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The heat of formation of a substance is defined as the enthalpy change when 1 mole of that substance is formed from its constituent elements in their gaseous standard states. Which of the following reactions does NOT represent the heat of formation?

1. 1/2 N2(g) + 3/2 H2(g) -> NH3(g)
2. C(s) + O2(g) -> CO2(g)
3. H2(g) + O2(g) -> H2O(l)
4. Xe(g) + 2F2(g) -> XeF4(s)

Correct answer: H2(g) + O2(g) -> H2O(l)

Solution

Option A: 1/2 N2 + 3/2 H2 -> NH3 is balanced and forms 1 mol NH3. Option B: C(s) + O2(g) -> CO2(g) is balanced and forms 1 mol CO2 from elements in standard states. Option C: H2(g) + O2(g) -> H2O(l) is NOT balanced (2 O atoms in, 1 O atom out) and does not represent a valid formation reaction. Option D: Xe(g) + 2F2(g) -> XeF4(s) is balanced and forms 1 mol XeF4. Hence option C does not represent heat of formation.

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