Activation Energy
Definition of Activation Energy
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.
Think of activation energy as the “entry fee” that particles must pay to initiate a chemical reaction.
The Activated Complex (Transition State)
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
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 ≥ $E_a$.
- Consider a reaction with an activation energy of $50 \text{ kJ mol}^{-1}$.
- 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 $≥ 50 \text{ kJ mol}^{-1}$, leading to a faster reaction rate.
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 $E_a$.
