1. Enthalpy of formation - ΔHºf
Enthalpy changes also occur when a compound is formed from elements.
Standard enthalpy of formation, ΔHºf, is the heat released or absorbed when one mole of a compound is formed by combination of its elements at standard conditions (25˚C, 1 atm).
The more negative the value of ΔHºf, the more stable the compound. Note that ΔHºf for free elements is zero (0.00 kJ/mol).

3. Enthalpy of formation - ΔHºf
ΔHºf is useful because the values can be used to calculate heat of reaction (ΔHºrxn) under standard conditions following the law of conservation of energy:
ΔHºrxn = ΣΔnprodHºf(products) – ΣΔnreactHºf(reactants)

4. ΔHºrxn = ΣΔHºf (products) – ΣΔHºf (reactants)
1. calculate the heat of reaction of sulfur trioxide: 2 SO2(g) + O2(g)  2 SO3(g)
from the reference table: ΔHºf SO2(g) = kJ/mol
ΔHºf O2(g) = 0.00 kJ/mol
ΔHºf SO3(g) = kJ/mol

5. ΔHºrxn = ΣΔHºf (products) – ΣΔHºf (reactants)
2. calculate the heat of reaction of the following reaction:
2 NH3(g) + 3 Cl2(g)  N2(g) + 6HCl(g)

6. Assignment
Complete calculations on page 548:
#21-23
#33