Question

What are the most favourable conditions for the reaction;
${SO}_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\leftrightharpoons {SO}_3\left(g\right);\Delta H^o=-ve$ to occur?

• A. Low temp and high press
• B. Low temp and low press
• C. High temp and low press
• D. High temp and high press

Answer 1

The correct option is A Low temp and high press

Negative value of enthalpy change suggests exothermic reaction. Such reactions are favored at low temperature as heat released during the reaction raised the temperature and nullifies the effect of low temperature.
$\Delta n=\text{number of moles of gaseous products}-\text{number of moles of gaseous reactants}$
$\Delta n=1-(1+\frac{1}{2})$
$\Delta n=-\frac{1}{2}$
Since$\Delta n<0$, the reaction is favored at high pressure. Increasing the pressure will shift the reaction towards forward direction (one with less number of moles of gaseous species). This will nullify the effect of increase in pressure.