In the previous section we saw Reaction Profiles for endo- and exothermic reactions.

In both cases, we have to put energy in before any energy is released.

The minimum amount of energy needed for the reaction to start is the Activation Energy or E_a

This is the energy we have to put in to break bonds in the reactants.

Then some energy is released when new bonds form in the products.

If less energy is released than was put in then the reaction is endothermic.

So this will cool down the surroundings by removing energy.

And it will make us feel colder

If more energy is released than was put in then the reaction is exothermic.

Energy is released to the surroundings making us feel warmer.

In either case, the energy given out or taken in overall is the difference between the starting energy and the final energy.

For some reason, the examiner really wants to see the curved line that represents the progress of the reaction.

It doesn't matter how much energy reactants have unless they are touching.

So for a reaction to happen, reactants must collide.

If they do not have enough energy then they may collide but no reaction will happen - this is an unsuccessful collision

But even if the collision happens with enough energy (equal to or more than E_a) there is no guarantee of a successful collision because the particles may collide in the wrong orientation (from the wrong side).

We will cover this again in the Rates topic.