Hess' Law

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 (ΔH_f) of a substance.

Example - Find ΔH of C_(s) + 1/2O_2(g) → CO_(g), given:

ΔH = -393.1 kJ for C_(s) + O_2(g) → CO_2(g)

ΔH = -282.7 kJ for CO_(g) + 1/2O_2(g) → CO_2(g)

Imagine:

ΔH = -393.1 kJ ; C_(s) + O_2(g) → CO_2(g)

Then:

ΔH = +282.7 kJ ; CO_2(g) → CO_(g) + 1/2O_2(g)

Net:

ΔH = -110.4 kJ ; C_(s) + 1/2O_2(g) → CO_(g)