Molar Enthalpies of Formation

1. Molar Enthalpies of Formation Chemistry 30 – Chapter 11.5 Freiman

2. Hess’s Law can be expressed mathematically: ΔrH = ∑ΔfH(products) – ∑ΔfH(reactants) or ΔrH = ∑ΔfpH- ∑ΔfrH • The procedure for finding enthalpy change of a reaction is essentially the same as Hess’s Law, except you are omitting the step where you write out all of the formation equations. • Step 1 – Balanced equation • Step 2 – Plug enthalpy of formation values into equation and solve for ΔH. • Step 3 – Solve for ΔHm if necessary

3. Using Molar Enthalpies of Formation • The molar enthalpy of formation refers to the energy required to form a compound from its elements • The molar enthalpy of formation for many compounds is found in your data book • Since this is enthalpy of FORMATION, and this type of reaction cannot produce elements, the ΔfHm for any element is ZERO.

4. Example • Find the molar enthalpy of combustion for propane using molar enthalpies of formation. C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) 1. ΔH = ∑ΔfpH - ∑ΔfrH [3 mol (-393.5 kJ/mol) + 4 mol (-241.8 kJ/mol] - [1 mol (-103.8 kJ/mol)] = -2043.9 kJ

5. 2. ΔHm= ΔH propane n = -2043.9 kJ 1 mol = -2043.9 kJ/mol

6. Example 2 • Given the following reaction, what is the molar enthalpy of reaction for HCl? Ca(OH)2(s) + 2HCl(g) CaCl2(s) + 2H2O(l)

8. Example 3 • If the molar enthalpy of combustion of CS2 is -1075.1 kJ/mol, what is the molar enthalpy of formation for carbon disulfide? • CS2 + 3O2  CO2 + 2SO2