2
(d) (i) conc HNO3 slowly decomposes releasing brown NO2 which dissolves in the acid
to give the yellow colour.
(ii) HNO3 is an oxidizing acid. Any hydrogen produced is oxidized to water.
(e) (i) 2Cu (NO3) 2(s) (heat) 2CuO(s) + 4NO2(g) + O2(g)
(ii) NH4 NO3(s) (heat) N2O(g) + 2H2O(g)