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Given that the standard enthalpy of formation of NH₃ is -46.0 kJ mol⁻¹, and the enthalpy of formation of H₂ from gaseous atoms is -436 kJ mol⁻¹ while that of N₂ from gaseous atoms is -712 kJ mol⁻¹, what is the average N–H bond enthalpy in NH₃?

1. -964 kJ mol⁻¹
2. +352 kJ mol⁻¹
3. -1056 kJ mol⁻¹
4. -1102 kJ mol⁻¹

Correct answer: +352 kJ mol⁻¹

Solution

The average N–H bond enthalpy in NH₃ can be calculated by considering the enthalpy changes associated with the formation of NH₃ from its elements. The positive value indicates that energy is required to break the N–H bonds, which is consistent with bond enthalpy being a measure of bond strength.

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