Question

The enthalpies of combustion in a constant-volume calorimeter for fumaric and maleic acids are -1337.21kJ mol^-1 and -1306.43 kJ mol ^-1, respecively at approximately 25^oC.

a) Calculate the enthalpies of formation of these isomers.

b) What is the difference in enthalpy between these isomers.

Answer 1

formula for fumaric and maleic acids is C4H4O4
Their conbustion reaction is:
C4H4O4 + 3O2   ------> 4CO2 (g) + 2H2O (l)
deltaHf (CO2) = -393.51 KJ/mol
deltaHf (H2O) = -285.83 KJ/mol
deltaHf (O2) = 0 KJ/mol
a)
for fumaric acid:
C4H4O4 + 3O2   ------> 4CO2 (g) + 2H2O (l)
delta H(combustion) = 4*deltaHf (CO2) + 2*deltaHf (H2O) - delta Hf (C4H4O4) - 3*delta Hf (O2)
-1337.21 = 4* (-393.51) + 2*(-285.83) - delta Hf (C4H4O4) - 3*0
delta Hf (C4H4O4) = -808.49 KJ/mol   <-----Answer

for maleic acid:
C4H4O4 + 3O2   ------> 4CO2 (g) + 2H2O (l)
delta H(combustion) = 4*deltaHf (CO2) + 2*deltaHf (H2O) - delta Hf (C4H4O4) - 3*delta Hf (O2)
-1306.43 = 4* (-393.51) + 2*(-285.83) - delta Hf (C4H4O4) - 3*0
delta Hf (C4H4O4) = -839.27 KJ/mol <----Answer

b)
difference in enthalphy = -808.49 KJ/mol - ( -839.27 KJ/mol ) = 30.78 KJ/mol