In a chemical reaction, chemical equilibrium is defined as the state where there is no further change in the concentration of the reactants or products. Chemical equilibrium is also called as a “steady state reaction”. This doesn’t mean that the chemical reaction has stopped occurring, but the formation and consumption of substances have reached a balanced condition.
The concept of Chemical Equilibrium:
Consider the following reversible reaction with the reactants A and B forming the products C and D and vice-versa.
A + B ⇌ C+ D
With the passage of time, the concentration of products (C and D) increases and reactants (A and B) decreases. This leads to an increase in the rate of backward reaction and decrease in the forward reaction rate. Eventually, both forward and backward reaction rates become equal, the concentrations of the products and the reactants remain constant, leading to the formation of a visible standstill state called chemical equilibrium.
At this state, Rate of forward reaction = Rate of backward reaction.
The state of reversible reaction at which the concentration of the reactants and products don’t change with time is known as the state of Chemical Equilibrium. The equilibrium concentrations of the reactants will always remain same whether we begin with the reactants or the products. The variation of concentration of reactants and products over the time can be represented by the following figure:
This state is not static, the reactions still proceed at the same rate i.e. the number of moles of product produced in the forward reaction is equal to the number of moles of substance disappearing per second in the backward reaction.
After attaining the state of equilibrium the reaction does not stop though it may appear as if it has stopped. In fact, both the forward and backward reactions continue even after attaining the equilibrium. But the rate the forward reaction becomes equal to the rate of backward reaction. This means that if some products are formed from the reactants than an equal amount of products are converted back into reactants. As a result of this, the concentration of both the reactants and products become constant. Hence it is known as dynamic equilibrium.
When a chemical reaction continues to proceed but the number of reactants and products remains constant, it is called as dynamic equilibrium.
Types of Chemical Equilibrium
- Homogeneous equilibrium: In an equilibrium reaction when both the product and the reactants are present in the same phase, it is called as homogeneous equilibrium. For example,
N2(g) + 3H2(g) ⇋ 2NH3 (g)
2SO2(g) + O2(g) ⇋ 2SO3 (g)
- Heterogeneous equilibrium: In an equilibrium reaction when both the product and the reactants are present in a different phase, it is called as homogeneous equilibrium. For example,
H2O(l) ⇋ H20(g)
CaCO3(s) ⇋ CaO(s) + CO2(g)
At constant temperature and pressure, the rate of a chemical reaction is directly proportional to the molar concentration of the reactants to the molar concentration of the products both raised to a power equal to the corresponding stoichiometric coefficients as represented by the balanced chemical equations.
Characteristics of Chemical Equilibrium:
- At a given temperature, the measurable properties like pressure, concentration, density or color remain constant in chemical equilibrium.
- Chemical equilibrium can be attained from either side (forward and backward reactions).
- It can be attained in less time in the presence of a catalyst; however, a catalyst cannot affect chemical equilibrium as it only affects the rate of both the reactions, forward and backward, to the same extent.
- It is dynamic in nature.
Factors affecting chemical equilibrium:
According to Le Chatelier’s principle if a chemical reaction in equilibrium is subjected to certain factors, the equilibrium shifts. The factors affecting equilibrium are mentioned below:
- Change in concentration
- Change in pressure
- Change in Temperature
- Effect of a Catalyst
- Effect of addition of an Inert Gas
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