AP Chemistry Questions on Elementary Steps and Rate Laws

📚 Quick Study Guide

• 🧪 Elementary Step: An elementary step is a single molecular event in a reaction mechanism.
• ➡️ Rate Law for Elementary Steps: The rate law for an elementary step can be written directly from the stoichiometry of that step. For the elementary step $A + B \rightarrow products$, the rate law is $rate = k[A][B]$.
• 🧮 Rate-Determining Step: The slowest step in a reaction mechanism is the rate-determining step, which determines the overall rate of the reaction.
• 🌡️ Arrhenius Equation: The Arrhenius equation, $k = Ae^{-\frac{E_a}{RT}}$, relates the rate constant ($k$) to the activation energy ($E_a$), temperature ($T$), and the pre-exponential factor ($A$).
• ⚛️ Reaction Mechanism: A reaction mechanism is a series of elementary steps that describe the overall reaction.
• 📈 Overall Reaction Order: The sum of the exponents in the rate law.

Practice Quiz

1. Question 1: For the elementary step $2A + B \rightarrow C$, what is the rate law?
   1. $rate = k[A][B]$
   2. $rate = k[A]^2[B]$
   3. $rate = k[A][B]^2$
   4. $rate = k[C]$
2. Question 2: The rate law for a reaction is determined experimentally because:
   1. Rate laws can be predicted from the balanced equation.
   2. Rate laws can be predicted from the reaction mechanism.
   3. Rate laws cannot be predicted from the stoichiometry of the overall reaction.
   4. Rate laws are always first order.
3. Question 3: What is the molecularity of the elementary reaction $A + B \rightarrow C$?
   1. Unimolecular
   2. Bimolecular
   3. Termolecular
   4. Quadmolecular
4. Question 4: Which of the following factors does NOT affect the rate constant, $k$?
   1. Temperature
   2. Activation Energy
   3. Concentration of Reactants
   4. Frequency Factor
5. Question 5: If the rate-determining step of a reaction is $A \rightarrow B$, what is the rate law for the overall reaction?
   1. $rate = k[B]$
   2. $rate = k[A]^2$
   3. $rate = k[A]$
   4. $rate = k[A][B]$
6. Question 6: A proposed mechanism for a reaction is: Step 1: $A + B \rightleftharpoons C$ (fast equilibrium) Step 2: $C + A \rightarrow D$ (slow) What is the rate law predicted by this mechanism?
   1. $rate = k[A][B]$
   2. $rate = k[A]^2[B]$
   3. $rate = k[D]$
   4. $rate = k[A][C]$
7. Question 7: For the reaction $A + B \rightarrow C$, the rate law is $rate = k[A]^2$. What is the overall order of the reaction?
   1. 0
   2. 1
   3. 2
   4. 3