5. For which of the following reactions would the heat of reaction be labelled ∆Hf°? a) PCl3 + ½ O2(g) ->POCl3(g) b) CaO(s) + SO2(g) -> CaSO3(s) c) Al(s) + 3/2O2(g) + 3/2H2(g) -> Al(OH)3(s) d) ½ N2O(g) + 1/4 O2(g) -> NO(g)

The reaction Al(s) + 3/2O2(g) + 3/2H2(g) -> Al(OH)3(s) can be labeled as  standard enthalpy of formation  (∆Hf°).

In chemistry, the standard enthalpy of formation refers to the heat evolved or absorbed when a substance is formed from its constituent elements under standard conditions.

As such, we can see that the compound Al(OH)3(s) in option C was formed from its components; aluminium, oxygen and hydrogen therefore, the reaction Al(s) + 3/2O2(g) + 3/2H2(g) -> Al(OH)3(s) can be labeled as  standard enthalpy of formation  (∆Hf°).

Learn more about standard enthalpy of formation : link

C

Explanation:

∆H°f means the enthalpy change of formation of one mole of substance by its constituent elements under standard conditions.

So in an equation for ∆H°f, we must see 2 or more elements as reactants combining to form a compound.

In the 4 answers, only C represents elements forming a substance (Al(s) + 3/2O2(g) + 3/2H2(g) -> Al(OH)3(s)),

while the others include compounds as one of their reactants.

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