AP Chemistry Unit 5 Cheat Sheet: Kinetics
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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:
Compare two trials where only one reactant changes.
Observe how the rate changes.
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:
Particles must collide
Collision energy must be at least the activation energy
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.
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