Reaction Profiles and Activation Energy

Chemical reactions only happen when particles collide with each other, and only some of those collisions cause a reaction. A collision leads to a reaction only if the particles hit with enough energy to break the existing bonds.

The activation energy is the minimum amount of energy that colliding particles must have for a reaction to occur. If particles collide with less than this, they simply bounce apart unchanged.

Activation energy = the minimum energy needed to start a reaction. It explains why some reactions need a spark or heating before they begin, even when the products store less energy than the reactants.

A mixture of petrol vapour and air, for example, can sit unreacted until a spark supplies the activation energy; then combustion runs on its own. The activation energy is the barrier that has to be overcome before reactants can turn into products, and a reaction profile is the diagram that shows this barrier.

A reaction profile (energy level diagram) plots energy on the vertical axis against the progress of the reaction on the horizontal axis. It shows three things at once:

• the relative energies of the reactants and the products,
• the activation energy, and
• the overall energy change of the reaction.

The line starts at the energy of the reactants, rises to a peak, and falls to the energy of the products. The peak represents the point where bonds are breaking and forming.

Feature on the profile	What it represents
Left-hand level	Energy of the reactants
Right-hand level	Energy of the products
Height from reactants to the peak	Activation energy
Vertical gap between reactants and products	Overall energy change

The curve rising to the peak and then falling shows the energy as the reaction proceeds. Reading a profile is a matter of comparing the two levels and measuring the hump, as the next two slides show for each type of reaction.

In an exothermic reaction the products are at a lower energy than the reactants. Energy is released to the surroundings, so the overall energy change is negative. The curve ends below where it started.

The diagram shows the reactants starting high, the curve climbing over the activation energy barrier, and the products settling at a lower level. The vertical drop from the reactant level to the product level is the energy given out to the surroundings.

The activation energy is still the height from the reactants up to the peak, even though the products end up lower. A reaction can be exothermic overall and still need an initial input of energy to start.

Because the products are lower in energy, an exothermic reaction transfers energy to the surroundings and the temperature of the surroundings rises. This is the profile shape for combustion, neutralisation and most oxidation reactions.

In an endothermic reaction the products are at a higher energy than the reactants. Energy is taken in from the surroundings, so the overall energy change is positive. The curve ends above where it started.

Here the curve rises over the activation energy barrier and then settles at a level higher than the reactants. The vertical rise from the reactant level to the product level is the energy absorbed from the surroundings.

For an endothermic reaction the activation energy is measured from the reactants up to the peak, and the overall energy change is the upward step to the products. Both are positive quantities on this diagram.

Because the products are higher in energy, energy is taken in from the surroundings and the temperature of the surroundings falls. Thermal decomposition follows this shape.

A common exam task is to draw a reaction profile and label it. Marks are awarded for the correct shape and the correct labels, so a neat sketch with clear arrows scores well.

Follow this checklist:

1. Draw axes: energy going up the side, "progress of reaction" along the bottom.
2. Draw a level line for the reactants and a level line for the products at the correct relative heights (products lower for exothermic, higher for endothermic).
3. Join them with a curve that rises to a peak between the two levels.
4. Mark the activation energy as the vertical arrow from the reactant level up to the peak.
5. Mark the overall energy change as the vertical arrow between the reactant and product levels.

A frequent error is measuring the activation energy from the bottom of the graph instead of from the reactant level. It is measured from the reactants up to the peak.

Worked reading. A profile shows reactants at 300 units and products at 120 units, with the peak at 460 units. The activation energy is $460-300=160$ units. The overall energy change is $120-300=-180$ units, a negative value, so the reaction is exothermic.

1. Measuring activation energy from the axis

Activation energy is the height from the reactant level to the peak, not from the bottom of the diagram. Draw the arrow starting at the reactants, not at zero energy.

2. Putting the products on the wrong side

For an exothermic reaction the products must be lower than the reactants; for an endothermic reaction they must be higher. Drawing them the wrong way round reverses the meaning of the whole diagram.

3. Forgetting the curve over the peak

The line must rise to a hump between the reactants and products, not go straight across. The hump is what represents the activation energy barrier as bonds break and form.

4. Confusing activation energy with the overall energy change

Activation energy is the barrier to starting the reaction. The overall energy change is the difference between reactants and products. They are two separate arrows on the profile.

5. Saying no energy is needed for an exothermic reaction

Even exothermic reactions need the activation energy to get started. A reaction can release energy overall yet still require an initial input to overcome the barrier.