A final thought: why does equilibrium happen in some processes and not in others? Theoretically all reactions are reversible. Practically speaking many are not. Those which are have moderate positive or negative standard free energy changes (roughly in the -60 to +60 kJ/mol range). These ΔG^o values depend, of course, on the enthalpy and entropy changes that occur during the reactions. When TΔS^o is similar in magnitude and sign to ΔH^o then the reaction is likely to reach equilibrium rather than proceed to completion under standard conditions since at equilibrium:

0 = ΔG = ΔH - TΔS

The fact that equilibrium conditions can be expressed in this way leads to our next topic: how to manipulate equilibrium to obtain different amounts of reactants and products.