The following data were collected at 293.15 K for thereaction acetone (C 3 H 6 O

Question

The following data were collected at 293.15 K for thereaction

acetone (C3H6O) + H+ (aq) +Br2 (aq) ? bromoacetone(C3H5BrO)

The time required to produce a small but constant amount ofbromoacetone was measured. The small constant amount correspondedto a concentration of bromoacetone of 1.0 x 10-4 M.

Initial Rate Data

[Br2] (M, initial) [acetone] (M,initial) [H+] (M, initial) time, sec

0.010 0.25 0.10 35

0.010 0.13 0.10 71

0.020 0.25 0.10 36

0.020 0.13 0.40 18

What is the order of the reaction with respect to each of the threereactants?

Explanation / Answer

Ex. of 0th Order - Double [X], rate unchanged Ex. of 1st Order - Double [X], rate doubles Ex. of 2nd order - Double [X], rate quadruples Ex. of 3rd order - Double [X], rate increased by a factor of 3 etc... 2nd row - When [Acetone]is halved, the rate is halved (35s ÷ 71s). The rateis 1st order with respectto [Acetone] 3rd row - When[Br2] is doubled, the rate is unaffected (rate remains~35s), the rate is 0thorder with respect to [Br2] 4th row - When [Acetone]is halved, and [H+] is quadrupled, the rate quadruples(36s x 2 ÷ 18s), the rate is 1st order with respect to [H+] Overall the reaction is 2ndorder (bimolecular) Rate =k[Acetone]1[H+]1 This makes sense if you know some Organic Chem. For this reaction,the rate determining step is theformation of the Enol, which is independent of the bromine. Alsoonly one molecule of acid and one molecule of ketone is involved inthe transition state, hence the bimolecular nature of thereaction.

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