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).

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 = 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

2. calculate the heat of reaction of the following reaction:2 NH3(g) + 3 Cl2(g) N2(g) + 6HCl(g)

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