(a)(i) Explain what is meant by acid anhydride and give one example (ii) State three chemical properties of hydrochloric acid. (b) Explain each of the follo...

(a)(i) An acid anhydride is an oxide which reacts with water to form an acid; it may be regarded as an acid with water removed. For example:

\[ SO_3 + H_2O \rightarrow H_2SO_4 \]

Therefore, sulphur(VI) oxide, \(SO_3\), is an acid anhydride.

(a)(ii) Three chemical properties of hydrochloric acid are:

• It reacts with metals above hydrogen in the reactivity series to form a chloride salt and hydrogen gas: \[ Zn + 2HCl \rightarrow ZnCl_2 + H_2 \]
• It neutralises bases or alkalis to form a chloride salt and water: \[ HCl + NaOH \rightarrow NaCl + H_2O \]
• It reacts with carbonates or hydrogencarbonates to form a chloride salt, water and carbon(IV) oxide: \[ Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2 \]

(b)(i) Tetraoxosulphate(VI) acid, \(H_2SO_4\), is dibasic: it has two ionisable hydrogen ions. It can therefore form:

• an acid salt, such as \(NaHSO_4\); and
• a normal salt, such as \(Na_2SO_4\).

Trioxonitrate(V) acid, \(HNO_3\), is monobasic because it has only one ionisable hydrogen ion. It forms only normal salts, such as \(NaNO_3\).

(b)(ii) Iron is above hydrogen in the reactivity series, so it reacts with dilute sulphuric acid and displaces hydrogen gas:

\[ Fe + H_2SO_4 \rightarrow FeSO_4 + H_2 \]

Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from dilute \(H_2SO_4\). However, hot concentrated sulphuric acid is an oxidising agent, so it reacts with both copper and iron. Thus, only iron reacts with the dilute acid.

(b)(iii) Zinc dust has a greater surface area than zinc granules of the same mass. More zinc particles are exposed to the acid, so collisions between zinc and acid particles occur more frequently. The reaction is therefore faster and hydrogen gas is evolved more vigorously.

(c)(i) Activation energy is the minimum energy which reacting particles must possess for a collision to result in a chemical reaction.

(c)(ii) The energy profile must show reactants, products, activation energy, and the enthalpy change. Since the question gives \(\Delta H=x\text{ kJ mol}^{-1}\) without stating whether \(x\) is positive or negative, the relative positions of products and reactants depend on its sign. The diagram below shows the endothermic case, where \(x\) is positive.

If \(x\) is negative, the reaction is exothermic and the products should be drawn below the reactants, with \(\Delta H=-x\text{ kJ mol}^{-1}\).

(c)(iii) Mineral acids such as hydrochloric acid, nitric acid and sulphuric acid are strong acids. In dilute solution, they ionise completely to produce \(H^+\) ions. Sodium hydroxide also ionises completely to produce \(OH^-\) ions. Therefore, the net ionic equation is always:

\[ H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)} \]

Since the same reaction—the formation of water—occurs each time, the heat of neutralisation is approximately constant, about \(-57\text{ kJ mol}^{-1}\) of water formed.

(d)(i) \(Q_{(l)}\) has the higher entropy. In a liquid, particles are less ordered and have more freedom of movement than particles in a solid, which are held in fixed positions. The symbol \(O_{(l)}\) in the question appears to be a typographical error for \(Q_{(l)}\).

(d)(ii) Melting, \(Q_{(s)} \rightleftharpoons Q_{(l)}\), is endothermic in the forward direction. A decrease in temperature favours the exothermic reverse reaction, freezing. The equilibrium therefore shifts to the left, producing more \(Q_{(s)}\).