Question

For the reaction MgSO3(s) → MgO(s) + SO2(g), which is spontaneous only at high temperatures, one would predict thata. ΔH˚ is negative and ΔS˚ is negativeb. ΔH˚ is positive and b. ΔS˚ is negativec. ΔH˚ is positive and ΔS˚ is positived. ΔH˚ is negative and ΔS˚ is positive

Answer 1

Answer: c. ΔH˚ is positive and ΔS˚ is positive

Explanation:

According to Gibb's equation:

$\Delta G=\Delta H-T\Delta S$

$\Delta G$ = Gibbs free energy  

$\Delta H$ = enthalpy change

$\Delta S$ = entropy change  

T = temperature in Kelvin

$\Delta G$= +ve, reaction is non spontaneous

$\Delta G$= -ve, reaction is spontaneous

$\Delta G$= 0, reaction is in equilibrium

Thus for $\Delta G=-ve$

Case :

$T\Delta S$ > $\Delta H$

when $\Delta S\text{ and }\Delta H$ both have positive values.

$\Delta G=(+ve)-T(+ve)$

$\Delta G=(+ve)(-ve)=-ve$

Reaction is spontaneous only at at high temperatures.