Factors Affecting Reaction Rates

Why This Matters

Chemical kinetics is fundamentally about understanding why reactions happen at different speeds—and that's exactly what AP Chemistry exams test. You're not just being asked to list factors that affect reaction rates; you're being tested on the underlying collision theory and energy concepts that explain how each factor works. Whether it's an MCQ asking why grinding a solid speeds up a reaction or an FRQ requiring you to analyze rate data, the examiners want to see that you understand the mechanism behind each effect.

Every factor affecting reaction rates connects back to two core principles: collision frequency (how often particles meet) and activation energy (the energy barrier particles must overcome). Some factors work by increasing how often collisions happen, others by making collisions more energetic, and catalysts take a completely different approach by lowering the energy barrier itself. Don't just memorize that "higher temperature = faster reaction"—know that temperature affects both collision frequency and collision energy, which is why its effect is so dramatic.

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Factors That Increase Collision Frequency

These factors speed up reactions by simply making reactant particles encounter each other more often. More collisions per second means more opportunities for successful reactions.

Concentration of Reactants

• Higher concentration increases collision frequency—more particles per unit volume means reactants encounter each other more often
• Reaction order determines the mathematical relationship between concentration and rate; zero-order reactions show no concentration dependence
• Rate laws quantify this effect—for a first-order reaction, doubling concentration doubles the rate; for second-order, it quadruples

Surface Area of Solid Reactants

• Grinding or powdering solids exposes more particles to the other reactant, dramatically increasing the reaction interface
• Heterogeneous reactions are surface-limited—only molecules at the solid's surface can participate in collisions
• Particle size inversely correlates with rate—smaller particles = larger total surface area = faster reaction

Pressure (for Gaseous Reactions)

• Increasing pressure compresses gas molecules closer together, effectively increasing concentration without adding more reactant
• Only affects reactions involving gases—pressure changes have negligible effects on solids and liquids
• Connects to the ideal gas law—at constant temperature, $PV = nRT$ shows that higher pressure means higher $\frac{n}{V}$ (concentration)

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Factors That Increase Collision Energy

These factors don't just create more collisions—they make each collision more likely to succeed by ensuring particles have enough energy to overcome the activation energy barrier.

Temperature

• Higher temperature increases average kinetic energy—molecules move faster and collide with greater force
• The Arrhenius equation $k = Ae^{-E_a/RT}$ shows the exponential relationship between temperature and rate constant
• Temperature affects both frequency AND energy—this dual effect explains why temperature changes have such dramatic impacts on reaction rates

Light (for Photochemical Reactions)

• Photons provide activation energy directly to reactant molecules, initiating reactions that wouldn't occur thermally
• Wavelength determines energy—shorter wavelengths (higher frequency) deliver more energy per photon according to $E = h\nu$
• Photocatalysis combines light absorption with catalytic pathways—used in applications from water splitting to self-cleaning surfaces

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Factors That Lower the Activation Energy Barrier

Rather than making collisions more frequent or energetic, these factors change the reaction pathway itself to require less energy for success.

Presence of Catalysts

• Catalysts provide an alternative reaction mechanism with lower activation energy—they don't change thermodynamics, only kinetics
• Catalysts are regenerated at the end of the reaction cycle and don't appear in the overall stoichiometry
• Selectivity is a key feature—catalysts can speed up one reaction pathway while leaving others unaffected, crucial in industrial chemistry

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Factors Related to Reactant Properties

Some rate effects come from the inherent chemical nature of the substances involved, not external conditions you can easily manipulate.

Nature of Reactants

• Bond strength and molecular structure determine intrinsic reactivity—weak bonds break more easily than strong ones
• Ionic reactions are typically fast because they involve electrostatic attractions rather than bond breaking; covalent bond rearrangements are slower
• Phase matters—reactions between gases or in solution are generally faster than those involving solids due to molecular mobility

Solvent Effects

• Solvent polarity affects reaction pathways—polar solvents stabilize charged intermediates and transition states through solvation
• Viscosity impacts diffusion rates—in highly viscous solvents, reactants encounter each other less frequently
• Solvent can participate in the mechanism—protic solvents can donate protons, while aprotic solvents cannot, affecting reaction outcomes

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Quick Reference Table

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Self-Check Questions

1. Which two factors both increase collision frequency but apply to different phases of matter? How does the underlying mechanism differ?

2. A student claims that adding a catalyst and increasing temperature both "give molecules more energy." Explain why this statement is incorrect for catalysts.

3. Compare and contrast how temperature and concentration affect reaction rate. Which factor has a more dramatic effect when doubled, and why?

4. An FRQ shows a reaction between a solid metal and an aqueous acid. Which two factors from this guide would most directly increase the reaction rate, and what would you do experimentally to apply each?

5. Why do ionic reactions in aqueous solution typically proceed faster than reactions requiring covalent bond rearrangement? Connect your answer to activation energy concepts.