Question

In the reaction, $2S{O}_2\left(g\right)+O_2\left(g\right)\rightleftharpoons 2{SO}_3\left(g\right)+Xcal$ most favourable conditions of temperature and pressure for greater yield of ${SO}_2$ are:

• A. Low temperature and low pressure
• B. High temperature and low pressure
• C. High temperature and high pressure
• D. Low temperature and high pressure

Answer 1

Answer: (D)

Low temperature and high pressure

In the reaction, $2S{O}_2\left(g\right)+O_2\left(g\right)\rightleftharpoons 2{SO}_3\left(g\right)+Xcal$ most favourable conditions of temperature and pressure for greater yield of ${SO}_2$ are high pressure and low temperature.
High pressure is maintained because the moles of the product are less than the moles of reactants.
$\Delta n_g=2-(2+1)=-1$
$\Delta n_g<0$
When pressure is increased, the equilibrium will shift to product side (which contains less number of moles of gaseous species). This nullifies the effect of increase in pressure. Hence, more and more product will be formed.
According to Le Chatelier's principle, when a system at equilibrium is disturbed, the position of the equilibrium shifts in a direction so that the effect of the change is nullified.
Low temperature favours exothermic reaction. Heat is evolved during the reaction (positive value of enthalpy change).