Equilibrium is the state of a closed system at which two opposing reactions are simultaneously occurring at the same rate. In other words, in an equilibrium, the concentrations of all chemicals are constant.
(Note: even though the two reaction rates are equal to each other, it doesn't mean that the concentrations of the chemicals are equal)
When two chemicals in a chemical equation are put together, would there always be an equilibrium?
No. Sometimes a reaction may go to completion, meaning that there wouldn't be any reverse reaction. For example, if you react HCl and NaOH, the chemicals are just going to keep reacting until they're all depleted; the NaCl and H2O that are produced are not going to react to re-make HCl and NaOH.
Other times, a reaction may not occur at all. For example, if you put NaCl and H2O in the same beaker, they're not going to react to form HCl and NaOH.
So how can one determine whether a reaction will go to completion, reach equilibrium, or not occur at all?
There are two things that one can look at: ΔS and ΔH (entropy and enthalpy)
- A positive ΔS favours the forward reaction, and vice versa
- A negative ΔH favours the backward reaction, and vice versa
And using these two rules, one can determine whether a reaction will go to completion, reach equilibrium, or not occur at all:
- If both rules favour the forward reaction, then the reaction will go to completion
- If both rules favour the backward reaction, then the reaction will not occur
- Otherwise, the reaction will reach equilibrium