Enthalpy of formation - ΔHºf

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Presentation transcript:

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).

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)

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) = -296.8 kJ/mol ΔHºf O2(g) = 0.00 kJ/mol ΔHºf SO3(g) = -395.7 kJ/mol

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)

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