Question

How can you calculate the heating value of liquid propane? Please show an equation

Answer 1

C3H8(g) + 5O2(g) ---> 3CO2(g) + 4H2O(l) ...dH = -2220.1 kJ/mol

Delta H subscript (f) means "the enthalpy of formation" of the substance under the standard state conditions (25 C and 1 atm).

Enthalpies of formations of gaseous CO2 and liquid water are given. Heat of combustion (nethalpy of combustion) of propane is also known, then;
delta Hf for C3H8 can be calculated from the enthalpy of the reaction. By definition; the enthalpy of a reaction (delta Hr);
delta Hr = Total delta Hf (products) - Total delta Hf (reactants)

Note: The enthalpy of formation of the most stable form of the elements is zero.

dH(combustion) = [3dHf (CO2) + 4dHf (H2O)] - [5dHf (O2) + dHf (C3H8)]

-2220.1 = [3(-393.5) + 4(-285.3)] - [ 0 + dHf (C3H8)]

-2220.1 = [-1180.5) - 1141.2] - dHf (C3H8)
-2220.1 = - 2321.7 - dHf (C3H8)
dHf (C3H8) = 2220.1 - 2321.7 = - 101.6 kJ/mol