AP Chemistry Unit 5: Kinetics

TLDR

• Kinetics explains how fast reactions occur, not whether they occur.

• Learn how to write and interpret rate laws using experimental data.

• Master integrated rate laws, half-life, and the Arrhenius equation.

• Understand activation energy, energy diagrams, and reaction mechanisms.

• Download the Unit 5 cheat sheet for quick practice and review.

Why This Unit Matters

Unit 5 is one of the most heavily tested reasoning units on the AP Chemistry exam. You are not just asked to plug numbers into equations. You are asked to interpret data, justify trends, and connect graphs to mechanisms.

Strong kinetics skills help later with thermodynamics, equilibrium, and electrochemistry. If you can look at a table of data and immediately infer a rate law or mechanism, you are operating at a high-scoring level.

1. What Reaction Rate Really Means

Reaction rate measures how quickly reactants are consumed or products are formed.

• Rate = change in concentration ÷ time

• Units are typically M/s

• Rates are always written as positive values

Reaction rate depends on:

• Reactant concentration

• Temperature

• Surface area

• Presence of a catalyst

• Nature of reactants

2. Rate Laws

A rate law shows how rate depends on reactant concentrations.

General form:
rate = k[A]ᵐ[B]ⁿ

• m and n are reaction orders

• Overall order = m + n

• Orders are determined experimentally, never from coefficients

Key interpretations:

• Zero order: rate independent of concentration

• First order: rate proportional to concentration

• Second order: rate proportional to concentration squared

Exam insight:
Doubling a reactant increases rate by 2ᵐ, not always by 2.

3. Determining Rate Laws from Data

On the AP exam, rate laws are found using the method of initial rates.

Steps:

1. Compare two trials where only one reactant changes.

2. Observe how the rate changes.

3. Solve for the reaction order.

Example logic:

• If doubling [A] quadruples rate → second order in A.

• If doubling [A] has no effect → zero order in A.

4. Integrated Rate Laws

Integrated rate laws allow you to calculate concentration after time has passed.

Zero order:
[A] = −kt + [A]₀
Straight line when [A] is plotted vs time.

First order:
ln[A] = −kt + ln[A]₀
Straight line when ln[A] is plotted vs time.

Second order:
1/[A] = kt + 1/[A]₀
Straight line when 1/[A] is plotted vs time.

Exam shortcut:
If you see ln[A] on a graph, it is first order.

5. Half-Life

Half-life is the time required for concentration to decrease by half.

Only first-order reactions have a constant half-life.

Formula:
t₁/₂ = 0.693 / k

Zero- and second-order half-lives depend on initial concentration and change over time.

6. Rate Constant (k)

The rate constant reflects reaction speed.

• Larger k → faster reaction

• Units depend on reaction order

  • Zero order: M/s

  • First order: s⁻¹

  • Second order: M⁻¹·s⁻¹

k increases as temperature increases.

7. Collision Theory

For a reaction to occur:

1. Particles must collide

2. Collision energy must be at least the activation energy

3. Particles must have proper orientation

Not all collisions lead to reactions.

8. Activation Energy and the Arrhenius Equation

Activation energy is the energy barrier that must be overcome for a reaction to proceed.

Arrhenius equation:
k = A · e^(−Ea / RT)

Linear form:
ln k = −(Ea / R)(1/T) + ln A

• Ea is found from the slope of ln k vs 1/T

• R = 8.314 J·mol⁻¹·K⁻¹

9. Energy Diagrams

Energy diagrams visualize reaction progress.

• Peak height = activation energy

• Difference between reactants and products = ΔH

• Exothermic reactions end lower than they start

• Endothermic reactions end higher

Catalysts:

• Lower activation energy

• Do not change ΔH or ΔG

• Affect forward and reverse reactions equally

10. Reaction Mechanisms

A mechanism is a series of elementary steps.

• The slow step controls the overall rate

• The rate law comes from the slow step only

• Intermediates appear in the mechanism but not the overall equation

Exam rule:
If a species appears in the rate law, it must be involved in the rate-determining step.

Common Pitfalls

• Using coefficients as reaction orders

• Forgetting that rate laws are experimental

• Applying the wrong integrated rate law

• Saying catalysts change ΔH or equilibrium

• Confusing activation energy with ΔH

Tutor Tip

On AP free-response questions, always justify kinetics answers using data-driven language.

Example:
“Doubling the concentration of A increases the rate by a factor of four, indicating second-order dependence.”

This reasoning-based explanation earns more points than simply stating the answer.

Ready to Master Kinetics?

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