Q. For a reaction A → B, if the rate constant k is 0.1 s^-1, what is the time required for the concentration of A to decrease to 25% of its initial value?
A.
10 seconds
B.
20 seconds
C.
30 seconds
D.
40 seconds
Solution
For a first-order reaction, t = (ln(2)/k) * ln([A]0/[A]t). To reach 25%, t = (ln(2)/0.1) * ln(4) = 20 seconds.
Q. In a first-order reaction, if the half-life is 10 minutes, what will be the concentration after 30 minutes if the initial concentration is 1 M?
A.
0.125 M
B.
0.5 M
C.
0.75 M
D.
0.25 M
Solution
For a first-order reaction, the concentration after n half-lives is given by [A] = [A]0 * (1/2)^n. After 30 minutes (3 half-lives), [A] = 1 M * (1/2)^3 = 0.125 M.
Q. In a reaction A → B, if the rate of formation of B is 0.5 M/s, what is the rate of disappearance of A?
A.
0.5 M/s
B.
1.0 M/s
C.
0.25 M/s
D.
0.75 M/s
Solution
For a 1:1 stoichiometry, the rate of disappearance of A is equal to the rate of formation of B. Therefore, it is 0.5 M/s, but since A is disappearing, it is 1.0 M/s.
Q. What is the relationship between the rate constant k and temperature T according to the Arrhenius equation?
A.
k increases with decreasing T
B.
k decreases with increasing T
C.
k increases with increasing T
D.
k is independent of T
Solution
According to the Arrhenius equation, k increases with increasing temperature due to the exponential dependence on the negative activation energy divided by temperature.
