Equilibrium Concepts

Reversible Reactions and Equilibrium

In a reversible reaction, there comes a point when the rate of the forward reaction becomes equal to the rate of the reverse reaction. The system reaches equilibrium at this condition. At equilibrium, no further change occurs in the concentrations of products or reactants unless the system is disturbed.
A chemical reaction occurs when one or more reactants form one or more products. A chemical reaction can be represented as:
A+BC+D\rm{A}+\rm{B}\rightarrow\rm{C}+\rm{D}
The products of this reaction are C and D. As these products form, the reverse reaction also starts to take place. The reverse reaction for this example would be:
C+DA+B(reverse reaction)\rm{C}+\rm{D}\rightarrow\rm{A}+\rm{B}\hspace{25pt}\text{(reverse reaction)}
In some reactions, the reverse reaction happens very slowly. Because of this, it can effectively be ignored. In such reactions, the reaction is said to proceed to completion. A reversible reaction is a reaction in which the products of the reaction can also react to produce the reactants at a nonnegligible rate. A reversible reaction can be represented using double arrows in the reaction equation.
A+BC+D\rm{A}+\rm{B}\rightleftarrows\rm{C}+\rm{D}
For example, consider the Haber-Bosch process, which breaks down the nitrogen-nitrogen bond in N2 and converts nitrogen into ammonia (NH3). This process, which is important in many industries for producing ammonia from atmospheric nitrogen and hydrogen gas (H2), is a reversible reaction.
N2(g)+3H2(g)2NH3(g){\rm{N_2}}(g)+3{\rm{H_2}}(g)\rightleftharpoons2{\rm{NH_3}}(g)
In the forward reaction, nitrogen and hydrogen gases combine to form ammonia gas. The reverse reaction is decomposition of ammonia to form hydrogen and nitrogen gases again.

If a reversible reaction continues, the system reaches a state of equilibrium in which the rates of the forward and reverse reactions are equal. The chemical equation representing a reversible reaction at equilibrium uses double arrows with partial arrowheads (called harpoons) instead of a single arrow. The concentrations of the reactants and the products no longer change at this point. When a dynamic equilibrium is reached, both the forward and reverse reactions still occur, but because the rates are the same, it appears as if the reaction has stopped.

When a reversible reaction begins, the forward reaction is dominant. Over time, concentrations of reactants will reduce, and concentrations of products will increase. The changing concentrations affect the rates of reaction. As concentrations of the reactants decrease, the rate of the forward reaction decreases. At the same time, as the concentrations of the products increase, the rate of the reverse reaction increases. At chemical equilibrium, the rates of the forward reaction and the reverse reaction equal each other. The concentrations of reactants and products do not vary, and the reaction proceeds in both directions at the same rate.

Mathematically, at chemical equilibrium
rate of forward reaction=rate of reverse reaction\text{rate of forward reaction}=\text{rate of reverse reaction}
When the concentrations of reactants and products versus time are plotted on a graph, the concentration of reactants will decrease and the concentration of products will increase over time. The change in concentration will gradually slow down, and at equilibrium, both the product and reactant concentrations stop changing.

Concentration of Products and Reactants at Equilibrium

Prior to equilibrium, the concentration of the products is increasing, and the concentration of the reactants is decreasing. At equilibrium, the rates of the forward and reverse reactions are equal, so the concentrations of the reactants and the products no longer change.
If the rates of the forward and reverse reactions are plotted versus time, a different graph is obtained. The rates of forward and reverse reactions become equal over time.

Rates of Forward and Reverse Reactions over Time and at Equilibrium

The rates of forward and reverse reactions become equal over time. When the rates of forward and reverse reactions are equal, the reaction system is at equilibrium.