Chemical Equilibrium

Consider the following reaction occurring in a closed container (so that no material can go in or out):

H₂ + I₂ → 2 HI

This is simply the reaction between elemental hydrogen and elemental iodine to make hydrogen iodide. The way the equation is written, we are led to believe that the reaction goes to completion, that all the H₂ and the I₂ react to make HI.

However, this is not the case. The reverse chemical reaction is also taking place:

2 HI → H₂ + I₂

It acts to undo what the first reaction does. Eventually, the reverse reaction proceeds so quickly that it matches the speed of the forward reaction. When that happens, any continued overall reaction stops: the reaction has reached chemical equilibrium (sometimes just spoken as equilibrium; plural equilibria), the point at which the forward and reverse processes balance each other’s progress.

Because two opposing processes are occurring at once, it is conventional to represent an equilibrium using a double arrow, like this:

H₂ + I₂ ⇄ 2 HI 

The double arrow implies that the reaction is going in both directions. Note that the reaction must still be balanced.

One thing to note about equilibrium is that the reactions do not stop; both the forward reaction and the reverse reaction continue to occur. They both occur at the same rate, so any overall change by one reaction is cancelled by the reverse reaction. We say that chemical equilibrium is dynamic, rather than static. Also, because both reactions are occurring simultaneously, the equilibrium can be written backward. For example, representing an equilibrium as

H₂+ I₂ ⇄ 2 HI 

is the same thing as representing the same equilibrium as

2 HI ⇄ H₂ + I₂

The reaction must be at equilibrium for this to be the case, however.