Question

Given the data

?G ? f ?H? f

CuO(s) ?129.7 ?157.3

HCN(g) +124.7 +135.1

NH3(g) ?16.45 ?46.11

PCl5(g) ?305.0 ?374.9

which compound will become more stable with respect to its elements if the temperature is raised?

1. NH3(g) 2. PCl5(g) 3. HCN(g) 4. CuO(s)

Answer 1

According to Le chatelier principle

         If the reaction is exothermic (?H is negative, puts energy out), heat is included as a product, and the reaction is reactants favored when temperature is raised. If the reaction is endothermic (?H is positive, takes energy in), heat is included as a reactant and the reaction is products favored when temperature is raised.

From the given data all the given compounds except HCN(g) have negative enthalpy of formation. That means their formation reactions are exthermic reactions. So these three compounds (NH3, PCl5 and CuO) are less stable with respect to its elements if the temperature is raised.

HCN(g) has positive enthalpy of formation so it is more stable with respect to its elements if the temperature is raised.

Hence option 3. HCN(g) is correct