7.6 Catalysis

This topic explains catalysis — the process by which a catalyst speeds up a chemical reaction. It describes how catalysts work, the difference between homogeneous and heterogeneous catalysis, and provides an example involving ozone depletion.

Definition

Catalyst: A substance which accelerates a chemical reaction but remains chemically unchanged at the end of the reaction.

Catalysis: The phenomenon in which a catalyst speeds up a chemical reaction.

Main Idea

Many industrial reactions are carried out at high temperature to maximize the amount of product that can be synthesized in a given time. High temperature reactions introduce safety concerns and many chemical species are not stable at high temperature. Thus, a different method for increasing the rates of chemical reactions would be useful. Another way to increase reaction rate is to change its mechanism in a way that lowers the activation energy of the rate-determining step. This can be done by introducing a catalyst.

Core Concept

• Catalyst: Speeds up a reaction but remains chemically unchanged.
• Catalysis: The phenomenon of speeding up a reaction.
• How it works: Provides a new mechanism with lower activation energy.
• Effect on ΔH: No effect on the total thermodynamic or enthalpy of the reaction.
• Cannot bring about a reaction that is not thermodynamically favoured.

Detailed Explanation

What is a Catalyst?

• A substance which accelerates a chemical reaction but remains chemically unchanged at the end of a reaction is called a catalyst.
• The phenomenon is called catalysis.
• A catalyst provides a new mechanism for the reaction with low energy of activation.
• Thus, catalyst increases the rate of reaction by decreasing its energy of activation.
• A catalyst has no effect on the total thermodynamic or enthalpy of the reaction.
• For this reason, a catalyst cannot be used to bring about a chemical reaction which is not favoured thermodynamically.

How a Catalyst Works

• A catalyst provides an alternative pathway with lower activation energy.
• This allows more molecules to have enough energy to react.
• The catalyst is not consumed in the reaction.

Example: Ozone Depletion

• In the stratosphere, conversion of an ozone molecule by an oxygen atom into two O₂ molecules occurs.
• This reaction has higher energy of activation.

O₃ + O → 2O₂ E_a = 17.1 kJ mol^-1

• Chlorofluorocarbon compounds diffuse up into the stratosphere.
• These compounds absorb short wavelength ultraviolet light from the sun, which breaks carbon-chlorine bonds and produces chlorine atoms.
• Cl atom catalyzes the mechanism requiring less energy of activation.

O₃ + Cl → O₂ + ClO E_a = 2.1 kJ mol^-1

O + ClO → O₂ + Cl E_a = 0.4 kJ mol^-1

Net reaction: O₃ + O → 2O₂

E_a = 2.5 kJ mol^-1

- It shows that the direct reaction between O₃ and O has a substantially higher activation energy than the chlorine catalyzed reaction.

Types of Catalysis

Homogeneous Catalysis: - The catalyst and the reactants are in the same phase.

Heterogeneous Catalysis: - The catalyst and reactants are in different phases.

Enzymes: - Catalysts in living organisms.

Intermediate Level

Catalyst and Catalysis

Example: Ozone Depletion

Direct reaction: O₃ + O → 2O₂ E_a = 17.1 kJ mol^-1

Catalyzed reaction (Cl): O₃ + Cl → O₂ + ClO E_a = 2.1 kJ mol^-1 O + ClO → O₂ + Cl E_a = 0.4 kJ mol^-1

Net reaction: O₃ + O → 2O₂ E_a = 2.5 kJ mol^-1

Types of Catalysis

Key Points

• Catalyst lowers Eₐ.
• No effect on ΔH.
• Cannot make non-spontaneous reactions occur.

Important Points

• 1. Catalyst: Speeds up reaction, unchanged.
• 2. Catalysis: Phenomenon.
• 3. Catalyst lowers Eₐ.
• 4. No effect on ΔH.
• 5. Cannot make non-spontaneous reactions occur.
• 6. Example: Ozone depletion, Cl catalyst.
• 7. Types: Homogeneous, heterogeneous, enzymes.

Remember

• - Catalyst: Speeds up reaction, unchanged
• - Catalysis: Phenomenon
• - Catalyst lowers Eₐ
• - No effect on ΔH
• - Example: Ozone depletion, Cl catalyst
• - Types: Homogeneous, heterogeneous, enzymes

Common Mistakes

• 1. Thinking catalyst is consumed: It remains unchanged.
• 2. Thinking catalyst changes ΔH: It does not.
• 3. Thinking catalyst can make any reaction happen: Only thermodynamically favoured reactions.
• 4. Confusing homogeneous and heterogeneous: Same phase vs. different phases.

Quick Revision

• Catalyst: Speeds up reaction, unchanged.
• Catalysis: Phenomenon.
• Catalyst lowers Eₐ.
• No effect on ΔH.
• Cannot make non-spontaneous reactions occur.
• Example: Ozone depletion, Cl catalyst.
• Types: Homogeneous, heterogeneous, enzymes.

60-Second Revision

• Catalyst: Speeds up reaction, unchanged
• Catalysis: Phenomenon
• Catalyst lowers Eₐ
• No effect on ΔH
• Example: Ozone depletion, Cl catalyst
• Types: Homogeneous, heterogeneous, enzymes

QUALITY VALIDATION

• Topic: 7.6 Catalysis
• Chapter: 7 (Chemical Kinetics)

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