AP Chemistry Unit 5 study games — Kinetics.
This unit covers reaction rates, rate laws, activation energy and catalysts — essential concepts for AP Chemistry. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers reaction rates, rate laws, activation energy and catalysts — essential concepts for AP Chemistry. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Reaction Rates
Reaction rate measures how quickly reactants are consumed or products are formed, expressed as change in concentration over time. Students must be able to calculate average and instantaneous rates from concentration-time data and understand how stoichiometry relates the rates of different species in a reaction. Rate is always expressed as a positive value, so a negative sign is used for reactants.
Key Points
- Rate = −(1/a)(Δ[A]/Δt) = (1/b)(Δ[B]/Δt) for aA → bB; stoichiometric coefficients appear as fractions
- Average rate uses Δ[X]/Δt over an interval; instantaneous rate is the slope of the tangent to a concentration-time curve
- Rate of disappearance of reactant equals rate of appearance of product adjusted by molar ratios
- Increasing temperature, concentration, or surface area generally increases reaction rate
For 2N₂O₅ → 4NO₂ + O₂, if [N₂O₅] decreases from 0.80 M to 0.60 M over 200 s, find the rate of formation of NO₂.
First calculate the rate of disappearance of N₂O₅: −Δ[N₂O₅]/Δt = −(0.60 − 0.80)/200 = 1.0 × 10⁻³ M/s. The overall rate of reaction is (1/2)(1.0 × 10⁻³) = 5.0 × 10⁻⁴ M/s. Since NO₂ has a coefficient of 4, rate of formation of NO₂ = 4 × 5.0 × 10⁻⁴ = 2.0 × 10⁻³ M/s.
2 Rate Laws
A rate law expresses the relationship between reaction rate and reactant concentrations: rate = k[A]^m[B]^n, where m and n are reaction orders determined experimentally, not from stoichiometry. Students must be able to determine reaction orders from initial rate data, calculate the rate constant k, and use integrated rate laws to find concentration at any time or determine half-life. The units of k depend on the overall reaction order.
Key Points
- Reaction orders are determined from experiments only — never from balanced equation coefficients
- Zero order: [A] = [A]₀ − kt; First order: ln[A] = ln[A]₀ − kt; Second order: 1/[A] = 1/[A]₀ + kt
- First-order half-life: t₁/₂ = 0.693/k (independent of concentration); second-order t₁/₂ = 1/(k[A]₀)
- Doubling [A] and observing rate double → first order in A; rate quadruples → second order in A
Given: Experiment 1: [A]=0.10 M, [B]=0.10 M, rate=2.0×10⁻⁴ M/s. Experiment 2: [A]=0.20 M, [B]=0.10 M, rate=4.0×10⁻⁴ M/s. Experiment 3: [A]=0.10 M, [B]=0.20 M, rate=2.0×10⁻⁴ M/s. Determine the rate law and k.
Comparing Experiments 1 and 2: [A] doubles while rate doubles → first order in A. Comparing Experiments 1 and 3: [B] doubles while rate is unchanged → zero order in B. The rate law is rate = k[A]. Solving for k using Experiment 1: k = (2.0×10⁻⁴)/(0.10) = 2.0×10⁻³ s⁻¹.
3 Activation Energy
Activation energy (Eₐ) is the minimum energy required for reactants to convert to products, representing the energy barrier of the reaction. Students must interpret energy diagrams, apply the Arrhenius equation to relate k and temperature, and use the two-point Arrhenius equation to calculate Eₐ or k at a new temperature. A higher Eₐ means a slower reaction at a given temperature.
Key Points
- Arrhenius equation: k = Ae^(−Eₐ/RT); ln k = ln A − Eₐ/RT, where R = 8.314 J/mol·K, T in Kelvin
- Two-point form: ln(k₂/k₁) = −(Eₐ/R)(1/T₂ − 1/T₁) — used when two k-T pairs are known
- On an energy diagram: Eₐ = energy of transition state minus energy of reactants; ΔH = products minus reactants
- A plot of ln k vs. 1/T is linear with slope = −Eₐ/R (commonly tested as a graph interpretation question)
A reaction has k = 1.5×10⁻³ s⁻¹ at 300 K and k = 6.0×10⁻³ s⁻¹ at 340 K. Calculate the activation energy.
Apply the two-point Arrhenius equation: ln(6.0×10⁻³/1.5×10⁻³) = −(Eₐ/8.314)(1/340 − 1/300). The left side is ln(4) = 1.386. The right side bracket equals −3.92×10⁻⁴ K⁻¹. Solving: Eₐ = 1.386 / (3.92×10⁻⁴ / 8.314) = 1.386 × 8.314 / 3.92×10⁻⁴ ≈ 2.94×10⁴ J/mol = 29.4 kJ/mol.
4 Catalysts
A catalyst increases reaction rate by providing an alternative reaction pathway with a lower activation energy, and is not consumed in the overall reaction. Students must distinguish homogeneous from heterogeneous catalysts, explain catalyst effects on energy diagrams, and understand that a catalyst does not change ΔH, equilibrium position, or the equilibrium constant. Enzymes are biological catalysts and follow the same principles.
Key Points
- Catalyst lowers Eₐ for both forward and reverse reactions equally — ΔH and Keq are unchanged
- Homogeneous catalyst: same phase as reactants (e.g., H⁺ in aqueous solution); heterogeneous: different phase (e.g., Pt solid with gas reactants)
- On an energy diagram, a catalyst shows a lower transition state peak but identical reactant and product energy levels
- A catalyst increases k (rate constant) by decreasing Eₐ in the Arrhenius equation; it does not shift equilibrium
A student claims that adding a catalyst to a reaction at equilibrium will shift the equilibrium to favor products. Is this correct? Explain using activation energy concepts.
The student is incorrect. A catalyst lowers the activation energy of both the forward and reverse reactions by the same amount, so both rates increase equally. Because k_forward/k_reverse = Keq remains unchanged, the equilibrium position does not shift and the ratio of products to reactants at equilibrium stays the same. The catalyst only allows the system to reach equilibrium faster.
Questions, answered.
What is Kinetics?
Kinetics is Unit 5 of AP Chemistry, covering reaction rates, rate laws, activation energy and catalysts.
How to study for AP Chemistry Unit 5?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.