Physical Chem - Reaction Mechanisms QUESTION: The decomposition of hydrogen pero

Question

Physical Chem - Reaction Mechanisms

QUESTION: The decomposition of hydrogen peroxide, H2O2 (aq), is catalyzed by halide ions, such as Br-. Given the following predicted mechanism:

H2O2 (aq) + Br- (aq) k1 H2O (l) + BrO- (aq) (slow)

H2O2 (aq) + BrO- (aq) k2 H2O (l) + O2 (g)+Br- (aq) (fast)

(i) What is the overall reaction stoichiometry? Which is the rate-determing step?

(ii) Are there any short-lived intermediates predicted by this mechanism?

(iii) Derive the rate law from the above mechanism, in terms of d[H2O2 ]/dt

(iv) The experimentally-determined rate law is first order in H2O2 and first order in catalyst. Show that your rate law is consistent with this result.

Thanks =)

Explanation / Answer

i.

Overall reaction is,

2 H2O2 (aq.) ----------> 2H2O (l) + O2 (g)

Ii.

Yes. BrO^-

Iii.

Since rate if reaction depends on only slow step,

Rate = k1[H2O2][Br-]

But since Br- here acts as catalyst , no net consumption of it is observed in the overall reaction and hence rate is independent of Br-.

Therefore, rate = k1[H2O2]

Iv.

So, the reaction is first order with respect to H2O2.

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