Chemical Equilibrium

Introduction

Chemical equilibrium is defined as the state in which both products and reactants are present in the present concentrations which have no further tendency to change with time so that there is no observable change in the properties of the system which is present. This results in the state when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the backward and forward reactions are generally not zero, but equal. Thus, no net changes are seen in the concentrations of reactants and products. Such a state is called a dynamic equilibrium state.

Historical Introduction

Berthollet developed a chemical equilibrium in 180, he found that some chemical reactions are reversible. Forward and backward (reverse) reaction rates are equal for any reaction mixture to exist at equilibrium. Arrows pointing both ways to indicate equilibrium in the following chemical equation, where A and B are reactant chemical species, product species are S and T, and the stoichiometric coefficients α, β, σ, and τ are of the respective reactants and products.

"Far to the right" is said to lie to the equilibrium concentration position of a reaction if, at equilibrium, nearly all the reactants are consumed.  "Far to the left" is said Conversely to the equilibrium position if hardly any product is formed from the reactants.

Based on Berthollet's ideas, Guldberg and Waage in 1865, proposed the law of mass action, where A, B, S, and T are active masses and k+ and k− are rate constants.

The numerator is formed from the products by convention. For the reaction of one-step, the mass law action is only valid, that proceed through a single transition state and is not valid in general because the rate of equations do not, in general, follow the reaction of stoichiometry as Guldberg and Waage had proposed (see, for example, nucleophilic aliphatic substitution by SN1 or reaction of hydrogen and bromine to form hydrogen bromide). However, for chemical equilibrium, the equality of forwarding and backward reaction rate is a necessary condition, though it is not sufficient to explain why equilibrium occurs.

The constant of equilibrium for a reaction is indeed a constant even despite the failure of this derivation, activities independent of any of the various species involved, though it is temperature-dependent as observed by the van 't Hoff equation. A catalyst addition will affect both the forward reaction and the reverse reaction in the same way and will not have an effect on the equilibrium constant. Both reactions will speed up by the catalyst and increase the speed at which the equilibrium is reached.

Types

Reactions do occur at the molecular level, Although the macroscopic equilibrium concentrations are constant in time. For example, when the acetic acid dissolved in water it  forms acetate and hydronium ions,

CH3CO2H + H2O ⇌ CH3CO−2 + H3O+

Types of Chemical Equilibrium

There are two types of chemical equilibrium:

• Homogeneous Equilibrium

• Heterogeneous Equilibrium

Homogeneous Chemical Equilibrium

In Homogeneous Equilibrium, the reactants and the products of chemical equilibrium are all in the same phase. It can be further divided into two types: 

a) Reactions in which the number of molecules of the products is equal to the number of molecules of the reactants. For example,

• H2 (g) + I2 (g) ⇌ 2HI (g)

• N2 (g) + O2 (g) ⇌ 2NO (g)

b) Reactions in which the number of molecules of the products is not equal to the total number of reactant molecules. For example,

• 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

• COCl2 (g) ⇌ CO (g) + Cl2 (g)

Heterogeneous Chemical Equilibrium

In, Heterogeneous Equilibrium the reactants and the products of chemical equilibrium are present in different phases. Examples of heterogeneous equilibrium are listed below.

• CO2 (g) + C (s) ⇌ 2CO (g) 

• CaCO3 (s) ⇌ CaO (s) + CO2 (g)

Thus, the different types of chemical equilibrium are based on the phase of the reactants and products.

Factors Affecting Chemical Equilibrium

Temperature, pressure, and concentration of the system which affect equilibrium are several factors. Some of the important factors affecting chemical equilibrium are discussed below.

Change in Pressure:

Due to the change in the volume, the change in pressure happens. The gaseous reaction can be affected If there is a change in pressure, as the total number of gaseous reactants and products are now different. As per Le Chatelier’s principle, in heterogeneous chemical equilibrium, as the volume is independent of pressure, the change of pressure in both liquids and solids can be ignored.

Change in Temperature:

The temperature effect on chemical equilibrium depends upon the sign of ΔH of the reaction and follows Le-Chatelier’s Principle.

• The equilibrium constant of an exothermic reaction decreases as temperature increases.

• The equilibrium constant increases in an endothermic reaction with an increase in temperature.

The rate of reaction is also affected along with equilibrium constant, by the change in temperature. According to Le Chatelier’s principle, the equilibrium shifts towards the reactant side when the temperature increases in case of exothermic reactions, for endothermic reactions the equilibrium shifts towards the product side with an increase in temperature.