Chemical equilibria

Reversible reactions
A reversible reaction is one where there is a forward and backward reaction occurring:
The double arrow signifies a reversible reaction.
If in the above reaction the concentrations of A, B, C, D do not change, although the reaction is still in progress, then the forward rate must equal the backward rate. A situation known as dynamic equilibrium has been reached.
Equilibrium constants
Any dynamic equilibrium can be described in terms of its equilibrium constant, Kc.
The equilibrium constant is the product of the molar concentrations of the products raised to the power of its coefficient in the stoichiometric equation, divided by the product of molar concentrations of the reactants, each raised to the power of its coefficient in the stoichiometric equation.
So for the reaction:
The equilibrium constant is given by:
Where [] represents the concentration of the species in moldm-3.
For gaseous systems, we use Kp instead of Kc. Here, the species are shownin the equilibrium equation in terms of their partial pressures. (In a mixture of gases, the proportion of the total pressure due to a particular gas is dependant on its mole fraction).

2. Le Chatelier Principle
Introduction
Le Chateliers principle states:
The position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions.
This principle applies to any reaction that is in equilibrium.
The effect of concentration changes on equilibrium
Changing concentration of a reactant or product does not change the numerical value of the equilibrium constant, but it does change the position of the equilibrium.
In general, the position of the equilibrium is shifted towards the right if the concentration of a reactant is increased or to the left if the concentration of a product is increased.
At the start, when the change is made, the mixture is not at equilibrium, but equilibrium is eventually restored.
The effect of pressure changes on equilibrium
For a reaction involving gases, altering the pressure may cause a change in the position of the equilibrium.
For a reaction where there is an increase in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the left.
Where there is a decrease in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the right. The equilibrium constant remains the same.
The effect of temperature changes on equilibrium
The change that takes place when temperature is changed depends upon whether the forward reaction is exothermic or endothermic.
If the forward reaction is exothermic then the backward one is endothermic.
If the temperature is increased, the equilibrium moves to the left, since an endothermic reaction will tend to reduce the temperature.
Conversely, if the temperature is decreased then the equilibrium, moves to the right.
The effects of catalysts on equilibrium
A catalyst has no effect on the position of the equilibrium. However, it does increase the rate of both the forward and backward reactions, decreasing the time taken to reach equilibrium.