A-LevelChemistryReaction kinetics9701/41 · May/June 2022 · Q315 Marks

The rate of reaction between 2-chloro-2-methylpropane, (CH3)3CCl, and methanol is investigated. When a large excess of methanol is used, the overall reaction is first order. (CH3)3CCl + CH3OH → (CH3)3COCH3 + HCl Fig. 3.1 shows the results obtained. [Figure 3.1]

(a)(i)Use the graph to determine the rate of reaction at 40s. Show all your working. rate[1]
(a)(ii)Use the graph to show that the overall reaction is first order. Explain your answer.[2]
(b)In a different reaction, which is also a first order reaction, 75% of the reactant is consumed in 320 s. Calculate the rate constant, k, for this reaction. State the units for k. k = units =[2]
(c)(i)Define standard electrode potential, E®.[1]
(c)(ii)A salt bridge is used in an electrochemical cell. State the function of the salt bridge. Explain your answer.[1]
(c)(iii)Complete the diagram of the apparatus that can be used to measure the Eº of the Cr2O72-(aq), H+(aq)/Cr3+(aq) electrode against the standard hydrogen electrode. Your diagram should be fully labelled to identify all apparatus, substances and conditions. salt bridge[3]
(c)(iv)The E® of the Cr2O72-(aq), H+(aq)/Cr³+(aq) electrode is +1.33V. Label the negative electrode and the direction of electron flow in the external circuit when the current flows in your diagram in (c)(iii).[1]
(d)(i)Table 3.1 lists relevant electrode potentials for some electrode reactions for use in (d)(i) and (d)(ii). Table 3.1 electrode reaction E°/V Cr2O72- + 14H+ + 6e¯ = 2Cr3+ + 7H2O +1.33 CH3CHO + 2H+ + 2e¯ = CH3CH2OH -0.61 CH3COOH + 2H+ + 2e¯ = CH3CHO + H₂O -0.94 O2 + 4H+ + 4e¯ ⇌ 2H₂O +1.23 Ethanal is oxidised to ethanoic acid in the presence of Cr2O72- ions. Construct the ionic equation for the oxidation of ethanal to ethanoic acid using dichromate(VI) in acid conditions. Calculate the Ecell for this reaction. ionic equation Ecell[2]
(d)(ii)In an ethanol-oxygen fuel cell, CH3CH2OH(l) and O2(g) are in contact with two inert electrodes immersed in an acidic solution. The cell reaction for the oxidation of ethanol by oxygen is shown. 2CH3CH2OH + O2 → 2CH3COOH + 2H2O Ecell = +2.01 V Calculate ∆G, in kJ mol¯¹, for the oxidation of ethanol by oxygen. ∆G° =[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 (May/June 2022). The mark or mark type is shown in brackets.

[1] evidence of tangent drawn at t = 40 s and calculation of gradient
= 0.000170 (mol dm⁻³ s⁻¹) min 2sf
[2] M1 evidence of construction lines and calculation of two t1/2
OR evidence of construction lines and times for halving of concentration
M2 deduction: constant half-life / constant time between halving of concentration 1st order
[2] M1 two half-lives in 320 s so t1/2 = 160 s
M2 k = 0.693/160 = 0.00433 s⁻¹ ecf min 2sf
[1] voltage / EMF / potential difference when a half-cell is connected to a (standard) hydrogen electrode under standard conditions
[1] ions move (from the salt bridge) to maintain charge balance / complete the circuit
[3] (Diagram as shown in MS: Pt, H2(g), good delivery system, 298 K, 1 atm; Pt, Cr2O72-(aq), H+(aq) / Cr3+(aq), 1 mol dm-3 seen once, voltmeter; three for one mark, six for two marks, nine for three marks)
[1] SHE labelled negative AND arrow in the external circuit moving away from this electrode
[2] M1 Cr2O72- + 3CH3CHO + 8H+ = 2Cr3+ + 3CH3COOH + 4H2O
ALLOW Cr2O72- + 3CH3CHO + 5H+ = 2Cr3+ + 3CH3COO- + 4H2O
M1 Ecell = +2.27 (V)
[2] M1 ∆G® = -nFEcell
M2 ∆G® = -4 × 96 500 × 2.01 = −775 860 J mol-1
∆G® = -776 kJ mol-1 min 3sf