Hess' Law

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Heat Summation

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This method is used to determine the change in enthalpy (ΔH) of a reaction or the enthalpy/heat of formation (ΔHf) of a substance.

Example - Find ΔH of C(s) + 1/2O2(g) → CO(g), given:

ΔH = -393.1 kJ for C(s) + O2(g) → CO2(g)
ΔH = -282.7 kJ for CO(g) + 1/2O2(g) → CO2(g)

Imagine:

ΔH = -393.1 kJ ; C(s) + O2(g) → CO2(g)

Then:

ΔH = +282.7 kJ ; CO2(g) → CO(g) + 1/2O2(g)

Net:

ΔH = -110.4 kJ ; C(s) + 1/2O2(g) → CO(g)