Reaction profiles
Activation energy
Reactions only happen when particles collide with enough energy. The minimum energy that colliding particles need to react is the .
This is why many reactions need starting off. A fuel won't burn until a spark or flame gives some particles enough energy to react. Once an exothermic reaction starts, the energy it releases can keep it going.
Key terms
Reading a reaction profile
A is a graph of energy (up the side) against the progress of the reaction (along the bottom).
- A line on the left shows the .
- The line rises to a peak, then falls to the on the right.
- The is the height from the reactants up to the peak.
- The is the difference between the reactants and the products.
When you draw one, label the reactants, products, activation energy and overall energy change. Draw the arrows: activation energy from the reactants up to the peak, and the energy change from the reactants to the products.
Key terms
Exothermic and endothermic profiles
| Exothermic | Endothermic | |
|---|---|---|
| Products compared with reactants | Lower in energy | Higher in energy |
| Energy change | Released to the surroundings | Taken in from the surroundings |
| Overall arrow | Points down | Points up |
Both kinds of profile have a peak, because both kinds of reaction need activation energy.
A gives a profile with a lower peak — a lower activation energy — but the reactants and products stay at the same energy, so the overall energy change is unchanged.