Page 141 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

Chemical Kinetics I 125
Solution:

1. A + B + C+Products
d[Al
2. Rate = --- = - -- d[B1 = --- d[C1 = k[A]X[B]Y[C]", where x,
1 dt 1 dt 1 dt
y and z are the unknown partial orders of the reaction.
3 and 4
Rate 1 /Rate 2 = { l~(0.856)~ (0.1 98)' (0.699)') / { k( 0.593)x (0.1 98)'
(0.699)")
= (0.856/0.593)" = (2.94 x 10-4/9.76 x
+ (1.4435076)" = 3.012295.
Then, 1n(1.4435076)X = In (3.012295) and x (In 1.4435076) =
In 3.012295 + x = 3
Rate 2/Rate 3
= { k(0 .593)3 (0.198)" (0.699)') / { k( 0.39 1) (0.699)' (0.699)")
= 3.4884612 x (0.2832618)y = (9.76 x 10-5/6.91 x =
1.4124457
(0.2832618)'' = 0.4048908.
Then, ln(0.2832618)y = In (0.4048908) and y In (0.2832618) =
In (0.4048908) 3 y = 0.72
Rate 3/Rate 4 =
5
(k(0.39 1)3(0. 699)0.72(0. 699)") /{ k(0.496)3 (0.325)0*72(0. 1 1)")
= 0.8502622 x (1.3679061)' = (6.91 x 10-5/1.37 x loA4) =
0.5043796 +- (1.3679061)" = 0.59320487.
Then, In (1.3679061)" = In (0.5932048) =+ z = -1.67
5. Overall order of the reaction = x + y + z = 3 + 0.72 - 1.67 =
2.05, i.e. approximately 2.

Answer: The approximate overall order of the reaction is second-
order.

Note: Fractional orders normally imply that the mechanism of a
reaction is more complicated then expected, i.e. consecutive reactions
may be occurring.

HOW CONCENTRATION DEPENDENCE, i.e. THE ORDER OF
A REACTION, CAN BE USED TO DEVELOP A MECHANISM
FOR A REACTION
,
The bromination of propanone (acetone), in the presence of acid Hf
is a second-order reaction, whose rate was found to be independent of
the concentration of the bromine Br2, i.e.