Lesson 7 of 12 · 6 min
Hess's law
NCERT § "Hess's Law of Constant Heat Summation"
Riya's stove sometimes burns with a yellow, sooty flame. That means incomplete burning, and carbon monoxide. Its formation enthalpy cannot be measured directly, yet the tables list it.
The lesson in notes
In short
Because enthalpy is a state function, the total enthalpy change of a reaction is the same whether it happens in one step or in several.
Thermochemical equations can therefore be added, subtracted and scaled like algebraic equations to obtain an unknown ΔᵣH.
A classic use is finding ΔfH° of CO from the combustion enthalpies of carbon to CO₂ and of CO to CO₂, because carbon cannot be converted cleanly to CO alone.
Combustion data give formation enthalpies: for a hydrocarbon, ΔcH° = Σ ΔfH°(CO₂ and H₂O formed) − ΔfH°(hydrocarbon), since ΔfH° of O₂ is zero.
When combining equations, carry each species' physical state; H₂O(l) and H₂O(g) have different ΔfH° values.
Watch a class
Prefer a video? Watch this
Hess cycles worked through step by step
Najam Academy · English · Solved problems · Open on YouTube