Reactions that have a negative ∆G release free energy and are called exergonic reactions.
Handy mnemonic: EXergonic means energy is EXiting the system.
A negative ∆G means that the reactants, or initial state, have more free energy than the products, or final state.
Exergonic reactions are also called spontaneous reactions, because they can occur without the addition of energy.
Reactions with a positive ∆G (∆G > 0), on the other hand, require an input of energy and are called endergonic reactions.
In this case, the products, or final state, have more free energy than the reactants, or initial state. Endergonic reactions are non-spontaneous, meaning that energy must be added before they can proceed. You can think of endergonic reactions as storing some of the added energy in the higher-energy products they form.
It’s important to realize that the word spontaneous has a very specific meaning here: it means a reaction will take place without added energy, but it doesn’t say anything about how quickly the reaction will happen. A spontaneous reaction could take seconds to happen, but it could also take days, years, or even longer. The rate of a reaction depends on the path it takes between starting and final states (the purple lines on the diagrams below), while spontaneity is only dependent on the starting and final states themselves. We’ll explore reaction rates further when we look at activation energy.