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AP Chemistry: Kinetics — Practice Questions & Answers
How fast reactions go: rate laws, reaction order, half-life, the Arrhenius equation, mechanisms, and catalysis.
610 practice questions available for this unit — here are 10 with full answers and explanations.
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Practice questions with answers
1
Multiple choice · Easy
Which of the following best describes the rate of a chemical reaction?
The change in concentration of a reactant or product per unit time
The total amount of product formed when the reaction is complete
The difference in energy between products and reactants
The temperature increase during the reaction
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Why Reaction rate is the change in concentration of a reactant or product per unit time, with reactants decreasing and products increasing.
2
Multiple choice · Easy
For the reaction 2 A -> B, how does the rate of disappearance of A compare to the rate of appearance of B?
A disappears twice as fast as B appears
A disappears at the same rate B appears
A disappears half as fast as B appears
A and B rates are unrelated
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Why By stoichiometry, A disappears twice as fast as B appears, so rate of A loss = 2 times rate of B formation.
3
Multiple choice · Easy
Which factor generally increases the rate of a reaction?
Increasing the temperature
Decreasing the concentration of reactants
Increasing the activation energy
Decreasing the surface area of a solid reactant
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Why Increasing temperature gives molecules more kinetic energy and more frequent, more energetic collisions, raising the rate.
4
Multiple choice · Easy
A catalyst increases reaction rate by which mechanism?
Providing an alternative pathway with lower activation energy
Increasing the activation energy of the reaction
Making the reaction more exothermic overall
Increasing the concentration of the products
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Why A catalyst provides an alternative reaction pathway with a lower activation energy; it is not consumed and does not change the overall enthalpy.
5
Multiple choice · Easy
For a first-order reaction, what does the half-life depend on?
Only the rate constant k
Only the initial concentration
Both k and the initial concentration
The temperature only, not k
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Why The half-life of a first-order reaction is t(1/2) = 0.693/k, which depends only on the rate constant and is independent of initial concentration.
6
Fill in the blank · Easy
In a reaction energy profile, the minimum energy that colliding molecules must have to react is called the
Check answer
Answer:
activation
Why Activation energy (Ea) is the energy barrier between reactants and the transition state that must be overcome for a reaction to proceed.
7
Multiple choice · Medium
The rate law for a reaction is rate = k[A][B]^2. What is the overall order of the reaction?
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Why Overall order is the sum of the exponents: 1 + 2 = 3, so the reaction is third order overall.
8
Multiple choice · Medium
For the rate law rate = k[A]^2, what are the units of k when concentration is in mol/L and time is in seconds?
L mol^-1 s^-1
s^-1
mol L^-1 s^-1
L^2 mol^-2 s^-1
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Why For a second-order rate law, rate (M/s) = k times M^2, so k must have units of M^-1 s^-1 (L mol^-1 s^-1).
9
Multiple choice · Medium
In the method of initial rates, doubling [A] doubles the rate while [B] is held constant. What is the order with respect to A?
First order
Zero order
Second order
Third order
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Why When doubling [A] doubles the rate (factor of 2 = 2^1), the order with respect to A is 1 (first order).
10
Multiple choice · Medium
A reaction is first order in A. If a plot of ln[A] versus time is made, what does the graph look like?
A straight line with slope -k
A straight line with slope k
A curve that levels off
A straight line with slope -1/k
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Why For a first-order reaction, ln[A] versus time is a straight line with slope -k, since ln[A] = ln[A]0 - kt.
Key terms in Kinetics
Chemical Kinetics: The branch of chemistry that studies the rates of chemical reactions and the factors that influence them.
Reaction Rate: The change in concentration of a reactant or product per unit time during a chemical reaction.
Average Rate: The change in concentration of a species divided by the total time interval over which the change occurs.
Instantaneous Rate: The reaction rate at a single specific moment, found from the slope of the tangent to a concentration-versus-time curve.
Initial Rate: The instantaneous rate measured at the very start of a reaction before significant product accumulates.
Rate of Disappearance: The rate at which a reactant is consumed, expressed as the decrease in its concentration over time.
Rate of Appearance: The rate at which a product is formed, expressed as the increase in its concentration over time.
Stoichiometric Rate Relationship: The connection that lets a single reaction rate be expressed for any species by dividing its concentration change by its coefficient.
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