# 5.3 Hess's Law and Enthalpies of Formation

PRE-HEALTH PREP

## Hess’s Law

3NO(g) ➔ N2O(g) + NO2(g)           ΔH°rxn = ?

THERMODYNAMIC DATA
2NO2(g) ➔ 2NO(g) + O2(g) ΔH° = +113kJ
2N2(g) + O2(g) ➔ 2N2O(g) ΔH° = +163kJ
N2(g) + O2(g) ➔ 2NO(g) ΔH° = +181kJ

C2H4(g) + 6F2(g) ➔ 2CF4(g) + 4HF(g)          ΔH°rxn = ?

THERMODYNAMIC DATA
H2(g) + F2(g) ➔ 2HF(g) ΔH° = -537kJ
C(s) + 2F2(g) ➔ CF4(g) ΔH° = -680kJ
2C(s) + 2H2(g) ➔ C2H4(g) ΔH° = +52.3kJ

C3H8(g) + 5O2(g) ➔ 3CO2(g) + 4H2O(l)     ΔH°rxn = ?

THERMODYNAMIC DATA
2C3H8(g) + 7O2(g) ➔ 6CO(g) + 8H2O(g) ΔH° = -2390kJ
CO(g) + ½O2(g) ➔ CO2(g) ΔH° = -283kJ
H2O(l) ➔ H2O(g) ΔH° = +40.7kJ

## Enthalpies of Formation

ΔH°rxn = ∑nΔH°products - ∑mΔH°reactants

Given the following standard enthalpy of formation values, calculate the ΔH°rxn of the following reaction:

3NO(g) ➔ N2O(g) + NO2(g)          ΔH°rxn = ?

ΔHf°    (kJ/mol)
NO(g) 90.4
N2O(g) 81.6
NO2(g) 33.8

### What is a Formation Reaction?

1) Form 1 mole of a single product

2) Reactants are elements in their standard states.

STANDARD STATES
Gases He, Ne, Ar, Kr, Xe, Rn, H2, N2, O2, F2, Cl2
Liquids Br2, Hg
Solids Everything Else
Special C(graphite), S8