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Rate equations (A-level only)

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Question 7

Reactants X X\,X and Y Y\,Y react together in the presence of an acid catalyst:

X(aq)+Y(aq)→Z(aq)+W(aq) X(aq) + Y(aq) \rightarrow Z(aq) + W(aq) X(aq)+Y(aq)→Z(aq)+W(aq)

The rate equation for this reaction is:

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

Table 1 shows how the relative initial rate of the reaction varies with different concentrations of each species at a constant temperature.

Table 1

Experiment[X] / mol dm−3[X] \,/\, \text{mol dm}^{-3}[X]/mol dm−3[Y] / mol dm−3[Y] \,/\, \text{mol dm}^{-3}[Y]/mol dm−3[H+] / mol dm−3[H^+] \,/\, \text{mol dm}^{-3}[H+]/mol dm−3Relative initial rate
10.400.150.201.20
20.800.150.20
30.400.200.80
40.600.300.10
a.

Complete Table 1 by calculating the missing values.

[3]
b.

A suggested mechanism for this reaction is:

  • Step 1: Y+H+→YH+Y + H^+ \rightarrow YH^+Y+H+→YH+
  • Step 2: YH++H+→YH22+YH^+ + H^+ \rightarrow YH_2^{2+}YH++H+→YH22+​
  • Step 3: YH22++X→Z+W+2H+YH_2^{2+} + X \rightarrow Z + W + 2H^+YH22+​+X→Z+W+2H+

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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