A-LevelChemistryEquilibria9701/42 · Feb/Mar 2024 · Q213 Marks

Water is an amphoteric compound that also acts as a good solvent of polar and ionic compounds.

(a)Equation 1 shows water acting as a Brønsted-Lowry acid. equation 1 H2O + NO2¯ ⇌ HNO2 + OH-
(a)(i)Identify the two conjugate acid-base pairs in equation 1. acid I H2O conjugate base of acid I acid II conjugate base of acid II[1]
(a)(ii)Water also behaves as a Brønsted-Lowry acid when it dissolves CH3NH2. Explain the ability of CH3NH2 to act as a base.[1]
(a)(iii)Write an equation to show water acting as a base with CH3COOH.[1]
(b)The ionic product of water, Kw, measures the extent to which water dissociates. H2O(l) ⇌ H+(aq) + OH¯(aq) Fig. 2.1 shows how Kw varies with temperature. [Figure 2.1]
(b)(i)Write an expression for Kw.[1]
(b)(ii)Use information from Fig. 2.1 to deduce whether the dissociation of water is an exothermic or an endothermic process. Explain your answer.[1]
(b)(iii)An aqueous solution has pH = 7.00 at 30°C. Use information from Fig. 2.1 to explain why this solution can be considered to be alkaline at 30 °C.[2]
(c)The three physical states of H2O have different standard entropies, Sº, associated with them. Table 2.1 shows these Sª values. Table 2.1 | state of H2O | standard entropy, S°/JK-1 mol-1 | |:-------------|:----------------------------------| | solid | +48.0 | | liquid | +70.1 | | gas | +188.7 |
(c)(i)Explain the difference in the Sº values of H2O(s) and H2O(l).[1]
(c)(ii)Explain why the increase in Sº is much greater when H2O boils than when it melts.[1]
(c)(iii)The energy changes for H2O(s) → H2O(l) are shown. ∆G = 0.00 kJ mol-1 ∆H = +6.03 kJ mol-1 Use these data to show that the melting point of H2O(s) is 0°C.[1]
(d)Metal-air batteries are electrochemical cells that generate electrical energy from the reaction of metal anodes with air. The standard electrode potentials for the zinc-air battery are shown. [Zn(OH)4]2- + 2e- ⇌ Zn + 4OH- Eº = -1.22V ½O2 + H2O + 2e¯ ⇌ 2OH- Eº = +0.40 V
(d)(i)Calculate the standard cell potential, Ecel, of the zinc-air battery. Ecel =[1]
(d)(ii)The zinc-air battery usually operates at pH11 and 298K. The overall cell potential is dependent on [OH-]. The Nernst equation shows how the electrode potential at the cathode changes with [OH-]. E = 0.40 - (0.059/z)log([OH-]²) Calculate the electrode potential, E, at pH 11. E =[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2024). The mark or mark type is shown in brackets.

[1] • conjugate base of acid I = OH- • acid II = HNO2 • conjugate base of acid II = NO2-
[1] lone pair on the N can be donated to a proton /H+ OR lone pair on the N can accept/gain a proton /H+ OR lone pair on the N can form a dative bond to a proton/H+
[1] H2O + CH3COOH → H3O+ + CH3COO-
[1] (Kw) = [H+][OH-] ALLOW (Kw) = Ka × Kb
[1] endothermic AND equilibrium position moves right OR as water dissociates more OR Kw increases with temperature
[1] M1 (Kw increases with temperature so) pH of neutral solution decreases OR (from graph) Kw = 1.50 × 10–14 = [H+]2 ∴ neutral pH = −½ log (1.50 × 10-14) = 6.91 OR [H+] = 10-7.00 ∴ [OH-] = 1.50 × 10–14 / 10–7.00 = 1.50 × 10–7
[1] M2 pH 7 is therefore above neutral pH / is alkaline OR [OH-] > [H+] (so alkaline)
[1] H2O(l) particles / molecules has more randomness / disorder OR H2O(l) has more ways to arrange particles / energy (than in solid)
[1] H2O(g) particles / molecules has much more randomness / disorder OR H2O(g) has many more ways to arrange particles / energy (than in liquid)
[1] +6030 / (70.1 – 48.0) = 272.85 / 272.9 / 273 K (which is 0 °C)
[1] (+)1.62 V
[1] [OH-] = 10–14/10–11 = 1 × 10–3
[1] E = +0.40 – 1/2 × 0.059 × log (10-3)2 = +0.40 – 0.059(−3) = +0.577 (V) min 2sf