AP Chemistry Practice Quiz: Elementary Reactions
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: July 2026
Test your understanding with short quizzes. This quiz has 9 questions to check your progress.
Question 1 of 9
All Questions (9)
A) Rate = k[A][B]
B) Rate = k[C]
C) Rate = k[A]^2
D) The rate law cannot be determined from the given information.
Correct Answer: A
For an elementary reaction, the rate law can be inferred directly from the stoichiometry of the reactants. Since one particle of A and one particle of B participate in the collision, the rate is first order with respect to [A] and first order with respect to [B], making the overall rate law Rate = k[A][B].
A) Rate = k[NO]
B) Rate = k[NO]²
C) Rate = k[N₂O₂]
D) Rate = k[N₂O₂] / [NO]²
Correct Answer: B
The rate law of an elementary reaction is determined by the stoichiometry of the reactants. In this reaction, two particles of NO collide. Therefore, the reaction is second order with respect to NO, and the rate law is Rate = k[NO]².
A) Only if the reaction is reversible.
B) Only if the reaction is an elementary reaction.
C) Only if the reaction is catalyzed.
D) This is never possible; rate laws must always be found experimentally.
Correct Answer: B
The rate law can be inferred from the stoichiometry of the particles participating in a collision only when the reaction is an elementary step. For multi-step reactions, the overall rate law is determined by the slowest step (rate-determining step) and must be determined experimentally.
A) The activation energy for such collisions is infinitely high.
B) The products of such reactions are typically unstable.
C) The statistical probability of three or more particles colliding at the same instant with the correct orientation and energy is very low.
D) Such reactions violate the law of conservation of energy.
Correct Answer: C
The rate of an elementary reaction depends on the frequency of effective collisions. The probability of two particles colliding is significant, but the probability of three or more particles all arriving at the same point in space, at the exact same time, and with the proper orientation for a reaction is extremely low, making such events rare.
A) Rate = k[A][B]
B) Rate = k[A]²
C) Rate = k[A]²[B]
D) The rate law must be determined experimentally.
Correct Answer: C
Assuming this is an elementary reaction, the rate law is derived from the reactant stoichiometry. The coefficient of A is 2, and the coefficient of B is 1. Therefore, the rate law would be Rate = k[A]²[B].
A) O₃ → O₂ + O
B) Cl + CH₄ → HCl + CH₃
C) 2NO₂ → N₂O₄
D) 2H₂ + O₂ → 2H₂O
Correct Answer: D
The reaction 2H₂ + O₂ → 2H₂O involves the collision of three particles (two H₂ and one O₂) simultaneously. Such a termolecular collision is a very rare event. The other options represent unimolecular (A) and bimolecular (B, C) steps, which are much more common.
A) the final equilibrium concentrations.
B) the change in enthalpy of the reaction.
C) the probability of the reactant particles colliding.
D) the total number of bonds broken and formed.
Correct Answer: C
For an elementary reaction, the rate is directly proportional to the frequency of collisions between the reacting particles. The concentration terms in the rate law reflect the statistical probability that the required particles will collide to initiate the reaction.
A) The reaction must be at a high temperature.
B) The reaction must have a low activation energy.
C) The reaction must be a single elementary step.
D) The concentrations of X and Y must be equal.
Correct Answer: C
A rate law can only be written directly from the stoichiometry of the overall reaction equation if that reaction occurs in a single elementary step. If it were a multi-step process, the rate law would be determined by the slowest step, which may not match the overall stoichiometry.
A) Because elementary steps cannot have three different reactants.
B) Because the simultaneous collision of three distinct particles is a highly improbable event.
C) Because the product D cannot be formed in a single step from three reactants.
D) Because elementary steps must be reversible.
Correct Answer: B
The proposed step is termolecular, meaning it requires the simultaneous collision of particles A, B, and C. Such events are extremely rare because the probability of three particles colliding at the same instant with sufficient energy and correct orientation is very low. Reaction mechanisms typically consist of more probable unimolecular or bimolecular steps.