Hess's Law Lesson #5

Hess’s Law: Hess believed that the change in enthalpy of a reaction would always be the same no matter how many steps it took to get the desired product.

For any chemical change the net H is equal to the sum of the H values of the separate steps.

Hess’s Law can be used when the molar enthalpy may not be measured with a calorimeter.

Two rules to remember:

1. when you reverse an equation you need to reverse the sign of H (multiply by -1)

2. when you multiply the coefficients of an equation, you need to multiply H by the same number

Heat flow: q=mcΔT

Enthalpy changes: ΔH=nΔH_r

Hess’s Law: ΔH_total = ΣΔH_known