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Which of the following equations corresponds to the definition of enthalpy of formation at \( 298 K ? \)

1. \( C(\text {graphite})+2 H_{2}(g)+1 / 2 O_{2}(l) \rightarrow C H_{3} O H(g) \)
2. \( C(\text { diamond })+2 H_{2}(g)+1 / 2 O_{2}(g) \rightarrow C H_{3} O H(l) \)
3. \( 2 C(\text { graphite })+4 H_{2}(g)+O_{2}(g) \quad \rightarrow 2 C H_{3} O H(l) \)
4. \( C(\text {graphite})+2 H_{2}(g)+1 / 2 O_{2}(g) \rightarrow C H_{3} O H(l) \)

Correct answer: \( C(\text {graphite})+2 H_{2}(g)+1 / 2 O_{2}(g) \rightarrow C H_{3} O H(l) \)

Solution

The standard enthalpy of formation is written for forming exactly 1 mole of a compound from its constituent elements in their standard states at 298 K. For methanol, that means carbon as graphite, hydrogen as H2(g), and oxygen as O2(g), producing CH3OH(l).

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