Exothermic and Endothermic Reactions » Reaction Profiles

Reactions need a collision with enough energy

Chemical reactions can only happen when the reacting particles collide with each other. But a collision on its own is not enough. The particles must also hit each other with enough energy. If they bump together too gently, they just bounce apart and nothing changes.

Think about pushing a ball up a small hill. If you push too softly, it rolls back down. You need to give it enough energy to get over the top. Reacting particles are the same. They need a minimum amount of energy before they can turn into products.

Key terms:

• Activation energy: the minimum amount of energy that particles must have to react.
• Reaction profile: a diagram that shows the relative energies of the reactants and products, the activation energy and the overall energy change of a reaction. It is also called an energy level diagram.

What is a reaction profile?

A reaction profile is a graph that follows a reaction from start to finish.

• The vertical axis shows the energy of the chemicals.
• The horizontal axis shows the progress of the reaction, from the reactants on the left to the products on the right.
• A curved line shows how the energy changes as the reaction goes on.

The line starts at a flat level for the reactants. It then rises to a peak. Then it falls to a flat level for the products. The peak is the point where the particles have just enough energy to react.

The parts of a reaction profile

The activation energy is always drawn as an arrow that goes up from the reactants to the top of the peak. The overall energy change is drawn as an arrow between the level of the reactants and the level of the products.

Exothermic reaction profiles

In an exothermic reaction, energy is transferred to the surroundings. So the products end up with less energy than the reactants.

To draw the profile:

1. Draw the axes. Label the vertical axis "Energy" and the horizontal axis "Progress of reaction".
2. Draw a flat line on the left for the reactants. Label it.
3. Draw a curve that rises to a peak, then falls to a flat line on the right that is lower than the start. Label this line as the products.
4. Draw an arrow up from the reactants to the peak. Label it "activation energy".
5. Draw an arrow down from the reactants level to the products level. Label it "overall energy change".

The products are lower than the reactants, so the overall energy change arrow points downwards.

Endothermic reaction profiles

In an endothermic reaction, energy is taken in from the surroundings. So the products end up with more energy than the reactants.

The drawing steps are the same, but the end of the curve is different. The curve rises to a peak, then falls only a little way, and the flat line for the products is higher than the start. The overall energy change arrow points upwards.

Using a profile to identify the reaction type

You may be given a reaction profile and asked what type of reaction it shows. Do not try to guess from the size of the peak. Just compare the two flat lines:

• If the products are lower than the reactants, the reaction is exothermic.
• If the products are higher than the reactants, the reaction is endothermic.

Every reaction profile has an activation energy peak, whether the reaction is exothermic or endothermic. Even a reaction that gives out lots of energy needs a push to get going. This is why a pile of coal does not burst into flames by itself. It needs a spark or a flame to give the first particles enough energy. Once the reaction starts, the energy it gives out helps to keep it going.

Exam-style question

The reaction profile for a reaction shows the products at a higher energy level than the reactants. (a) Is the reaction exothermic or endothermic? Give a reason. (b) What does the difference in height between the reactants and the top of the peak represent? (c) Describe what happens to the temperature of the surroundings.