Solution :-
Balanced reaction equation
      Â
             C4H4N2O2(s)
   +   4 O2(g) ------
> 4 CO2(g) Â Â Â +
    2 H2O(g)
    +
     N2(g)
Delta Hf   Â
-424.4Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
0Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
      -393.5            Â
-241.8Â Â Â Â Â Â Â Â
        0
      Â
In kJ/mol
Now lets calculate the enthalpy change for the reaction
Delta H rxn = sum of delta Hf of product – sum of delta Hf or
reactant
                   Â
=[(CO2*4)+(H2O*2)] – [C4H4N2O2*1]
                  Â
= [(-393.5*4)+(-241.8*2)] – [ -424.4*1]
                  Â
= -1633.2 kJ
Therefore the enthalpy of the combustion of the uracil is
-1633.2 kJ per mol
Negative sign is because combustion process is exothermic that
means it gives off heat.
So we can also write it as
Enthalpy of combustion of uracil = 1633.2 kJ per mol
