What is the standard enthalpy of formation (ΔH_f^ext{°}) for H_2O(l) in kJ/mol?

• A. 0
• B. -285.8
• C. +285.8
• D. -241.8

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 (usually 1 atm and 25°C).
2. Identify the elements and their standard states for water (\( \text{H}_2\text{O}(l) \)): hydrogen gas (\( \text{H}_2(g) \)) and oxygen gas (\( \text{O}_2(g) \)) are the elements in their standard states.
3. Write the formation reaction for liquid water from its elements: \[ \text{H}_2(g) + \frac{1}{2} \text{O}_2(g) \rightarrow \text{H}_2\text{O}(l) \]
4. Recall that the standard enthalpy of formation for any element in its standard state is zero, so \( \Delta H_f^{\circ} \) for \( \text{H}_2(g) \) and \( \text{O}_2(g) \) is zero.
5. Use tabulated thermodynamic data or a reliable source to find the value of \( \Delta H_f^{\circ} \) for \( \text{H}_2\text{O}(l) \), which is the enthalpy change for the reaction above, typically given in kJ/mol.