1. For each of the following rate equations, describe what would happen to the r
ID: 1002321 • Letter: 1
Question
1. For each of the following rate equations, describe what would happen to the rate if the concentration of reactant A was tripled and the concentration of reactant B is halved.
a. Rate = k[A][B]
b. Rate = k[A]2 [B]
c. Rate = k[A]2 [B]2
d. Rate = k[A][B]
2. If a reactant is used up according to a first order rate equation, and the initial concentration of the reactant is 3.2 mol L-1 , what is the concentration of the reactant after two half-lives have passed, and after six half-lives have passed?
Explanation / Answer
1. Let [A] = 1 M, and [B] = 1 M
a) r1 = k[A][B] = k (1)(1) = k
When concentration of reactant A was tripled and the concentration of reactant B is halved,
r2 = k[3(1)][(1)/2]
= 3/2 k
= 3/2 r1
Rate will become 3/2 of earlier rate
b) r1 = k[A]2[B]
When concentration of reactant A was tripled and the concentration of reactant B is halved,
r2 = k[3(1)]2[1/2]
= 9/2 k
= 9/2 r1
Rate will become 9/2 of earlier rate
c) r1 = k[A]2[B]2
When concentration of reactant A was tripled and the concentration of reactant B is halved,
r2 = k[3(1)]2[1/2]2
= 9/4 k
= 9/4 r1
Rate will become 9/4 of earlier rate.
d) Same as part a.
2. Initial concentration = [A]o = 3.2 mol L-1
Final concentration = [A]
Number of half lives = n
[A] = [A]o (1/2)n
For n = 2
[A] = (3.2) (1/2)2
= 3.2/4 = 0.8 mol L-1
For n = 6
[A] = (3.2) (1/2)6
= 3.2/64 = 0.05 mol L-1
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