Rate equations (A-level only)

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Question 24
Medium

P and Q react together in the presence of an acid catalyst.

P(aq)+2Q(aq)→R(aq)+S(aq)P(aq) + 2Q(aq) \rightarrow R(aq) + S(aq)P(aq)+2Q(aq)→R(aq)+S(aq)

The rate equation for this reaction is

rate=k[Q]2[H+]\text{rate} = k[Q]^2[H^+]rate=k[Q]2[H+]

Table 1 shows how the values of the relative initial rate vary with different concentrations of each reagent at the same temperature.

Table 1

Experiment[P] / mol dm−3[P] \,/\, \text{mol dm}^{-3}[P]/mol dm−3[Q] / mol dm−3[Q] \,/\, \text{mol dm}^{-3}[Q]/mol dm−3[H+] / mol dm−3[H^+] \,/\, \text{mol dm}^{-3}[H+]/mol dm−3Relative initial rate
10.300.200.101.00
20.450.200.10
30.300.100.49
40.450.300.08
1.

Complete Table 1 by calculating the missing values.

[5]
2.

A suggested mechanism for the reaction is shown:

  • Step 1: Q+H+→QH+Q + H^+ \rightarrow QH^+Q+H+→QH+
  • Step 2: QH++Q→Q2H+QH^+ + Q \rightarrow Q_2H^+QH++Q→Q2​H+
  • Step 3: Q2H++P→R+S+H+Q_2H^+ + P \rightarrow R + S + H^+Q2​H++P→R+S+H+

Deduce the rate-determining step for this reaction and give a reason for your answer.

[2]

Rate equations (A-level only) Questions

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