Two questions decide what happens in a reaction. Does it go? That is decided by the free energy change, ΔG: the drop from reactants to products. How fast? That is decided by the activation energy, Ea: the height of the barrier in between. Work through the three stages in order.
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Choose a reaction:
Catalyst:
1. Which energy profile matches this reaction?
Remember
ΔG < 0 means the reaction is spontaneous: it can go by itself. It says nothing about how fast.
The higher the activation energy, the fewer molecules have enough energy to get over the barrier, and the slower the reaction.
A catalyst (an enzyme in biology) provides a route with a lower barrier. It makes a spontaneous reaction faster, but it does not change ΔG, and it cannot make a non-spontaneous reaction go.
A non-spontaneous reaction (ΔG > 0) needs energy to be supplied: electricity, light, heat, or coupling to a spontaneous reaction.
Activation energies and times in this model are representative values chosen to match what is observed at 25 °C, so treat the times as orders of magnitude.