what is hess's law
Hess’s law says that the overall enthalpy change of a reaction is the same whether it happens in one step or many steps, as long as you start and finish with the same substances under the same conditions.
Quick scoop definition
- Hess’s law (law of constant heat summation) states: the enthalpy change of a chemical reaction depends only on the initial and final states, not on the path taken.
- In symbols, if a reaction can go from A to D directly or via intermediates B and C, then
ΔHA→D=ΔHA→B+ΔHB→C+ΔHC→D\Delta H_{A\to D}=\Delta H_{A\to B}+\Delta H_{B\to C}+\Delta H_{C\to D}ΔHA→D=ΔHA→B+ΔHB→C+ΔHC→D.
Why this works
- Enthalpy is a state function: it depends only on the state (temperature, pressure, composition), not on the history of how you got there.
- This is a consequence of energy conservation (first law of thermodynamics): you cannot get extra energy by choosing a “clever route” that just returns you to where you started.
Simple example
Imagine converting solid carbon to carbon dioxide:
- One-step route: \ce{C(s)+O2(g)->CO2(g)} with some \Delta H.
- Two-step route:
- \ce{C(s)+1/2O2(g)->CO(g)} with \Delta H_1
- \ce{CO(g)+1/2O2(g)->CO2(g)} with \Delta H_2.
Hess’s law says \Delta H=\Delta H_1+\Delta H_2; the total heat change is identical either way.
What it’s used for
- Finding enthalpy changes that are hard to measure directly by adding and subtracting known thermochemical equations like algebra.
- Calculating heats of formation, combustion, and reaction, and supporting energy analysis in areas like fuel combustion and engine efficiency.
In short, Hess’s law lets you “build” tough energy changes from easier ones, knowing the total will always add up the same.
TL;DR: Hess’s law: total reaction enthalpy is path-independent; add the enthalpy changes of individual steps to get the overall change.
Information gathered from public forums or data available on the internet and portrayed here.