The minimum amount of energy that particles must have to react is called…

The idea

Particles collide constantly, but most collisions do nothing because the particles simply bounce apart. For a reaction to happen, the colliding particles must have at least a certain amount of kinetic energy so that existing bonds can begin to break and new arrangements can form. This threshold is the activation energy. Picture two molecules approaching: if their combined energy is below the threshold, they rebound unchanged; if it equals or exceeds the threshold, they can react. Activation energy is measured in joules, commonly kJ/mol, and is shown on a reaction profile as the vertical gap from the reactant energy level up to the peak of the curve. A high activation energy means few collisions succeed, so the reaction is slow at a given temperature; adding a catalyst provides an alternative pathway with a lower activation energy.

What earns the marks

• Activation energy is defined as the minimum energy that colliding particles must possess for a reaction to occur.
• On a reaction profile it is represented by the vertical distance from the reactants' energy level to the highest point of the curve.
• Collisions with energy below the activation energy do not lead to reaction; the particles separate unchanged.
• Activation energy is usually expressed in kJ/mol and is not the same as the overall energy change of the reaction.
• A catalyst lowers the activation energy by providing an alternative reaction pathway.

Common mistakes and how to put them right

• Confusing activation energy with the overall energy change.

  Put it right: The overall change is the difference between reactant and product energy levels, whereas activation energy is measured from the reactants up to the peak.

• Thinking that every collision causes a reaction.

  Put it right: Only collisions with energy at or above the activation energy are successful.

• Believing a catalyst increases the energy of the particles.

  Put it right: A catalyst lowers the activation energy instead, leaving particle energies unchanged.

Exam tips

• When labelling a profile, draw the activation energy arrow from the reactant level to the top of the curve, not from the products.
• Use the phrase 'minimum energy' in definitions; answers that say only 'the energy needed' may be treated as incomplete.
• Check the units: if the axis is in kJ/mol, give activation energy values in kJ/mol rather than J.