Hess's LawJEE Main

Enthalpy is a state function — paths don't matter — interactive Chemistry simulation for IIT-JEE.

Concept

Enthalpy is a state function: ΔH between two states is fixed no matter what route the reaction takes. So thermochemical equations can be added, reversed and scaled like algebra — measure the easy steps, deduce the impossible-to-measure one. Burning C to CO cleanly is impossible in a lab; Hess's law gets ΔH anyway.

Key formula

\Delta H_{direct} = \sum \Delta H_{steps}: \qquad \Delta H_1 = \Delta H_2 + \Delta H_3

Derivation

H depends only on the state (P, T, composition). Any closed cycle has $\oint dH = 0$ .

Operate on equations: reverse a step → flip the sign; multiply by k → scale ΔH by k; add equations → add ΔH values. The C/CO/CO₂ triangle is the canonical example: ΔH(C→CO) = ΔH(C→CO₂) − ΔH(CO→CO₂) = −393.5 − (−283) = −110.5 kJ/mol.

Scenarios to explore

Hess's Law — Enthalpy is a state function — add routes like algebra.

Real-world applications

Formation enthalpies of unstable species (CO, benzene from atomisation).
Born–Haber cycles (lattice enthalpy).
Fuel-value bookkeeping in metallurgy & biology.

JEE exam tips

ΔH_rxn = ΣΔH_f(products) − ΣΔH_f(reactants) — Hess in its most-used costume.
Via combustion: ΔH_rxn = ΣΔH_c(reactants) − ΣΔH_c(products) — note the flip.
Set up the cycle diagram first; arrows and signs stop being confusing.

Common mistakes

Forgetting to flip ΔH's sign when reversing an equation.
Adding equations without cancelling common species.
Mixing enthalpies of formation with combustion mid-calculation.

Exam traps to avoid

ΔH_f of any ELEMENT in its standard state is zero.
Bond-energy estimates of ΔH work only for GAS-phase reactions.