9. Influence of External Factors on the Position of the Equilibrium State

A chemical equilibrium will be disturbed by any change of certain external conditions. As a result the equilibrium will shift to the left or to the right in order to reach a new equilibrium state.

• Concentration

a A + b B c C + d D

Adding or extracting small amounts of reactants or products has the consequence that Q will change and will no longer be equal to K: the equilibrium state is broken.

For instance, by adding some reactant A, the concentration quotient Q will decrease. To reach a new equilibrium state, the reaction will occur in that sense in which Q will increase again until Q = K. The forward reaction will occur: a A + b B c C + d D. The equilibrium is shifting to the right.

On the other hand, by removing some reactant A the concentration quotient Q will increase. To reach a new equilibrium state, the reaction will occur in that sense in which Q will decrease again until Q = K. The reverse reaction will occur: c C + d D a A + b B. The equilibrium is shifting to the left.

On adding a component of a chemical equilibrium, the equilibrium will shift away from the added component. Removing a component of a chemical equilibrium, will result in a shift of the equilibrium toward the removed component.

• Volume and Pressure

a A(aq) + b B(aq) c C(aq) + d D(aq)

Adding solvent to a homogeneous equilibrium in that solvent, results in a decrease of all the concentrations (dilution). For instance, if the volume of the solution is doubled, all concentrations are reduced by half. The change of the reaction quotient Q depends on the exponents of the concentrations.

• (c+d) = (a+b)
  In this case the reaction quotient Q does not change. The equilibrium remains unchanged.
• (c+d) > (a+b)
  In this case the reaction quotient Q decreases. To reach a new equilibrium state, the reaction occurs in that sense in which Q will increase again until Q = K. The forward reaction will occur: a A + b B c C + d D. The equilibrium is shifting to the right.
• (a+b) > (c+d)
  In this case the reaction quotient Q increases. To reach a new equilibrium state, the reaction occurs in that sense in which Q will decrease again until Q = K. The reverse reaction will occur: c C + d D a A + b B. The equilibrium is shifting to the left.

Adding solvent (dilution) to a homogeneous equilibrium in that solvent will induce a shift of the equilibrium position toward the side of that reaction involving the larger number of solute molecules in the reaction equation. Extracting solvent (for example by evaporation) from a homogeneous equilibrium in that solvent will induce a shift of the equilibrium position toward that side of the reaction involving the smaller number of solute molecules in the reaction equation.

a A(g) + b B(g) c C(g) + d D(g)

Increasing of the pressure on a homogeneous equilibrium in the gaseous phase, results in a decrease of the volume and an increase of all the concentrations. For instance, if the volume is reduced by half, all concentrations are doubled. The change of the reaction quotient Q depends on the exponents of the partial pressures.

• (c+d) = (a+b)
  In this case the reaction quotient Q does not change. The equilibrium remains unchanged.
• (c+d) > (a+b)
  In this case the reaction quotient Q increases. To reach a new equilibrium state, the reaction occurs in that sense in which Q will decrease again until Q = K. The reverse reaction will occur: c C + d D a A + b B. The equilibrium is shifting to the left.
• (a+b) > (c+d)
  In this case the reaction quotient Q decreases. To reach a new equilibrium state, the reaction occurs in that sense in which Q will increase again until Q = K. The forward reaction will occur: a A + b B c C + d D. The equilibrium is shifting to the right.

Increasing the pressure on a homogeneous equilibrium in the gaseous phase, and thus decreasing the volume of the gas mixture, will result in a shift of the equilibrium position toward the side of the reaction involving the smaller number of gaseous molecules. Decreasing the pressure on a homogeneous equilibrium in the gaseous phase, and thus increasing the volume of the gas mixture, will result in a shift of the equilibrium position toward the side of the reaction involving the larger number of gaseous molecules.

• Temperature

We must be aware of the fact that the previous changes alter the equilibrium position, but do not alter the equilibrium constant.

On the other hand, changing the temperature will cause a change of the value of the equilibrium constant K.

For exothermic reaction equilibria the equilibrium constant K will decrease as temperature rises. So Q_eq will decrease: equilibrium position shifting to the left, thus in endothermic sense.

For endothermic reaction equilibria the equilibrium constant K will increase as temperature rises. So Q_eq will increase: equilibrium position shifting to the right, thus in endothermic sense.

Increasing the temperature will result in a shift of the equilibrium position toward the endothermic side of the reaction. Decreasing the temperature will result in a shift of the equilibrium position toward the exothermic side of the reaction.

Summary

Change	Equilibrium state
adding a component	shift to the other side
extracting a component	shift to the same side
volume increase	shift to the side with larger number of molecules
volume decrease	shift to the side with smaller number of molecules
pressure increase	shift to the side with smaller number of molecules
pressure decrease	shift to the side with larger number of molecules
temperature increase	shift in endothermic sense
temperature decrease	shift in exothermic sense