The enthalpy of formation of water is –285.8 kJ/mol. What does this value indicate?

• A. The enthalpy of formation of water is positive, indicating an endothermic reaction.
• B. The formation of 1 mol of H2O from its elements absorbs 285.8 kJ of energy.
• C. The formation of 1 mol of H2O from its elements releases 285.8 kJ of energy.
• D. 285.8 kJ of energy is required to decompose 1 mol of H2O into its elements.

Answer: (C)

Verified step by step guidance

1. Understand that the enthalpy of formation (\( \Delta H_f^\circ \)) of a compound is the heat change when 1 mole of the compound is formed from its elements in their standard states.
2. Note that the given enthalpy of formation for water is \( -285.8 \, \text{kJ/mol} \), where the negative sign indicates the direction of heat flow.
3. Recognize that a negative enthalpy of formation means the reaction is exothermic, so energy is released when water forms from hydrogen and oxygen.
4. Interpret the value \( -285.8 \, \text{kJ/mol} \) as the amount of energy released when 1 mole of water is formed from its elements under standard conditions.
5. Understand that the reverse process, decomposing 1 mole of water into hydrogen and oxygen, would require an input of \( 285.8 \, \text{kJ} \) of energy, making it endothermic.