
To find:
The rate law for the reaction 2BrO(g)→Br2(g)+O2(g) under each of the given reaction conditions.

Answer to Problem 13.51QA
Solution:
a) The rate doubles when [BrO] doubles; Rate=k[BrO]
b) The rate quadruples when [BrO] doubles; Rate=k[BrO]2
c) The rate is halved when [BrO] is halved; Rate=k[BrO]
d) The rate is unchanged when [BrO] doubles; Rate=k
Explanation of Solution
1) Concept:
Rate law is the equation, which determines the relation between the
2BrO(g)→Br2(g)+O2(g)
Consider the general rate law for this reaction, which is
Rate=k[BrO]n
where k is the rate constant and n is the order in BrO.
2) Calculation:
a) The rate doubles when [BrO] doubles.
Let the initial rate be Rate 1, and when the concentration of BrO doubles, the rate also doubles. Consider it as Rate 2.
i.e., Rate 2 = 2 (Rate 1) and [BrO] = 2 [BrO]
So the rate law for both will be
Rate 1=k[BrO]n
Rate 2=k(2[BrO])n
Take the ratio of Rate 2 to Rate 1,
Rate 2Rate 1=k(2[BrO])nk [BrO]n
Plug the value of Rate 2=2 (Rate 1),
Therefore,2 Rate 1Rate 1=k(2[BrO])nk [BrO]n
2=2n [BrO]n [BrO]n
2=2n
n = 1
That means reaction is first order in BrO. Hence the rate law can be written as
Rate=k[BrO]
b) The rate quadruples when [BrO] doubles.
Let the initial rate be Rate 1m, and when the concentration of BrO doubles, the rate quadruples. Consider it as Rate 2.
i.e., Rate 2 = 4 (Rate 1) when [BrO] = 2 [BrO]
So the rate law for both will be
Rate 1=k[BrO]n
Rate 2=k(2[BrO])n
Take the ratio of Rate 2 to Rate 1.
Rate 2Rate 1=k(2[BrO])nk [BrO]n
Plug the value of Rate 2=4 (Rate 1).
Therefore,4 Rate 1Rate 1=k(2[BrO])nk [BrO]n
4=2n [BrO]n [BrO]n
4=2n
n =2
i.e., reaction is second order in BrO, and the rate law will be
Rate=k[BrO]2
c) The rate is halved when [BrO] is halved.
Let the initial rate be Rate 1, and when the concentration of BrO halves, the rate also halves. Consider it as Rate 2.
i.e., Rate 2 = 1/2 (Rate 1) when [BrO] = 1/2 [BrO]
so the rate law for both will be
Rate 1=k[BrO]n
Rate 2=k(1/2[BrO])n
Take the ratio of Rate 2 to Rate 1.
Rate 2Rate 1=k(1/2 [BrO])nk [BrO]n
Plug the value of Rate 2=12(Rate 1).
Therefore,
1/2 Rate 1Rate 1=k(1/2[BrO])nk [BrO]n
1/2=(1/2)n [BrO]n [BrO]n
1/2= (1/2)2
n =1
Hence, reaction is first order in BrO, and rate law is
Rate=k[BrO]
d) The rate is unchanged when [BrO] doubles.
When rate the concentration of BrO doubles, the rate remain unchanged,
i.e., [BrO] =2 [BrO] , Rate 2 = Rate 1
So the rate law for both will be
Rate 1=k[BrO]n
Rate 2=k(2[BrO])n
Take the ratio of Rate 2 to Rate 1.
Rate 2Rate 1=k(2[BrO])nk [BrO]n
Since, Rate 2 = Rate 1, equation becomes
Rate 1Rate 1=k(2[BrO])nk [BrO]n
0=2n [BrO]n [BrO]n
0=2n
n = 0
Hence, the reaction is zero order in BrO.
The rate law will be
Rate=k[BrO]o
Rate=k
Conclusion:
The effect of change in concentration of reactants on rate depends on the order of reaction with respect to that reactant.
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Chapter 13 Solutions
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