Catalysts Homogeneous, Heterogeneous, Enzymatic Cheat Sheet

A printable reference covering catalyst types, activation energy, reaction pathways, enzymes, and homogeneous versus heterogeneous catalysis for grades 10-12.

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This cheat sheet covers homogeneous, heterogeneous, and enzymatic catalysts in chemistry. Students need it to compare catalyst types, understand reaction energy diagrams, and explain how catalysts speed reactions without being consumed. It is useful for kinetics, equilibrium, biochemistry, and industrial chemistry review. The focus is on clear definitions, common examples, and the energy changes that matter most.

Key Facts

A catalyst increases reaction rate by providing an alternate pathway with lower activation energy, so $E_{a,\text{cat}} < E_{a,\text{uncat}}$ .
A catalyst is not consumed overall, so it appears in a mechanism but cancels from the net chemical equation.
Catalysts lower activation energy but do not change $\Delta H$ , the products, the reactants, or the overall energy difference between them.
A catalyst increases the rate constant because the Arrhenius equation is $k = Ae^{-E_a/(RT)}$ , and a smaller $E_a$ gives a larger $k$ .
A homogeneous catalyst is in the same phase as the reactants, such as an aqueous acid catalyst reacting with aqueous molecules.
A heterogeneous catalyst is in a different phase from the reactants, often a solid surface where gas or liquid reactants adsorb, react, and desorb.
An enzyme is a biological catalyst with an active site that binds a substrate and often follows the model $E + S \rightleftharpoons ES \rightarrow E + P$ .
Catalysts speed both the forward and reverse reactions, so they help a system reach equilibrium faster but do not change $K_{eq}$ .

Vocabulary

Catalyst: A substance that increases reaction rate by lowering activation energy and is regenerated by the end of the reaction.
Activation Energy: The minimum energy needed for reacting particles to reach the transition state, usually written as $E_a$ .
Homogeneous Catalyst: A catalyst that is in the same phase as the reactants, such as a dissolved catalyst in a solution reaction.
Heterogeneous Catalyst: A catalyst that is in a different phase from the reactants, commonly a solid surface used with gases or liquids.
Enzyme: A biological catalyst, usually a protein, that speeds a specific biochemical reaction by binding substrates at an active site.
Active Site: The region of an enzyme where the substrate binds and the catalyzed reaction occurs.

Common Mistakes to Avoid

Saying a catalyst changes the amount of product at equilibrium is wrong because a catalyst changes rate, not $K_{eq}$ or equilibrium position.
Forgetting that a catalyst is regenerated is wrong because a true catalyst may be used in a step but must cancel from the overall reaction.
Thinking catalysts make impossible reactions possible is wrong because catalysts lower $E_a$ but do not make a reaction thermodynamically favorable if $\Delta G$ is not favorable.
Mixing up homogeneous and heterogeneous catalysts is wrong because the classification depends on phase, not whether the catalyst is natural or synthetic.
Assuming enzymes work best at any temperature is wrong because enzyme shape depends on conditions, and denaturation can reduce or stop activity.

Practice Questions

1 A reaction has $E_{a,\text{uncat}} = 80\ \text{kJ/mol}$ and $E_{a,\text{cat}} = 45\ \text{kJ/mol}$ . By how many $\text{kJ/mol}$ did the catalyst lower the activation energy?
2 For the mechanism $A + C \rightarrow AC$ and $AC + B \rightarrow AB + C$ , identify the catalyst and write the net reaction.
3 Classify each catalyst as homogeneous, heterogeneous, or enzymatic: solid $\text{Pt}$ in a catalytic converter, dissolved $H^+$ in ester hydrolysis, and catalase breaking down $H_2O_2$ .
4 Explain why adding a catalyst to a closed reaction mixture helps equilibrium form faster but does not increase the final equilibrium yield.

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