Activation Energy, Collision Theory and Arrhenius Equation — Part 1: Energy barriers and collisions: visual lesson, ISC Class 12 Chemistry

Lesson

Why reactions need a push

Part 1 — the energy barrier that molecules must cross, and why most collisions fail.

In this lesson you will

Explain energy barrier, threshold energy, activation energy and the activated complex
Draw and label energy profiles for exothermic and endothermic reactions, with and without a catalyst
Explain why only a fraction of collisions lead to reaction (energy and orientation)

Table of everyday observations explained by activation energy
Observation	Explanation (this chapter)
A matchstick needs striking before it burns	Energy is needed to start the reaction (activation energy)
Milk sours faster in summer	More molecules have enough energy at higher temperature
Enzymes work at body temperature	They lower the activation energy

Concept

Threshold energy and activation energy

Threshold energy: the minimum total energy that colliding molecules must have for the collision to produce a reaction. Activation energy (Ea): the extra energy that must be supplied to reactant molecules, beyond their average energy, to reach the threshold energy: Ea = threshold energy − average energy of reactants. Activated complex: the unstable, high-energy arrangement at the top of the barrier, in which old bonds are partly broken and new ones partly formed.

E_\text{a} = E_\text{threshold} - E_\text{reactants}

Table of terms used for the energy barrier
Term	Meaning
Energy barrier	The hump between reactants and products on an energy profile
Threshold energy	Energy level at the top of the barrier
Activation energy	Height of the barrier above the reactants
Activated complex	Short-lived species at the top; bonds partly broken and formed

Diagram

Energy profile of an exothermic reaction

Schematic, not to scale.

Products lie below the reactants (ΔH < 0). The reactants must still climb over the barrier. A catalyst (dashed path) provides a route with a lower activation energy; it does not change the energies of reactants and products, so ΔH is unchanged.

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