1. Calculating ΔH using molar heats of formation
Chem 12

2. If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation, Hof .
Standard conditions (standard state): 1 atm and 25 oC (298 K).
Standard enthalpy of formation: 1 mol of compound is formed from substances in their standard states.

3. Examples – write formation reactions for each:
*Remember the compounds are formed directly from their elements.
H2SO4
NH4Cl

4. You want only one mole of the product being formed.
Look up the ΔHf on the table
H2 + S + 2O2  H2SO4 ΔHf = -814 kJ/ mol
2. ½ N2 + 2H2 + ½ Cl2  NH4Cl
ΔHf = kJ/ mol

5. Enthalpies of Formation

6. Using Enthalpies of Formation for Calculating Enthalpies of Reaction
For a reaction

7. By definition, the enthalpy of formation of an element in its standard state is zero.
Example, oxygen (O2) and chlorine (Cl2) both have Hof of zero.

8. Sample Problem 1
Calculate ΔH for the following reaction using standard molar heats of formation, ΔH°f .
2NH3(g) + 3Cl2(g) → N2(g) + 6HCl(g)
ΔH = ?

9. ΔH°f for NH3(g) = -45.9 kJ/mol
ΔH°f for HCl(g) = kJ/mol
ΔH°f for Cl2(g) and N2(g) is 0
ΔHrxn = Σ nΔH°f(product) - Σ nΔH°f(reactant)
ΔHrxn= (0 + 6(-92.3 kJ)) - (2(-45.9 kJ) + 0)
= (-553 kJ) - (-91.8 kJ)
= -461 kJ

10. Examples
Use the tables in the back of your book to calculate ΔH for the following reactions:
4 CuO (s)  2 Cu2O (s) + O2 (g)
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l)
NH3 (g) + HCl (g)  NH4Cl (s)
kJ kJ kJ

11. Practice:
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