Catalysts - Chemistry

1. Fundamental Concepts

A catalyst is a substance that increases the rate of a chemical reaction without being consumed.
Catalysts work by providing an alternative reaction pathway with lower activation energy.
Catalysts do NOT change thermodynamic quantities:
- ΔH (enthalpy)
- ΔG (Gibbs free energy)
- Equilibrium constant $K$
A catalyst speeds up both the forward and reverse reactions, allowing equilibrium to be reached faster but not changing the equilibrium position.

2. Key Concepts

Lower Activation Energy
- More particles have sufficient energy to undergo effective collisions at the same temperature.
Reaction Mechanism
- A catalyst appears in one step of the mechanism and is regenerated in a later step.
Types of Catalysts
- Homogeneous catalyst: same phase as reactants (e.g., all aqueous or all gas).
- Heterogeneous catalyst: different phase (e.g., solid catalyst with gas reactants).
Enzymes
- Biological catalysts
- Highly specific to their substrates
- Activity depends on temperature and pH

3. Examples

Easy (Conceptual)

Question
Which statement about a catalyst is TRUE?

A. It increases the enthalpy change (ΔH)
B. It is consumed during the reaction
C. It lowers the activation energy
D. It shifts equilibrium toward products

Answer: C

Explanation:
A catalyst lowers activation energy but does not affect ΔH or equilibrium.

Medium (Reaction Mechanism)

Question
Given the mechanism:

$NO + O_3 \rightarrow NO_2 + O_2$
$NO_2 + O \rightarrow NO + O_2$

Which species acts as a catalyst?

Answer: NO

Explanation:
NO is consumed in Step 1 and regenerated in Step 2, so it is a catalyst.

Hard (Equilibrium & Energy Diagram)

Question
A catalyst is added to a reaction that is already at equilibrium. What happens?

A. The equilibrium constant increases
B. The reaction becomes exothermic
C. Both forward and reverse reaction rates increase
D. More products are formed

Answer: C

Explanation:
A catalyst increases the rates of both directions equally, without changing equilibrium.

4. Problem-Solving Techniques

Energy Diagram Questions
- Catalyzed pathway → lower peak (lower activation energy)
- Reactant and product energy levels remain unchanged (ΔH unchanged)
Identifying a Catalyst in a Mechanism
1. Appears in an early step
2. Reappears (is regenerated) in a later step
3. Does not appear in the overall balanced equation
Equilibrium Questions
- Presence of a catalyst → no change in $K$ , ΔH, or equilibrium position
Enzyme Questions
- Look for specificity
- Optimal temperature and pH
- Denaturation reduces catalytic activity