AP Chem - Unit 5 (Rate laws & Mechanisms)

Rate laws, mechanisms

If the rate doubles when [A] doubles and [B] stays constant, the order for [A] is?

1

If the rate quadruples when [A] doubles and [B] stays constant, the order for [A] is?

2

If the rate stays constant when [A] doubles and [B] stays constant, the order for [A] is?

0

Rate Law—equation that relates ____ (__/___), rate constant (k) and _____ (M) of reactants.

rate; (M/time); concentration

Integrated Rate Law—equation that relates _____ and ______ (M).

time; concentration

Focus on the ________ _____ of the rate laws not how they are determined

integrated forms

For integrated rate laws, you rearrange the change in concentration (the y-axis) until the plot of the graph is _____

linear

For first order reactions, the graph’s x-axis will be time, and the y-axis will be ____

ln [A]

For second order reactions, the graph’s x-axis will be time, and the y-axis will be ___

1/[A]

For zero order reactions, the graph’s x-axis will be time, and the y-axis will be [A]. What does that mean for the rate?

The rate is independent of the concentration of the reactant, so rate = k

What are reaction mechanisms?

Describes the steps by which reactions occur

What is an elementary reaction?

Reaction that occurs by a single step

What is the molecularity of an elementary reaction?

how many molecules are involved in that step of the mechanism

What are the different molecularities of elementary reactions?

• Unimolecular: involves a single molecule

• Bimolecular: two molecules collide

• Termolecular: three molecules collide

Termolecular steps require three molecules to simultaneously collide with the proper orientation and the proper energy, so they are…

slower than unimolecular or bimolecular steps

The overall reaction cannot occur faster than the _____ reaction in the mechanism.

slowest

What is the rate determining step?

The slowest reaction in the mechanism (slow step)

What are the requirements of a plausible mechanism?

• The rate law must be derived from the rate-determining step.

• The rate determining step is the slow step.

• Each step must balance, like any equation.

• All intermediates are made and used up.

• The stoichiometry must be obtained when all steps are added up.

• Any catalyst is used and regenerated.

An intermediate (is or is not?) a reactant or a product.

is not

Intermediates are stable, ____ ___ the transition state, and occur along the __________ ______

are not; reaction pathway

A _____ is a substance that changes the speed of a chemical reaction without undergoing a permanent chemical change itself.

catalyst

Are catalysts included in the rate law?

No, they are not explicitly included in the rate law because the rate law typically focuses on the fundamental relationship between reactant concentrations and reaction rate, and catalysts don’t appear in the balanced chemical equation.