An equilibrium occurs when the backward reaction is also possible, and therefore the system has a balance between two reactions. This can be seen in the next graph, in which the rate of the forward reaction is equal to the rate of the backward reaction.
Le Chatelier’s Principle
If a dynamic equilibrium is disturbed by changing the concentration, temperature, volume or partial pressure, the position of equilibrium moves to counteract the change and a new equilibrium is established.
If any system in the equilibrium is changed the reaction will try to shift the equilibrium in order to counteract that change.
This can be seen best using an example, such as:
N2 + 3 H2 ⇌ 2 NH3 ΔH = -92 kJ mol-1
If the concentration of nitrogen is increased the reaction will shift to the right to reduce the concentration of nitrogen.
If the pressure of the mixture is increased by either decreasing the volume or adding more reactants the reaction will shift to the right in order to reduce the pressure.
If the pressure of the mixture is increased by adding an inert gas there will be no effect on the equilibrium since this will not change the partial pressures of the individual component.
If the temperature is increased the reaction will go to the left since this would be an endothermic reaction.
If a catalyst is put in the reaction the equilibrium will not change, since the catalyst will increase the rate of both the forward and the backward reaction
Equilibrium Constants
The equilibrium constant is a number which defines the ratio of the products to the reactants. The higher the ratio the more the forward reaction is favored. Equilibrium constants are affected by temperature but not by concentrations and pressure.
Concentration constant
The concentration constant can be found by writing the products, to the power of their coefficient divided by the reactants to the power of their coefficient. It is important to note that the concentration is used in this case.
Once the equilibrium constant equation has been established then write the following table:
| aA | bB | cC | dD | |
| Initial | ||||
| Change | ||||
| Equilibrium | ||||
| MAKE SURE YOU HAVE USED CONCENTRATION | ||||
Check the units for the equilibrium constant.
Pressure constant
The pressure constant can be found by writing the products, to the power of their coefficient divided by the reactants to the power of their coefficient. It is important to note that the partial pressure is used in this case.
Once the equilibrium constant equation has been established then write the following table:
| aA | bB | cC | dD | |
| Initial | ||||
| Change | ||||
| Equilibrium | ||||
| MAKE SURE YOU HAVE USED PARTIAL PRESSURE | ||||
Check the units for the equilibrium constant.