Which of the following reactions represents the standard enthalpy of formation (ΔH_f^∘) for water (H_2O(l))?

• A. H_2(g) + O_2(g) → H_2O_2(l)
• B. H_2(g) + 1/2 O_2(g) → H_2O(l)
• C. 2 H_2(g) + O_2(g) → 2 H_2O(l)
• D. H_2O(l) → H_2(g) + 1/2 O_2(g)

Answer: (B)

Verified step by step guidance

1. Understand that the standard enthalpy of formation (\( \Delta H_f^\circ \)) of a compound is defined as the enthalpy change when one mole of the compound is formed from its elements in their standard states under standard conditions.
2. Identify the elements involved in forming water (\( \text{H}_2\text{O}(l) \)) and their standard states: hydrogen gas (\( \text{H}_2(g) \)) and oxygen gas (\( \text{O}_2(g) \)).
3. Write the formation reaction such that exactly one mole of water is produced from its elements in their standard states. This means the coefficients must reflect the formation of one mole of \( \text{H}_2\text{O}(l) \).
4. Balance the reaction so that the number of atoms of each element is conserved. Since water has two hydrogen atoms and one oxygen atom, the balanced reaction should have \( 1 \) mole of \( \text{H}_2\text{O}(l) \), \( 1 \) mole of \( \text{H}_2(g) \), and \( \frac{1}{2} \) mole of \( \text{O}_2(g) \).
5. Confirm that the reaction represents formation (elements to compound) and not decomposition (compound to elements), and that it produces exactly one mole of the compound, which matches the standard enthalpy of formation definition.