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The standard enthalpy of formation of ammonia, NH3, is −46.0 kJ mol−1. If the enthalpy change for forming H2 from gaseous atoms is −436 kJ mol−1 and that for forming N2 from gaseous atoms is −712 kJ mol−1, what is the average bond enthalpy of an N−H bond 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 bond enthalpy of an N−H bond in NH3 can be calculated by considering the enthalpy changes associated with the formation of NH3 from its elements in their gaseous state. The formation of NH3 releases energy, and by using the given enthalpy changes for the formation of H2 and N2, we can derive the bond enthalpy, which results in the correct answer of +352 kJ mol−1.

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