Catalysts are substances that speed up chemical reactions without being used up in the overall reaction. They matter because many useful reactions would be too slow at normal temperatures and pressures without them. Catalysts are essential in industry, biology, pollution control, and energy technology.
A catalyst works by giving reacting particles a more efficient way to turn into products.
The key idea is that a catalyst provides an alternative reaction pathway with a lower activation energy. On an energy profile diagram, the catalyzed pathway has a smaller energy peak, but the reactants and products start and end at the same energy levels as before. This means a catalyst does not change the overall energy difference, ΔH, or the equilibrium position of a reversible reaction.
Catalysts can be homogeneous, in the same phase as the reactants, heterogeneous, in a different phase, or biological catalysts called enzymes.
Key Facts
- A catalyst speeds up a reaction without being consumed in the overall chemical change.
- Catalysts lower activation energy: Ea,catalyzed < Ea,uncatalyzed.
- A catalyst provides an alternative reaction pathway, not extra energy for the reaction.
- Catalysts do not change ΔH: ΔH = Hproducts - Hreactants.
- Catalysts do not change the equilibrium constant K, but they help equilibrium be reached faster.
- Arrhenius equation: k = Ae^(-Ea/RT), so a lower Ea usually gives a larger rate constant k.
Vocabulary
- Catalyst
- A substance that increases the rate of a chemical reaction without being permanently consumed.
- Activation energy
- The minimum energy particles need to reach the transition state and react.
- Reaction pathway
- The sequence of steps and energy changes that connects reactants to products.
- Heterogeneous catalyst
- A catalyst that is in a different physical phase from the reactants, often a solid surface reacting with gases or liquids.
- Enzyme
- A biological catalyst, usually a protein, that speeds up specific reactions in living organisms.
Common Mistakes to Avoid
- Saying a catalyst is used up is wrong because a catalyst is regenerated by the end of the overall reaction, even if it temporarily forms intermediates.
- Drawing the catalyzed products at a different energy is wrong because a catalyst lowers the activation energy but does not change ΔH.
- Thinking catalysts make more product at equilibrium is wrong because catalysts do not change the equilibrium constant, only the speed of reaching equilibrium.
- Confusing homogeneous and heterogeneous catalysis is wrong because homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are in a different phase.
Practice Questions
- 1 An uncatalyzed reaction has an activation energy of 80 kJ/mol. A catalyst lowers the activation energy to 45 kJ/mol. By how many kJ/mol is the activation energy reduced?
- 2 For a reaction, Hreactants = 120 kJ and Hproducts = 70 kJ. Calculate ΔH. If a catalyst is added, what is the new ΔH?
- 3 A powdered solid catalyst makes a gas reaction faster than a single solid lump of the same catalyst. Explain why the powder is more effective using collision and surface area ideas.