The equilibrium concept
Every chemical reaction is in principle reversible. Once products are formed by reaction in the forward direction, the reverse reaction can occur. The extent to which the reverse reaction actually occurs depends on the particular reaction.
Consider the reaction N2(g) + 3H2(g) 
2NH3(g)
N2 is mixed with three times as much H2 in a flask.
The forward and reverse reaction shown above occur.
As the graph at the right shows,
As the graph below shows, if a mixture at equilibrium is allowed to stand, no further change in composition occurs.
The composition of a reaction mixture at equilibrium at a particular temperature depends on which components of the system were mixed at the outset and how much of each was used. For any chemical reaction there are an infinite number of equilibrium compositions.
The equilibrium state of a reaction at a particular temperature is characterised by a number known as the equilibrium constant (K) that is independent of how the equilibrium system was prepared. The equilibrium constant is a measure of the tendency of reactants in the forward direction to form products.
2NH3(g)N2 is mixed with three times as much H2 in a flask.
The forward and reverse reaction shown above occur.
As the graph at the right shows,
the rate of the forward reaction decreases with time.
the rate of the reverse reaction increases with time.
When these rates are equal, the system is said to be at equilibrium.the rate of the reverse reaction increases with time.
This is analogous to a situation in which during a specific time interval, two students enter a crowded classroom and two leave. Change is occurring, but the number of students in the classroom is constant.
The composition of a reaction mixture at equilibrium at a particular temperature depends on which components of the system were mixed at the outset and how much of each was used. For any chemical reaction there are an infinite number of equilibrium compositions.
The equilibrium state of a reaction at a particular temperature is characterised by a number known as the equilibrium constant (K) that is independent of how the equilibrium system was prepared. The equilibrium constant is a measure of the tendency of reactants in the forward direction to form products.
This means that each of the large number of possible different equilibrium compositions at a particular temperature is associated with a particular equilibrium constant that is characteristic of that reaction at that temperature.