Explain the reactions and properties of dinitrogen monoxide and nitrogen dioxide, including their preparation and physical differences.

a)

i) The gas being produced is dinitrogen monoxide (or nitrous oxide).

ii) The equation for the thermal decomposition of ammonium nitrate is:

$$NH_4NO_3(s) \longrightarrow N_2O(g) + 2H_2O(g)$$

iii) The gas is collected over hot water because dinitrogen monoxide is sparingly soluble in water, and its solubility decreases further in hot water. This method ensures maximum collection of the gas.

iv) When burning sulphur is lowered into a gas jar containing dinitrogen monoxide:

• Observation: The sulphur burns with a brighter blue flame.
• Explanation: Dinitrogen monoxide decomposes upon heating to produce nitrogen and oxygen ($2N_2O(g) \longrightarrow 2N_2(g) + O_2(g)$). The oxygen produced supports the combustion of sulphur, causing it to burn more vigorously.

b)

i) The catalyst used during catalytic oxidation of ammonia is platinum (or platinum-rhodium alloy).

ii) Nitrogen (IV) oxide ($NO_2$) is formed from nitrogen monoxide ($NO$) and oxygen. The chemical equation for its formation is:

$$2NO(g) + O_2(g) \longrightarrow 2NO_2(g)$$

iii) Two physical differences between Nitrogen (I) oxide ($N_2O$) and Nitrogen (IV) oxide ($NO_2$) are:

• Color: $N_2O$ is a colorless gas, while $NO_2$ is a reddish-brown gas.
• Odor: $N_2O$ has a sweet smell, while $NO_2$ has a pungent, suffocating smell.

c)

i) The apparatus must be made of glass because concentrated nitric acid is highly corrosive and reacts with many materials, especially metals. Glass is inert to concentrated nitric acid, preventing unwanted reactions and contamination.

ii)
(I) To identify the species oxidized, we determine the change in oxidation states:

• In $S_{(s)}$, the oxidation state of S is 0.
• In $H_2SO_{4(aq)}$, the oxidation state of S is +6.
• In $HNO_{3(aq)}$, the oxidation state of N is +5.
• In $NO_{2(g)}$, the oxidation state of N is +4. The oxidation state of sulphur increases from 0 to +6, indicating oxidation. The species oxidized is sulphur ($S$).

(II) Pure nitric acid is colorless, but the product during its preparation is usually pale yellow because nitric acid decomposes when exposed to light or heat, forming nitrogen dioxide ($NO_2$). Nitrogen dioxide is a reddish-brown gas that dissolves in the nitric acid, imparting a pale yellow color.
The decomposition reaction is:

$$4HNO_3(aq) \longrightarrow 4NO_2(g) + O_2(g) + 2H_2O(l)$$