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1 Atomic Structure and Properties 2 Compound Structure and Properties 3 Properties of Substances and Mixtures 4 Chemical Reactions 5 Kinetics 6 Thermochemistry 7 Equilibrium 8 Acids and Bases 9 Thermodynamics and Electrochemistry

5 Kinetics

5.1 Reaction Rates 5.2 Introduction to Rate Law 5.3 Concentration Changes Over Time 5.4 Elementary Reactions 5.5 Collision Model 5.6 Reaction Energy Profile 5.7 Introduction to Reaction Mechanisms 5.8 Reaction Mechanism and Rate Law 5.9 Pre-Equilibrium Approximation 5.10 Multistep Reaction Energy Profile 5.11 Catalysis

Catalysis

Learning Objective 5.11.A Explain the relationship between the effect of a catalyst on a reaction and changes in the reaction mechanism.

Quick Notes

  • A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy.
  • Catalysts are not consumed overall in the reaction. They appear in early steps and are regenerated later.
  • Catalysts can work by:
    • Improving orientation of reactants for a successful collision.
    • Lowering the energy barrier (activation energy, Ea).
    • Introducing new intermediates and new steps in the mechanism.
  • Catalysis types:
    • Homogeneous (same phase as reactants)
    • Heterogeneous (different phase, e.g., surface catalysis)
    • Enzyme or biological catalysis (specific binding to substrates)

Full Notes

What Is a Catalyst?

A catalyst is a substance that speeds up a chemical reaction without being permanently changed or consumed. It achieves this by creating a new reaction pathway that has a lower activation energy than the uncatalyzed reaction.

This increases the number of effective collisions between reacting particles and makes the reaction proceed faster.

How Catalysts Work Mechanistically

A catalyst introduces one or more new elementary steps in the reaction mechanism. These steps often involve a temporary intermediate that includes the catalyst.

Key features:

Reactants bind to the catalyst or interact with it in a way that lowers Ea.
The catalyst may help orient the reactants properly or stabilize a high-energy transition state.

AP Chemistry exam tip avatar for Matt
Matt’s exam tip

Look for the catalyst as a reactant in one step and a product in a later step. It should cancel out in the overall reaction.

Enzyme Catalysis (Biological)

Enzymes are biological catalysts, made up of proteins that have active sites with complimentary shapes that a substrate can bind to.

AP Chemistry enzyme catalysis diagram showing stereospecific active site and substrate binding

The binding reduces activation energy for the overall reaction, allowing it to occur at lower temperatures.

Acid-Base Catalysis

A reactant gains or loses a proton (H+), forming a new intermediate.
This allows the reaction to proceed through new elementary steps.

ExampleAcid-Catalyzed Decomposition of Hydrogen Peroxide

Reaction:
2 H2O2 (aq) → 2 H2O (l) + O2 (g)
Catalyst: H+ (acid)

Mechanism (Simplified):
Protonation step: H2O2 + H+ → H3O2+
Decomposition step: The unstable H3O2+ intermediate breaks down into water and an oxygen molecule: H3O2+ → H2O + O2 + H+
Catalyst regeneration: The H+ ion is regenerated and can catalyze more decomposition.

Surface Catalysis (Heterogeneous)

Occurs on a solid surface (often a metal).

Reactants adsorb to the surface, where bond-breaking or bond-making occurs.

A new set of intermediates is formed on the surface of the catalyst before deposition occurs and the products diffuse away from the surface.

AP Chemistry heterogeneous catalysis schematic showing gas reactants interacting with solid catalyst surface and active sites

Example Vanadium(V) oxide (V2O5) in the Contact Process:

Reaction: SO2 + ½O2 → SO3 (used to make H2SO4).

Role of Catalyst:

AP Chemistry contact process catalytic cycle using V2O5 and V2O4 for SO2 to SO3 conversion

Effect on Energy Profile

The energy profile for a catalyzed reaction will have an overall lower energy barrier (smaller activation energy). However it will show the same energy difference (ΔE) between reactants and products – catalysts do not change thermodynamics of a reaction, only the kinetics (how quickly the reaction happens).

AP Chemistry energy profile comparing uncatalyzed vs catalyzed pathway with lower activation energy (endothermic example)

Summary

Catalysts increase the rate of chemical reactions by offering an alternate mechanism with lower activation energy. They are not consumed overall and often operate by creating favorable orientations or intermediates. Types of catalysis include enzyme action, acid-base catalysis, and surface interactions. Understanding how a catalyst alters the reaction mechanism is key to interpreting kinetic behavior and energy profiles.