Question

± pH and Kinetics

The iodate ion, IO3−, is reduced by sulfite, SO32−, according to the following net ionic equation:

IO3−(aq)+3SO32−(aq)→I−(aq)+3SO42−(aq)

Part A

The reaction is found to be first order in IO3−, first order in SO32−, and first order in H+.

If [IO3−]=x, [SO32−]=y, and [H+]=z, what is the rate law for the reaction in terms of x, y, and z and the rate constant k?

Express the rate in terms of k, x, y, and z (e.g., kxy^3z^2).

Hints

rate =

k[x]1[y]1[z]1

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Correct

Expressed in terms of x, y, and z, the rate law is as follows:

rate=kxyz

By substituting the actual species from the reaction, the expression becomes

rate=k[IO3−][SO32−][H+]

Part B

By what factor will the rate of the reaction change if the pH decreases from 6.50 to 2.00?

Express your answer numerically using two significant figures.

Hints

Part C

The reaction is pH dependent even though the H+ ion does not appear in the overall reaction. Which of the following explains this observation?

Check all that apply.

Hints

Check all that apply.

H+ serves as a catalyst in the reaction.

The exponent of [H+] in the rate law is zero.

H+ is an intermediate in the reaction mechanism.

H+ enters the reaction mechanism after the rate-determining step.

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Answer 1

Part B

pH 6.5 means, the concentration of H+ is 3.2 x 10 ^-7 M (pH = - log [H+], [H+] = antilog -pH)

pH 2 means , the concentration of H+ is 1x 10 ^-2 M

Ratio of these two concentrations are 31645.5696203. Since the reaction is first order with respect to protons, the rate will increase by a factor of 31645.5696203

If we are rpresenting in 2 significant figures, it will be 3.2 x 10³

Part C

H+ serves as a catalyst in the reaction

and thus it is retained after the reaction