Determining the Standard Enthalpy of Formation

How can we determine the standard enthalpy of formation of a compound?

Given the standard enthalpies of combustion for hydrogen, graphite, and propane, how can we calculate the standard enthalpy of formation of propane?

Calculation of Standard Enthalpy of Formation

To determine the standard enthalpy of formation of a compound, we can use the standard enthalpies of combustion of its constituent elements. In this case, we are interested in finding the standard enthalpy of formation of propane using the given values for hydrogen, graphite, and propane.

The standard enthalpy of formation (ΔHf°) of a compound can be calculated using the standard enthalpies of combustion (ΔHc°) of its constituent elements. The equation used is ΔHf° = ΣΔHc°(products) - ΣΔHc°(reactants).

For propane, the combustion reaction is C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l), which has a ΔHc° of -2220 kJ/mol. The standard enthalpy of combustion for hydrogen (H2) is -241.8 kJ/mol, and for graphite (C) is -393.5 kJ/mol.

Using the equation, we can calculate the standard enthalpy of formation of propane as follows:

ΔHf°(propane) = ΣΔHc°(products) - ΣΔHc°(reactants)

= [3(-393.5 kJ/mol) + 4(-241.8 kJ/mol)] - (-2220 kJ/mol)

= -1036.9 kJ/mol

Therefore, the standard enthalpy of formation of propane is -1036.9 kJ/mol.