Question

# The oxidation of SO2 by O2 to SO3 is an exothermic reaction. The yield of SO3 will be maximum if

A

Temperature is increased and pressure is kept constant

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B

Temperature is reduced and pressure is increased

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C

Both temperature and pressure are increased

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D

Both temperature and pressure are reduced

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Solution

## The correct option is B Temperature is reduced and pressure is increased It states that if a system in equilibrium is subjected to a change of temperature, pressure or concentration, then the equilibrium shifts in such a way that the effect of the changed condition is nullified. Thus,according to Le Chatelier's principle, (i)If the temperature is raised, reaction will proceed in direction in which some heat can be destroyed (absorbed) so that temperature of the system remains constant. Thus, increase in temperature shifts the equilibrium in the forward direction of those reactions which proceed with absorption of heat (endothermic reactions), N2(g)+O2(g)+43.2 kcal⇌2NO(g) and in the backward direction of those reactions which proceed with the evolution of heat (exothermic reactions). N2(g)+3H2(g)⇌2NH3(g)+23 kcal (ii)If the pressure is increased, reaction will take place in a direction which will bring about lowering of pressure.This implies that the equilibrium will shift in the direction which produces the smaller number of gas molecules. For example, PCl5(g)⇌PCl3(g)+Cl2(g) (Backward reaction is favoured with increase of pressure) increase in pressure in the above type of reactions will favour the backward reaction and vice-versa. In case, the reaction proceeds with the decrease in the number of moles, as in N2(g)+3H2(g)⇌2NH3(g)+23 kcal (Forward reaction is favoured with increase of pressure) increase in pressure will favour the forward reaction. It is important to note that change in pressure has no effect on equilibrium if the total volume (number of moles) of the reactants and products is same, such as in the following systems. CO+H2O⇌CO2+H2 2HI⇌H2+I2 Since, increase in pressure does not affect the volume of solids and liquids to an appreciable extent, the number of moles per unit volume also remains practically constant. Hence, the equilibrium is not affected by change of pressure of reaction involving solid or liquid system. (iii)if concentration of the reactants is increased or product is removed, the reaction will take place in the forward direction. Thus in short NoChange made at equilibriumShift of Equilibrium to1Addition of productBackward Reaction2Removal of productForward direction3Removal of reactantBackward Reaction4Addition of reactantForward direction Since, effect of catalyst speeds up both the forward and backward reactions, in a reversible reaction, to the same extent, it does not have any effect on equilibrium point. Thus, it helps in achieving the equilibrium state quickly without shifting it in one direction or the other. (v)Effect of inert gas. For a reaction at constant volume, i.e., when the number of moles of reactants and products is same, the addition of inert gas has no effect. For a reaction at constant pressure, addition of an inert gas will shift the equilibrium in that direction in which there is increase in the number of moles of gases.The reaction is 2SO2(g)+O2(g)⇌2SO3(g);ΔH=−ve. Since the reaction is exothermic, more of SO3 will be formed if temperature is lowered. Since the reaction involves only gaseous phase reactants and products, increase in pressure will cause the formation of more of SO3 or decrease in total number of moles, thus compensating for the increase in pressure.

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