 Enthalpy changes also occur when a compound is formed from elements.  Standard enthalpy of formation, ΔHº f, is the heat released or absorbed when one.

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 Enthalpy changes also occur when a compound is formed from elements.  Standard enthalpy of formation, ΔHº f, 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 ΔHº f, the more stable the compound. Note that ΔHº f for free elements is zero (0.00 kJ/mol).

ΔHº f is useful because the values can be used to calculate heat of reaction (ΔHº rxn ) under standard conditions following the law of conservation of energy: ΔHº rxn = ΣΔHº f (products) – ΣΔHº f (reactants)

1. calculate the heat of reaction of sulfur trioxide: 2 SO 2 (g) + O 2 (g)  2 SO 3 (g) from the reference table: ΔHº f SO 2 (g) = kJ/mol ΔHº f O 2 (g) = 0.00 kJ/mol ΔHº f SO 3 (g) = kJ/mol

2. calculate the heat of reaction of the following reaction: 2 NH 3 (g) + 3 Cl 2 (g)  N 2 (g) + 6HCl(g)

 Complete calculations on page 548: ◦ #21-23 ◦ #33