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Question

The molar heat of formation of NH4 NO3(s) is 367.5KJ and those of N2O(g) and H2O(l) are +81.46KJ and 285.78KJ respectively at 25o C and 1 atmospheric pressure. The U for the reaction
NH4NO3(s)N2O(g) + 2H2O(l)


A

-125.03 kJ

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B

125.03 kJ

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C

120 kJ

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D

-120 kJ

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Solution

The correct option is A

-125.03 kJ


The change in enthalpy can be given by:
Ho=Hof(products)Hof(reactants)
=[Hof(N2O)+2×Hof(H2O)][Hof(NH4NO3)]
=81.46+2×(285.78)(367.5)
Ho=122.56KJ
We know H=U+nRT
U=HnRT
n=1,R=8.314×103kJmol1K1,T=298K
U=122.56(1)(8.314×103)(298)
U=122.562.477
U=125.037kJ


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