Chemistry / Unit 4 Chem 0715 short lesson, about 20 seconds
The Summation Form of Hess's Law
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The idea
A reaction's standard enthalpy change equals the summed formation enthalpies of its products minus those of its reactants, weighted by coefficient.
In practice
This summation form treats a reaction as decomposing every reactant to its elements, then reforming them into every product, in one equation.
Where people slip
Summing formation enthalpies without weighting each one by its own coefficient ignores how much of each substance actually reacts.
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