Question

Write the combustion reaction for uracil (C4H4N2O2 (s)) and calculate the standard enthalpy for the combustion reaction of uracil at 298.15 K, using only the standard formation enthalpies for uracil (s), CO2 (g) and H2O (l), which you will find online.

Answer 1

Solution :-

Balanced reaction equation

                     C₄H₄N₂O₂(s)    +    4 O₂(g) ------ > 4 CO₂(g)    +     2 H₂O(g)     +      N₂(g)

Delta Hf     -424.4                          0                      -393.5              -241.8                 0        

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

                     =[(CO2*4)+(H2O*2)] – [C4H4N2O2*1]

                    = [(-393.5*4)+(-241.8*2)] – [ -424.4*1]

                    = -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