AP Chemistry: Kinetics & Chemical Equilibrium 2 — Questions and Answers
Question 1: For the reaction rate = k[A]^2[B], what happens to the rate if [A] is tripled while [B] is held constant?
- Rate increases 3x
- Rate increases 6x
- Rate increases 9x (Correct answer)
- Rate is unchanged
Correct answer: Rate increases 9x
Since A is second order, tripling [A] gives 3^2 = 9 times the rate.
Question 2: A reaction is first order in a reactant with a half-life of 20 seconds. How much of the reactant remains after 60 seconds?
- 1/2
- 1/4
- 1/8 (Correct answer)
- 1/16
Correct answer: 1/8
60 seconds is three half-lives, so (1/2)^3 = 1/8 remains.
Question 3: Which statement about a catalyst is correct?
- It increases the equilibrium constant
- It is consumed in the reaction
- It lowers the activation energy of both forward and reverse reactions (Correct answer)
- It shifts equilibrium toward products
Correct answer: It lowers the activation energy of both forward and reverse reactions
A catalyst provides a lower-energy pathway, speeding both directions equally without changing K.
Question 4: For a first-order reaction, which plot yields a straight line?
- [A] versus time
- ln[A] versus time (Correct answer)
- 1/[A] versus time
- [A]^2 versus time
Correct answer: ln[A] versus time
A first-order integrated rate law gives a linear plot of ln[A] versus time.
Question 5: The rate constant k of a reaction increases with temperature primarily because
- activation energy decreases
- more molecules have energy exceeding the activation energy (Correct answer)
- the reaction becomes exothermic
- the order of reaction changes
Correct answer: more molecules have energy exceeding the activation energy
Higher temperature raises the fraction of collisions with sufficient energy to react.
Question 6: In the Arrhenius equation k = A·e^(-Ea/RT), a larger activation energy Ea results in
- a larger rate constant
- a smaller rate constant (Correct answer)
- no effect on k
- a negative rate constant
Correct answer: a smaller rate constant
A larger Ea makes the exponential term smaller, decreasing k.
Question 7: Units of the rate constant k for a second-order reaction are
- s^-1
- M·s^-1
- M^-1·s^-1 (Correct answer)
- M^-2·s^-1
Correct answer: M^-1·s^-1
Second-order rate constants have units of M^-1·s^-1.
AP Chemistry: Kinetics & Chemical Equilibrium 2
For the reaction rate = k[A]^2[B], what happens to the rate if [A] is tripled while [B] is held constant?