AP Chem reaction mechanisms and rate-determining step practice questions

Define a reaction mechanism.

A reaction mechanism is a series of steps that describes the process by which reactants are converted into products.

List the common steps in reaction mechanisms.

- Initiation - Propagation - Termination

True or False: The rate-determining step is always the first step.

False. The rate-determining step is the slowest step, not necessarily the first.

Fill in the blank: The __________ step is the slowest step in a reaction mechanism.

rate-determining

What role do intermediates play in a reaction mechanism?

Intermediates are species formed in one step and consumed in a subsequent step.

Compare elementary and overall reactions.

Elementary reactions occur in a single step; overall reactions are the sum of elementary reactions.

How do catalysts affect reaction mechanisms?

Catalysts provide an alternative pathway with a lower activation energy, speeding up the reaction.

Identify the type of elementary step: A + B → C.

This is a bimolecular elementary step, involving two reactant molecules.

List two factors that influence the rate of a reaction.

- Concentration - Temperature

What does a potential energy diagram illustrate?

It illustrates the energy changes during a reaction, including activation energy and stability of intermediates.

Describe the transition state.

The transition state is a high-energy, transient configuration during a reaction, unable to be isolated.

How can you identify the rate-determining step?

Identify the slowest elementary step based on reaction rates or mechanism details.

What is an elementary reaction?

An elementary reaction is a single-step process with a specific molecularity.

True or False: Intermediates appear in the overall balanced equation.

False. Intermediates are not present in the final balanced equation.

Explain why reactions may have multiple mechanisms.

Different conditions (temperature, pressure) can lead to different pathways and mechanisms.

What is molecularity?

Molecularity is the number of reactant molecules involved in an elementary step.

Example: Write the rate law for A + 2B → products.

Rate = k[A]^1[B]^2, where k is the rate constant.

Define activation energy.

Activation energy is the minimum energy required for reactants to form products.

List features of a catalyst.

- Lowers activation energy - Not consumed in reaction - Increases reaction rate

How does temperature affect reaction mechanisms?

Increasing temperature generally increases reaction rates by providing more energy to overcome activation barriers.

Rate-determining step definition?

The slowest step in a reaction mechanism that determines the overall reaction rate.

Identify the rate-determining step in the mechanism: A + B → C (slow), C → D (fast).

A + B → C is the rate-determining step.

True or False: A fast step can be rate-determining.

False. The rate-determining step is always the slowest step.

Effect of increasing concentration of reactants on rate-determining step?

Increases reaction rate if the reactants are in the rate-determining step.

If a catalyst speeds up the rate-determining step, what happens?

The overall reaction rate increases, lowering activation energy.

Fill in the blank: The rate law is derived from the __________.

rate-determining step of the mechanism.

Compare the roles of intermediates and transition states.

Intermediates are stable, formed during a reaction; transition states are unstable, at the maximum energy point.

What is the significance of the rate constant in kinetics?

It relates the rate of reaction to the concentration of reactants in the rate-determining step.

What affects the rate of the rate-determining step?

Temperature, concentration of reactants, and presence of catalysts.

How to identify the slowest step in a given mechanism?

Compare the activation energies of each step; the highest energy barrier is the slowest.

Rate law for a two-step mechanism: A + B → C (slow), C → D (fast)?

Rate = k[A][B]. Only the slow step affects the rate law.

The role of activation energy in the rate-determining step?

Higher activation energy leads to a slower rate; it defines the energy barrier.

True or False: Rate-determining steps always involve two reactants.

False. A rate-determining step can involve one or more reactants.

Effect of a temperature increase on the rate-determining step?

Increases kinetic energy, leading to a higher reaction rate.

What happens if the rate-determining step has a high activation energy?

The reaction rate will be slow and less favorable.

Identify a common example of a rate-determining step.

The formation of an activated complex in enzyme-catalyzed reactions.

Cause → Effect: High concentration of a reactant?

Increased rate of the rate-determining step, leading to a faster overall reaction.

What does a mechanism with multiple steps suggest?

There is likely a rate-determining step among the various steps.

How does a catalyst affect the activation energy of the rate-determining step?

It lowers the activation energy, increasing the reaction rate.

Example of a reaction with a complex rate law?

A + 2B → Products, where rate = k[A]^1[B]^2, indicating a rate-determining step with two B's.