Thermodynamics

Thermodynamics

Question

The standard enthalpy of formation of NH3 is– 46.0 kJmol–1. If the enthalpy of formation of H2 from its atoms is – 436 kJ mol–1 and that of N2 is – 712 kJ mol–1,the average bond enthalpy of N – H bond is NH3 is

  • -964 kJ mol-1

  • +352 kJ mol-1

  • +1056 kJ mol-1

  • -1102 kJ mol-1

Answer

B.

+352 kJ mol-1

NH subscript 3 space left parenthesis straight g right parenthesis space rightwards arrow with space on top space 1 half space straight N subscript 2 space left parenthesis straight g right parenthesis space space plus space 3 over 2 space straight H subscript 2 space left parenthesis straight g right parenthesis
increment straight H degree space equals space minus space increment minus straight H degree subscript straight f space left parenthesis NH subscript 3 right parenthesis space equals space minus space left parenthesis negative 46 right parenthesis space equals space 46 space kJ space mol to the power of negative 1 end exponent
Also comma space increment straight H degree space equals space 3 increment straight H subscript straight N minus straight H end subscript space plus 1 half increment straight H subscript straight N identical to straight N end subscript space plus 3 over 2 space increment straight H subscript straight H minus straight H end subscript
space 46 space equals space 3 space increment straight H subscript straight N minus straight H end subscript space plus 1 half space left parenthesis negative 712 right parenthesis space plus space 3 over 2 space left parenthesis negative 436 right parenthesis
increment straight H subscript straight N minus straight H end subscript space equals space 1 third left square bracket 1056 right square bracket space equals space plus 352 space kJ space mol to the power of negative 1 end exponent

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