Chemical Equilibrium:

Introduction

Chemical equilibrium is a state in a reversible reaction when the rate of forward reaction becomes equal to rate of backward reaction. At this state no further net change occurs to the system.

Reversible reaction

A reaction in which products under suitable conditions react to give back reactants are called reversible reactions. It can be expressed symbolically as

aA + bB cC + dD

where A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B.

Law of mass action:

At constant temperature the rate of a chemical reaction is directly proportional to the product of the molar concentrations of reacting species with each concentration term raised to the power equal to the numerical coefficients of that species in the chemical equation.

Guldberg and Waage showed that for a reaction aA + bB → cC + dD , the rate (speed) of the reaction in either direction is proportional to the "active masses" of the various components:

rate of forward reaction = k_f [A]^a [B]^b 

rate of backward reaction = k_r [C]^c [D]^d 

in which the proportionality constants k are called rate constants and the quantities in square brackets represent concentrations. As the reaction proceeds, the rate of the forward reaction diminishes while that of the backward reaction increases. Eventually the two processes are proceeding at the same rate, and the reaction is at equilibrium:

rate of forward reaction = rate of reverse reaction,

k_f [A]^a [B]^b  = k_r [C]^c [D]^d 

Characteristics of chemical equilibrium

• The observable properties of the system become constant at equilibrium remain unchanged thereafter.
• The equilibrium can be approached from either direction.
• It can be attained only if the system is closed.
• A catalyst does not alter the equilibrium point.

Le Chatelier principle:

If a system at equilibrium is subjected to a change of pressure, temperature, or the number of moles of a component, there will be a tendency for a net reaction in the direction that reduces the effect of this change.

Consider the reaction,

2HI → H₂ + I₂

Consider an arbitrary mixture of these three components at equilibrium, and assume that we inject more hydrogen gas into the container. Because the H2 concentration now exceeds its new equilibrium value, the system is no longer in its equilibrium state

The Le Chatelier principle states that the net reaction will be in a direction that tends to reduce the effect of the added H2. This can occur if some of the H2 is consumed by reacting with I2 to form more HI; in other words, "the equilibrium shifts to the left".

Homogenous and heterogenous equilibria

The equilibria in which all the substances are present in the same phase are known as homogenous equilibria.

The equilibria in which the substances are present in different phases are called heterogenous equilibria.

Characteristics of equilibrium constant

• It has a definite value for every reaction at a particular temperature.
• It is independent of initial concentrations of reacting species.
• For a reversible reaction the equilibrium constant for the backward reaction is inverse of the equilibrium constant for the forward reaction.
• It is independent of the presence of catalyst.
• When addition of two equilibria leads to equilibrium then, the product of their equilibrium constants gives the equilibrium constant of the resultant equilibrium.