Chemical Equilibrium

A chemical reaction system can have both a forward reaction and a reverse reaction.

Example reaction system: production of ammonia (NH3).

\schemestart \chemfig{N_2} + \chemfig{3H_2} \arrow{<=>}[,0.75] \chemfig{2NH_3} \schemestop

The double arrow shows that both forward and reverse reactions are taking place.

The forward reaction for this system looks like this:
\schemestart \chemfig{N_2} + \chemfig{3H_2} \arrow[,0.75] \chemfig{2NH_3} \schemestop

This forward reaction consumes N2 and H2 and produces NH3.

The reverse reaction for this system looks like this:
\schemestart \chemfig{2NH_3} \arrow[,0.75] \chemfig{N_2} + \chemfig{3H_2} \schemestop

This reverse reaction consumes NH3 and produces N2 and H2.
(The reverse reaction does the exact opposite of the forward reaction.)

Both reactions are happening at the same time.

If the forward and reverse reactions have the same rate, the reaction is at chemical equilibrium.

At chemical equilibrium, the concentrations of each chemical stay the same. Each substance is being produced and used up at the same rate.

Equilibrium Shift

The equilibrium can shift if conditions change to favor one of the two reactions (forward reaction or reverse reaction).

Scenario 1: Equilibrium Shifts Right

  • If the forward reaction is faster than the reverse reaction, the right side of the reaction is favored.
  • Substances on the right side are being produced by the forward reaction faster than they are being used up by the reverse reaction.
  • Concentration of substances on the right side is increasing as the equilibrium shifts to the right.
  • Substances on the left side are being used up by the forward reaction faster than they are being produced by the reverse reaction.
  • Concentration of substances on the left side is decreasing while the equilibrium shifts to the right.

Scenario 2: Equilibrium Shifts Left

  • If the reverse reaction is faster than the forward reaction, the left side of the reaction is favored.
  • Substances on the left side are being produced by the reverse reaction faster than they are being used up by the forward reaction.
  • Concentration of substances on the left side is increasing as the equilibrium shifts to the left.
  • Substances on the right side are being used up by the reverse reaction faster than they are being produced by the forward reaction.
  • Concentration of substances on the left side is decreasing while the equilibrium shifts to the right.

Ammonia Reaction: Example of Shifting to the Right

\schemestart \chemfig{N_2} + \chemfig{3H_2} \arrow{<=>}[,0.75] \chemfig{2NH_3} \schemestop

Let’s say we start with a jar with 10 molecules of each substance in this reaction:

# of N2 molecules 10
# of H2 molecules 10
# of NH3 molecules 10