R2.2.5 Catalysts

Catalysts and Energy Profiles

1. You're climbing a steep hill to reach your school.
2. Without assistance, the climb is exhausting and requires significant effort.
3. Now picture a shortcut—stairs that make the ascent easier and faster.

Catalysts in chemical reactions work in a similar way: they provide an alternative, more efficient pathway for the reaction to occur, reducing the "energy climb" required, all while remaining unchanged themselves.

How Catalysts Work: An Alternative Reaction Pathway with a Lower Activation Energy

To understand catalysts, let's revisit the concept of activation energy ($E_a$).

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy.

This means more particles now have enough energy to overcome the activation barrier, leading to a higher frequency of successful collisions.

Key Characteristics of Catalysts:

1. Not consumed: Catalysts participate in the reaction but are regenerated at the end, so they are not used up.
2. Do not alter equilibrium: A catalyst accelerates both the forward and reverse reactions equally, leaving the equilibrium position and the overall enthalpy change ($\mathrm{\Delta }H$) unchanged.
3. Specificity: Some catalysts are highly selective, working only for specific reactions (e.g., enzymes in biological systems).

Energy Profiles: Catalyzed vs. Uncatalyzed Reactions

• Energy profiles are graphical tools that help us visualize the energy changes during a chemical reaction.
• They provide a clear picture of how catalysts reduce activation energy.

Components of an Energy Profile:

1. Reactants: The starting substances, represented on the left side of the graph.
2. Products: The substances formed, shown on the right side.
3. Activation Energy ($E_a$): The energy barrier that reactants must overcome to reach the transition state.
4. Transition State: The peak of the energy barrier, representing the unstable arrangement of atoms during the reaction.
5. Enthalpy Change ($\mathrm{\Delta }H$): The difference in energy between reactants and products.

Comparing Catalyzed and Uncatalyzed Profiles:

1. In an uncatalyzed reaction, the activation energy is higher, meaning fewer particles have sufficient energy to react.
2. A catalyst lowers this barrier, enabling more particles to react and speeding up the process.

Types of Catalysts: Homogeneous and Heterogeneous

Catalysts are classified based on their phase (solid, liquid, gas) relative to the reactants.

Homogeneous Catalysts:

• Same phase as the reactants (e.g., both are in the liquid phase).

Heterogeneous Catalysts:

• Different phase from the reactants, typically solid catalysts interacting with gaseous or liquid reactants.

Biological Catalysts: Enzymes

• Enzymes are specialized proteins that function as biological catalysts, enabling essential biochemical reactions to occur efficiently at body temperature.
• Without enzymes, many reactions necessary for life would proceed too slowly to sustain life.

Unique Features of Enzymes:

1. Highly specific: Each enzyme typically catalyzes only one reaction or a group of closely related reactions.
2. Operate under mild conditions: Enzymes work efficiently at physiological temperatures and pH levels.
3. Efficiency: Enzymes can increase reaction rates by factors of ${10}^6$ or more.
4. Regulation: Enzyme activity can be controlled by inhibitors or activators, allowing the body to fine-tune reaction rates.

Visualizing Catalysts with Maxwell-Boltzmann Distributions

The Maxwell-Boltzmann energy distribution curve shows the spread of kinetic energies among particles in a reaction mixture.

Effect of a Catalyst:

• Without a catalyst, only particles with energy greater than or equal to the activation energy ($E_a$) can react.
• Adding a catalyst lowers $E_a$, increasing the proportion of particles with sufficient energy to react.

Reflection and Review