Problem: Find
δH for the reaction of sulfur dioxide with oxygen to form
sulfur trioxide given the following heats of formation:
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δHfSO2 = - 296.8 kJ / mole |
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δHfSO3 = - 395.7 kJ / mole |
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Solution: First write a balanced equation:
In this case, no conversion of moles or mole ratio is necessary. Based on the
coefficients, simply assume 2 moles
2SO2(g) and 1 mole
O2(g). You want to find
δH for the basic equation not a special case.
You probably noticed that the
δHf for
O2
was not given. It is important to realize that the heat of formation for any
element in its basic state is arbitrarily set to 0 kJ / mole. The diatomic
molecules
H2(g),
N2(g),
O2(g),
F2(g),
Cl2(g),
Br2(l), and
I2(s) are
included in the grouping of elements in their fundamental states. Knowing this,
let's find the components needed for the
δH equation.
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δH (reactants) = + = - 593.6 kJ |
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δHf products = = - 791.4 kJ |
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Now solve.
| δH = δHF (products) - δHF (reactants) = - 791.4 kJ - (- 593.6 kJ) = - 197.8 kJ |
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