6.9 Hess's Law/ AP CHEM

(Hess’s Law)

If a reaction is reversed…

inverse the sign of ΔH

(+) to (-)

(Hess’s Law)

If the coefficients of a reaction are multiplied…

multiply ΔH by the same factor

if A → B has ΔH = +50 kJ

then 2A → 2B has ΔH = +100 kJ

(Hess’s Law)

overall enthalpy change of a reaction (ΔH_rxn) is the…

sum of ΔH values of each other smaller steps

ΔH_step1 + ΔH_step2 = ΔH_rxn(overall)

Hess’s Law

total enthalpy change for a rxnis the same, even if its 1 or multiple steps

• enthalpy depends on initial and final states, not the path taken