On the basis of Le- Chatellier's principle ,discuss the condition for obtaining the maximum yield of `SO_3` in the following reactions :
` 2SO_2 (g) + O_2(g) hArr 2SO_3(g) , Delta H =- 42 ` k cal

Explanation

(i) Le Chatelier.s principle states that,
• When a constraint (stress) is applied on a system at equilibrium, the system behaves in such a way so as to counteract the constraint.
• When a chemical system at equilibrium is subjected to a change in concentration, pressure or temperature, the equilibrium shifts in the direction that counteracts the change.
Le-Chatelier.s principle is very useful qualitative prediction of the effect of change of concentration, pressure, or temperature.
• Effect of change of concentration: When the concentration of any reactant or product is changed, the system readjusts itself by shifting the equilibrium in a direction which counteracts the change in concentration that was made.
• Effect of change of pressure: When the pressure is increased or decreased on a system containing gaseous substances in equilibrium, then the equilibrium shifts in direction which tend to decrease or increase the pressure and hence to decrease or increase the number of moles. When the system proceeds with no change in the number of moles, there is no effect of increasing or decreasing pressure on the system.
• Effect of change of temperature: When the temperature of a system at equilibrium is increased, the equilibrium shifts in the direction in which heat is absorbed.
Le Chatelier.s principle has some important industrial applications. In the manufacture of sulphuric acid (Contact process), the main reaction involves is,
`2SO_(2) (g)+O_(2) (g) Leftrightarrow 2SO_(3) (g), triangle H=-42"kcal"`
The above reaction was highly exothermic and proceeded with decrease in number of moles. By applying Le Chatelier.s pandple the maximum yiekis of `SO_(3)` could be obtained at low temperature and high pressure. So, in actual practice, the reaction is carried out at `400-450^@C`, under a pressure of 2 to 3 atmospheres, using catalyst `V_(2)O_(5)`.
(ii) Acetic acid is a weak and it is partially ionized in the solution. As it is 5% dissociated, its degree of dissociation `alpha` is given by
`alpha=5//100=0.05`
The ionization of acetic acdic acid can be expressed as
`CH_(3)COOH (aq)+H_2O(l) Leftrightarrow underset(0)(H_(3)O^+ (aq))+underset(0)(CH_(3)COO^(-)(aq))`
conc,, at equilibrium 0.01 (1-0.05) `0.01 xx 0.05`
`[H_(3)O^+]=0.01 xx 0.05`
`=5 xx 10^(-4)"mol L^(-)`
Hence, `pH =-log_(10) (5 xx 10^(-4))`
=3.3