EQUILIBRIUM

                  1. Introduction

Chemical equilibria are important in numerous biological and environmental processes. For example, equilibria involving O2 molecules and the protein hemoglobin play a crucial role in the transport and delivery of O2 from our lungs to our muscles. Similar equilibria involving CO 

molecules and hemoglobin account for the toxicity of CO. When a liquid evaporates in a closed container, molecules with relatively higher kinetic energy escape the liquid surface into the vapour phase and number of liquid 

molecules from the vapour phase strike the liquid surface and are retained in the liquid phase. It gives rise to a constant vapour pressure because of an equilibrium in which the number of molecules leaving the liquid equals the number returning to liquid from the vapour. We say that the system has reached equilibrium state at this stage. However, this is not static equilibrium and there is a lot of activity at the 

boundary between the liquid and the vapour. Thus, at equilibrium, the rate of evaporation is equal to the rate of condensation. It may be represented by H2O (l) H2O (vapour) 

The double half arrows indicate that the processes in both the directions are going on simultaneously. The mixture of reactants and products in the equilibrium state is called an equilibrium mixture. 

 Equilibrium can be established for both physical

processes and chemical reactions. The reaction may be fast or slow depending on the experimental conditions and the nature of the reactants. When the reactants in a closed vessel 

at a particular temperature react to give products, the concentrations of the reactants keep on decreasing, while those of products keep on increasing for some time after which there is no change in the concentrations of either of 

the reactant or products. This stage of the system is the dynamic equilibrium and the rates of the forward and reverse reactions become equal. It is due to this dynamic equilibrium stage that there is no change in the concentrations of various species in the reaction mixture. Based on the extent to which the reactions proceed to reach the state of chemical equilibrium, these may be classified in three groups. 

(i) The reactions that proceed nearly to

completion and only negligible concentrations of the reactants are left. In some cases, it may not be even possible to detect these experimentally. 

(i i) The reactions in which only small amounts

of products are formed and most of the reactants remain unchanged at equilibrium stage. 

(i i i) The reactions in which the concentrations

of the reactants and products are

comparable, when the system is in

equilibrium.

The extent of a reaction in equilibrium

varies with the experimental conditions such

as concentrations of reactants, temperature,

etc. Optimisation of the operational conditions

is very important in industry and laboratory

so that equilibrium is favourable in the

direction of the desired product. Some

important aspects of equilibrium involving

physical and chemical processes are dealt in

this unit along with the equilibrium involving

ions in aqueous solutions which is called as

ionic equilibrium.

 2. EQUILIBRIUM IN PHYSICAL 

                   PROCESSES

The characteristics of system at equilibrium

are better understood if we examine some

physical processes. The most familiar examples are phase transformation processes, e.g., 

                     solid= liquid

                     liquid =gas

                      solid = gas

              3. Solids in liquids

We know from our experience that we can

dissolve only a limited amount of salt or sugar

in a given amount of water at room

temperature. If we make a thick sugar syrup

solution by dissolving sugar at a higher

temperature, sugar crystals separate out if we

cool the syrup to the room temperature. We

call it a saturated solution when no more of

solute can be dissolved in it at a given

temperature. The concentration of the solute

in a saturated solution depends upon the

temperature. In a saturated solution, a

dynamic equilibrium exits between the solute

molecules in the solid state and in the solution:

Sugar (solution) Sugar (solid), and the rate of dissolution of sugar = rate of crystallisation of sugar. 

Equality of the two rates and dynamic

nature of equilibrium has been confirmed with

the help of radioactive sugar. If we drop some

radioactive sugar into saturated solution of

non-radioactive sugar, then after some time

radioactivity is observed both in the solution

and in the solid sugar. Initially there were no

radioactive sugar molecules in the solution

but due to dynamic nature of equilibrium,

there is exchange between the radioactive and

non-radioactive sugar molecules between the

two phases. The ratio of the radioactive to non-

radioactive molecules in the solution increases

till it attains a constant value.

          4. Gases in liquids

When a soda water bottle is opened, some of

the carbon dioxide gas dissolved in it fizzes

out rapidly. The phenomenon arises due to

difference in sol ability of carbon dioxide at

different pressures. There is equilibrium

between the molecules in the gaseous state

and the molecules dissolved in the liquid

under pressure i.e., CO 2 (gas) CO 2 (in solution) . 

This equilibrium is governed by Henry’s

law, which states that the mass of a gas

dissolved in a given mass of a solvent at

any temperature is proportional to the

pressure of the gas above the solvent. This

amount decreases with increase of

temperature. The soda water bottle is sealed

under pressure of gas when its sol ability in

water is high. As soon as the bottle is opened,

some of the dissolved carbon dioxide gas

escapes to reach a new equilibrium condition

required for the lower pressure, namely its

partial pressure in the atmosphere. This is how

the soda water in bottle when left open to the

air for some time, turns ‘flat’. It can be

generalised that:

(i) For solid liquid equilibrium, there is

only one temperature (melting point) at

1 a™ (1.013 bar) at which the two phases

can coexist. If there is no exchange of heat

with the surroundings, the mass of the two

phases remains constant.

(i i) For liquid vapour equilibrium, the

vapour pressure is constant at a given

temperature.

(i i i) For dissolution of solids in liquids, the

sol ability is constant at a given

temperature.

(i v) For dissolution of gases in liquids, the

concentration of a gas in liquid is

proportional to the pressure

(concentration) of the gas over the liquid.

5. General Characteristics of Equilibria Involving Physical Processes

For the physical processes discussed above,

following characteristics are common to the

system at equilibrium:

(i) Equilibrium is possible only in a closed

system at a given temperature.

(i i) Both the opposing processes occur at the

same rate and there is a dynamic but

stable condition.

(i i i) All measurable properties of the system

remain constant.

(i v) When equilibrium is attained for a physical

process, it is characterised by constant

value of one of its parameters. 

(v) The magnitude of such quantities at any

stage indicates the extent to which the

physical process has proceeded before

reaching equilibrium.