Factors affecting equilibrium
How changes affect a chemical reaction equilibrium system can be predicted using Le Châtelier’s principle:
If a system at equilibrium is subjected to a change in conditions, the system will adjust to re-establish equilibrium in such a way as to partially counteract (oppose) the imposed change.
If an equilibrium system contains a solid or pure liquid, then a change in the mass of this substance will not put the system out of equilibrium unless the liquid changes the volume.
In an equilibrium system with the reaction
2CrO42-(aq) + 2H+(aq) ⇌ Cr2O72-(aq) + H2O (l)
Hydrochloric acid is added to it, resulting in an increased concentration of H+ ions.
- To oppose the change, some of the added hydrogen ions need to be removed;
- The equilibrium position will shift towards the products.
- The rate of the forward reaction is now larger than the reverse one, until equilibrium is achieved
- Thus, more of the reactants would react, producing more products.
- The concentration of H+ would be greater than before, since the change is only partially opposed.
- The addition of one of the reactants results in more of the products, more of the added reactant, and less of the other reactant
- The rates of the reactions are equal, but greater than before
- The value of K is constant
The changes in concentration can be predicted using molar ratios.
Since the ratio of reactants is 1:1, the decrease in concentration will be equal.
Because the ration of product to reactant is 1:2, the decrease in concentration of product will be half.
Using the same equilibrium system, except adding sodium hydroxide, inducing a decrease in the concentration of hydrogen ions.
- Since there is a decrease in [H+], there must be more hydrogen ions produced to achieve equilibrium.
- The equilibrium system would shift towards the reactants
- The reverse reaction becomes faster than the forward reaction, since there is less reactant
- Thus, more of the products will undergo the reverse reaction, producing more reactants
- Concentration of H+ would be less than before, since change is only partially opposed
- Removing one of the reactants results in less of the products, less of the removed reactant and more of the other reactant
- The rates of the reactions are equal, but lesser than before
- The value of K remains the same
Change in volume or pressure
Decrease in volume
The effect of change of volume is predicted using Le Châtelier’s principle, but is best done if a change in volume is interpreted as a change in pressure.
- If the volume is decreased, pressure increases.
- The system re-establishes equilibrium by reducing the total pressure.
- This is done by reducing the total amount of particles in the system,
- thus shifting the equilibrium position towards the side with the least particles.
- Since there are more particles one side of the reaction, the concentration would increase more.
- Since concentration increases more, more of those particles would react
- The concentration of the side with the most particles would increase overall, since change is never totally opposed (but decreases from when change happened)
- The concentration of the side with the least particles would increase
- The rates of the reactions would be equal, but greater than before.
- Value of K remains the same
Increase in volume
If volume is increased, pressure decreases, and the system attempts to increase the total pressure by increasing the number of particles. Hence, the equilibrium position would favour the side of the reaction with the most particles.
At equilibrium, the concentration of the side with the least particles would decrease. The concentration of the side with the most particles would decrease overall, since change is only partially counteracted, but would increase from when the change happened.
Change in temperature
The heat of the reaction must be known to predict the effect of change in temperature.
In the above example,
2NO2 (g) ⇌ N2O4 (g); ΔH = -57 kJ OR
2NO2 (g) ⇌ N2O4 + 57 kJ
The forward reaction is exothermic.
Note that since temperature has changed, so does the equilibrium constant.
|If the forward reaction is exothermic, increasing the temperature decreases K, but decreasing temperature increases K.|
|If the forward reaction is endothermic, increasing temperature increases K, but decreasing temperature decreases K.|
Increase in temperature
According to Le Châtelier’s principle
- If an equilibrium system is heated, the system will adjust by absorbing heat by favouring the endothermic reaction
- Thus, the concentration of the products of the endothermic reaction would increase, and the concentration of the products of the exothermic reaction would decrease
Decrease in temperature
According to Le Châtelier’s principle
- If an equilibrium system is cooled the system will adjust by producing heat by favouring the exothermic reaction
- Thus, the concentration of the products of the exothermic reaction would increase, and the concentration of the products of the endothermic reaction would decrease
Presence of a catalyst
- When a catalyst is added, it lowers the activation energy and increases the rate of reaction by providing an alternate reaction pathway.
- Thus, systems not in equilibrium reach it faster, but does not affect anything else.
- If a catalyst is added to a system in equilibrium, it will not be put out of equilibrium, but will increase the rates of the forward and reverse reaction by the same amount.