(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...
(ii) Three chemical properties of hydrochloric acid are:
It reacts with metals above hydrogen in the electrochemical series to form a chloride salt and hydrogen gas: \[\mathrm{Zn(s)+2HCl(aq)\rightarrow ZnCl_2(aq)+H_2(g)}\]
It neutralises bases or alkalis to form a chloride salt and water: \[\mathrm{HCl(aq)+NaOH(aq)\rightarrow NaCl(aq)+H_2O(l)}\]
It reacts with carbonates or hydrogencarbonates to produce carbon(IV) oxide, water and a chloride salt: \[\mathrm{Na_2CO_3(aq)+2HCl(aq)\rightarrow 2NaCl(aq)+H_2O(l)+CO_2(g)}\]
(b)(i) \(\mathrm{H_2SO_4}\) is dibasic because it has two replaceable hydrogen ions. It can therefore form an acid salt, for example \(\mathrm{NaHSO_4}\), and a normal salt, for example \(\mathrm{Na_2SO_4}\). \(\mathrm{HNO_3}\) is monobasic, having only one replaceable hydrogen ion, so it forms only normal salts such as \(\mathrm{NaNO_3}\).
(ii) Concentrated \(\mathrm{H_2SO_4}\) is an oxidising acid. It reacts with both iron and copper, including copper which is below hydrogen in the electrochemical series. For example:
Dilute \(\mathrm{H_2SO_4}\) behaves as an ordinary acid. Iron, being above hydrogen in the electrochemical series, displaces hydrogen from it, whereas copper, being below hydrogen, does not:
(iii) Zinc dust has a much larger surface area than zinc granules of the same mass. Hence, more zinc particles are in contact with the acid at a time, giving a faster reaction and more vigorous effervescence.
(c)(i) Activation energy is the minimum energy which reacting particles must possess for collisions between them to result in a chemical reaction.
(ii) Energy profile for \(\mathrm{A+B\rightarrow C+D}\), where \(\Delta H=+x\ \mathrm{kJ\,mol^{-1}}\):
Labelled energy profile diagram for the endothermic reaction \(\mathrm{A+B\rightarrow C+D}\).
(iii) Mineral acids such as hydrochloric acid, nitric acid and tetraoxosulphate(VI) acid are strong acids and are fully ionised in dilute aqueous solution. Their reaction with sodium hydroxide has the same net ionic equation:
\[\mathrm{H^+(aq)+OH^-(aq)\rightarrow H_2O(l)}\]
Since the same process, the formation of one mole of water, occurs in each case, the heat of neutralisation is approximately constant, about \(-57\ \mathrm{kJ\,mol^{-1}}\).
(d)(i) \(\mathrm{Q_{(l)}}\) has the higher entropy. In the liquid state, the particles have greater freedom of movement and a more random arrangement than in the solid state.
(ii) A decrease in temperature favours the reverse reaction, that is, the conversion of \(\mathrm{Q_{(l)}}\) to \(\mathrm{Q_{(s)}}\). The forward reaction is melting and is endothermic, so lowering the temperature favours the exothermic reverse change.
(ii) Three chemical properties of hydrochloric acid are:
It reacts with metals above hydrogen in the electrochemical series to form a chloride salt and hydrogen gas: \[\mathrm{Zn(s)+2HCl(aq)\rightarrow ZnCl_2(aq)+H_2(g)}\]
It neutralises bases or alkalis to form a chloride salt and water: \[\mathrm{HCl(aq)+NaOH(aq)\rightarrow NaCl(aq)+H_2O(l)}\]
It reacts with carbonates or hydrogencarbonates to produce carbon(IV) oxide, water and a chloride salt: \[\mathrm{Na_2CO_3(aq)+2HCl(aq)\rightarrow 2NaCl(aq)+H_2O(l)+CO_2(g)}\]
(b)(i) \(\mathrm{H_2SO_4}\) is dibasic because it has two replaceable hydrogen ions. It can therefore form an acid salt, for example \(\mathrm{NaHSO_4}\), and a normal salt, for example \(\mathrm{Na_2SO_4}\). \(\mathrm{HNO_3}\) is monobasic, having only one replaceable hydrogen ion, so it forms only normal salts such as \(\mathrm{NaNO_3}\).
(ii) Concentrated \(\mathrm{H_2SO_4}\) is an oxidising acid. It reacts with both iron and copper, including copper which is below hydrogen in the electrochemical series. For example:
Dilute \(\mathrm{H_2SO_4}\) behaves as an ordinary acid. Iron, being above hydrogen in the electrochemical series, displaces hydrogen from it, whereas copper, being below hydrogen, does not:
(iii) Zinc dust has a much larger surface area than zinc granules of the same mass. Hence, more zinc particles are in contact with the acid at a time, giving a faster reaction and more vigorous effervescence.
(c)(i) Activation energy is the minimum energy which reacting particles must possess for collisions between them to result in a chemical reaction.
(ii) Energy profile for \(\mathrm{A+B\rightarrow C+D}\), where \(\Delta H=+x\ \mathrm{kJ\,mol^{-1}}\):
Labelled energy profile diagram for the endothermic reaction \(\mathrm{A+B\rightarrow C+D}\).
(iii) Mineral acids such as hydrochloric acid, nitric acid and tetraoxosulphate(VI) acid are strong acids and are fully ionised in dilute aqueous solution. Their reaction with sodium hydroxide has the same net ionic equation:
\[\mathrm{H^+(aq)+OH^-(aq)\rightarrow H_2O(l)}\]
Since the same process, the formation of one mole of water, occurs in each case, the heat of neutralisation is approximately constant, about \(-57\ \mathrm{kJ\,mol^{-1}}\).
(d)(i) \(\mathrm{Q_{(l)}}\) has the higher entropy. In the liquid state, the particles have greater freedom of movement and a more random arrangement than in the solid state.
(ii) A decrease in temperature favours the reverse reaction, that is, the conversion of \(\mathrm{Q_{(l)}}\) to \(\mathrm{Q_{(s)}}\). The forward reaction is melting and is endothermic, so lowering the temperature favours the exothermic reverse change.
