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Using the thermochemical information below, and taking ΔfG° for H+ as 0: H2O(l) → H+(aq) + OH−(aq); ΔH = 57.32 kJ H2(g) + 1/2 O2(g) → H2O(l); ΔH = −286.20 kJ What is the standard enthalpy of formation of the OH− ion at 25°C?
- −22.88 kJ
- −228.88 kJ
- +228.88 kJ
- −343.52 kJ
Correct answer: −228.88 kJ
Solution
The standard enthalpy of formation for the OH− ion can be derived by rearranging the given reactions. By combining the enthalpy change of water formation and the dissociation of water into H+ and OH−, we find that the formation of OH− from its elements results in an enthalpy of −228.88 kJ.
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