Calculate the heat of formation of propane if the heat of combustion is -2220kj/mol.The heat of formation of carbon (IV) Oxide and water are -393.5kj/mol respectively.Show steps by using Hess's law diagram.
Let ΔHf be the heat of formation of propane (C3H8), which we want to calculate. The other two ΔH we know are the heat of formation of CO2 (-393.5 kJ/mol) and H2O (-285.8 kJ/mol). The heat of combustion of propane is -2220 kJ/mol.
Hess's law states that the total enthalpy change of a chemical reaction is independent of the path and depends only on the initial and final states. In other words, we can consider any chemical equation (or reactions) that lead us from some initial reactants to final products, and the ΔH values for each reaction will add up to the desired total.
We can break the combustion of propane into the following steps:
1) Formation of propane from its elements (carbon and hydrogen):
3 C(s) + 4 H2(g) -> C3H8(g) ΔH1 = ΔHf (propane)
2) Combustion of propane to form CO2 and H2O:
C3H8(g) + 5 O2(g) -> 3 CO2(g) + 4 H2O(l) ΔH2 = -2220 kJ/mol
3) Formation of CO2 and H2O from their respective elements:
3 C(s) + 6 H2(g) -> 3 CO2(g) + 4 H2O(l) ΔH3 = 3*-393.5 + 4*-285.8 = -2644.7 kJ/mol
Now, we will apply Hess's law. Since step 1 and step 2 add up to step 3:
ΔH1 + ΔH2 = ΔH3
ΔHf (propane) -2220 kJ/mol = -2644.7 kJ/mol
Now, we'll solve for the heat of formation of propane, ΔHf:
ΔHf (propane) = -2644.7 kJ/mol + 2220 kJ/mol = -424.7 kJ/mol
Thus, the heat of formation of propane is -424.7 kJ/mol.