Question

The standard heat of formation, ΔH∘f, is defined as the enthalpy change for the formation of one mole of substance from its constituent elements in their standard states. Thus, elements in their standard states have ΔH∘f=0. Heat of formation values can be used to calculate the enthalpy change of any reaction.

Consider, for example, the reaction

2NO(g)+O2(g)⇌2NO2(g)

with heat of formation values given by the following table:

Substance	ΔH∘f (kJ/mol)
NO(g)	90.2
O2(g)	0
NO2(g)	33.2

Then the standard heat of reaction for the overall reaction is

ΔH∘rxn===ΔH∘f(products)2(33.2)−114 kJ

For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)?

You do not need to look up any values to answer this question.

Check all that apply.

Hints

Check all that apply.

Na(s)+12Cl2(g)→NaCl(s)

Na(s)+12Cl2(l)→NaCl(s)

SO3(g)→12O2(g)+SO2(g)

2Na(s)+Cl2(g)→2NaCl(s)

SO(g)+12O2(g)→SO2(g)

S(s)+O2(g)→SO2(g)

Answer 1

ΔH^∘_rxn = ΔH^∘_f [product(s)]

This will be true only when all the reactants used in given chemical reaction are in their native state.

This is the case with reactions

Na(s)+12Cl2(g)→NaCl(s)

2Na(s)+Cl2(g)→2NaCl(s)

S(s)+O₂(g)→SO₂(g)

Where, Na (s), Cl₂ (g) S(s) & O₂(g) both are in the so-called elemental state.

Note: Cl2(l) or Cl2 (s) state are not elemental states.

Hence Answe options : (1), (4), (6).

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