In which of the following reactions does the heat change represent the heat of formation of water? - Chemistry

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MCQ

In which of the following reactions does the heat change represent the heat of formation of water?

Options

  • \[\ce{2H2_{(g)} + O2_{(g)} -> 2H2O_{(l)}}\]; ∆H = −116 kcal

  • \[\ce{H2_{(g)} + 1/2O2_{(g)} -> H2O_{(l)}}\]; ∆H = −58 kcal

  • \[\ce{H^+_{( aq.)} + OH^-_{( aq.)} -> H2O_{(l)}}\]; ∆H = −13.7 kcal

  • \[\ce{C2H2_{(g)} + 5/2O2_{(g)} -> 2CO2_{(g)} + H2O_{(l)}}\]; ∆H = −310 kcal

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Solution

\[\ce{H2_{(g)} + 1/2O2_{(g)} -> H2O_{(l)}}\]; ∆H = −58 kcal

Explanation:

Heat of formation is the formation of one mole of the substance from its constituent elements.

1. In 2 moles of water are formed hence it cannot be considered as heat of formation of water.

2. In 1 mole of water is formed but not from its elements in their standard state.

3. The ruled out as H2O is not formed from its constituent elements.

Concept: Enthalpy (H)
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