The Summation Form of Hess's Law
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A reaction's standard enthalpy change equals the summed formation enthalpies of its products minus those of its reactants, weighted by coefficient.
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This summation form treats a reaction as decomposing every reactant to its elements, then reforming them into every product, in one equation.
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Summing formation enthalpies without weighting each one by its own coefficient ignores how much of each substance actually reacts.