Activation Energy

Definition of Activation Energy

Definition

Activation energy

Activation energy ( $E_{a}$ ) is the minimum energy required for colliding particles to form an activated complex (also known as the transition state) and proceed to products.

When particles collide, they must possess sufficient energy to overcome the energy barrier that separates reactants from products.

This energy barrier corresponds to the activation energy.

If the colliding particles do not have energy equal to or greater than $E_{a}$ , the reaction will not proceed.

Tip

Think of activation energy as the “entry fee” that particles must pay to initiate a chemical reaction.

The Activated Complex (Transition State)

Definition

Activated complex

The activated complex is a high-energy, unstable arrangement of atoms that exists momentarily as reactants are transformed into products. It represents the "peak"; of the energy barrier in a reaction.

Graphical Representation: Maxwell–Boltzmann Energy Distribution

Definition

Maxwell–Boltzmann energy distribution curve

The Maxwell–Boltzmann energy distribution curve provides a visual representation of the distribution of kinetic energies among particles in a system.

It is particularly useful for understanding how temperature affects the proportion of particles with energy greater than or equal to the activation energy ( $E_{a}$ ).

Key Features of the Maxwell–Boltzmann Curve

X-axis: Represents the kinetic energy of particles.
Y-axis: Represents the number of particles with a given energy.
Shape: The curve is asymmetrical, with a peak corresponding to the most probable energy (the energy possessed by the largest number of particles).
Area under the curve: Represents the total number of particles in the system.

Effect of Temperature on the Curve

At lower temperatures: The curve is taller and narrower, with fewer particles having energy ≥ $E_{a}$ .
At higher temperatures: The curve flattens and spreads out, increasing the proportion of particles with energy ≥ $E_{a}$ .

Example

Imagine a reaction with an activation energy of 50 kJ/mol. At 300 K, only a small fraction of particles have enough energy to overcome this barrier. However, if the temperature is increased to 400 K, a significantly larger fraction of particles will have energy ≥ 50 kJ/mol, leading to a faster reaction rate.

Maxwell–Boltzmann energy distribution curve for lower (T1) and higher (T2) temperatures

Tip

Increasing temperature does not change the activation energy itself; it only increases the number of particles with sufficient energy to overcome it.

Common Mistake

Do not confuse the peak of the Maxwell–Boltzmann curve with the activation energy. The peak represents the most probable energy, not $E_{a}$ .

Energy Profiles for Chemical Reactions

Energy profiles illustrate the energy changes that occur during a chemical reaction.
They show the energy difference between reactants, products, and the activation energy barrier.

Components of an Energy Profile

Reactants: The starting substances of the reaction.
Products: The substances formed after the reaction.
Activation Energy ( $E_{a}$ ): The energy required to reach the activated complex.
Enthalpy Change ( $Δ H$ ): The difference in energy between reactants and products.

Endothermic vs. Exothermic Reactions

1. Exothermic Reactions

Definition: Reactions that release energy to the surroundings.
Energy Profile:
- The energy of the products is lower than that of the reactants.
- $Δ H$ is negative.

2.Endothermic Reactions

Definition: Reactions that absorb energy from the surroundings.
Energy Profile:
- The energy of the products is higher than that of the reactants.
- $Δ H$ is positive.

Energy profiles for exothermic and endothermic reactions, taking into account activation energy.

Analogy

Think of the activation energy as a hill that reactants must climb to reach the products. In an exothermic reaction, the hill leads to a lower valley (products), while in an endothermic reaction, the hill leads to a higher plateau (products).

Common Mistake

Students often confuse the activation energy ( $E_{a}$ ) with the enthalpy change ( $Δ H$ ). Remember, $E_{a}$ is the energy barrier, while $Δ H$ is the net energy change of the reaction.

Reflection

Self review

What is the role of activation energy in a chemical reaction?
How does increasing temperature affect the Maxwell–Boltzmann energy distribution curve?
Compare the energy profiles of exothermic and endothermic reactions.

Theory of Knowledge

How does the concept of activation energy relate to the broader idea of energy barriers in real-world systems, such as overcoming personal or societal challenges?

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R2.2.4 Activation energy

Activation Energy

Definition of Activation Energy

The Activated Complex (Transition State)

Graphical Representation: Maxwell–Boltzmann Energy Distribution

Key Features of the Maxwell–Boltzmann Curve

Effect of Temperature on the Curve

Energy Profiles for Chemical Reactions

Components of an Energy Profile

Endothermic vs. Exothermic Reactions

1. Exothermic Reactions

2.Endothermic Reactions

Reflection

You've read 2/2 free chapters this week.

Introduction to Activation Energy