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Hess's Law

Hess's Law is used to predict ΔH˚r in enthalpy changes.

Addition of chemical equations can lead to a net equation. If energy is included for each equation and added, the result will be the energy for the net equation.

Hess's Law says that heat of enthalpy are additive.
ΔH(net) = ΣΔH(r), equals the sum of enthalpies of reaction.
Two rules:
* If the chemical equation is reversed, the sign of ΔH changes.
* If coefficients are multiplied, multiply ΔH by the same amount.

see also Chemistry