Simulation · Chemistry · Class 11
Hess's law: add the steps
From the lesson Hess's law in Thermodynamics. Change the values and watch what happens.
Hess's law: add the stepsChemistry · Class 11
The idea behind it
NCERT § "Hess's Law of Constant Heat Summation"
- 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.