1. Thermochemistry Chapter 17:4 Hess’s Law and
Standard Enthalpies of Formation
Pages 527 – 532

2. A. Hess’s Law
Hess’s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction
2S(s) + 3O2(g)  2SO3(g) H = ?

3. A. Hess’s Law

4. A. Hess’s Law -292 kJ N2(g) + 2O2(g)  2NO2(g) H = ? Known:
N2 (g) + O2(g)  2NO(g) H = -180 kJ
2NO(g) + O2(g)  2NO2(g) H = -112 kJ
Build your equation:

5. If you reverse the reaction, then reverse the sign for ΔH
A. Hess’s Law
If you need to multiply your coefficients, then multiply the ΔH as well
2S(s) + 3O2(g)  2SO3(g) H = ?
-792 kJ
Known:
S(s) + O2(g)  SO2(g) H = -297 kJ
2SO3(g)  2SO2(g) + O2(g) H = 198 kJ
If you reverse the reaction, then reverse the sign for ΔH
Build your equation:

6. A. Hess’s Law 2H2O2(l)  2H2O(l) + O2(g) H = ? -196 kJ
Known:
2H2(g) + O2(g)  2H2O(l) H = -572 kJ
H2(g) + O2(g)  H2O2(l) H = -188 kJ
Build your equation:

7. A. Hess’s Law 36 kJ 2B(s) + 3H2(g)  B2H6(g) H = ? Known:
2B (s) + 3/2O2(g)  B2O3(s) H = kJ
B2H6(g) + 3O2(g)  B2O3 (s) + 3H2O (g) H = kJ
H2 (g) + ½ O2(g)  H2O (l) H = -286 kJ
H2O (l)  H2O (g) H = 44 kJ
Build your equation:

8. B. Standard Heats of Formation
Standard heat of formation, ΔHf, of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements
All substances at standard pressure and 25oC
ΔHf of free elements (and diatomic molecules) in standard state is set at 0

9. B. Standard Heats of Formation
Provides an alternative to Hess’s Law of calculating heat of reaction indirectly
For reactions occurring at standard conditions, calculate heat of reaction, ΔHo , using standard heats of formation (given to you on appendix copies)
ΔHo = ΔHf (products) – ΔHf (reactants)

10. B. Standard Heats of Formation
Write the thermochemical equation for the formation of sodium oxide.
Na(s) + O2(g)  Na2O(s) 4 2 2
ΔH=
-416 kJ

11. B. Standard Heats of Formation
What is the standard heat of reaction of CO(g) with O2(g) to form CO2 (g)?
2CO(g) + O2(g) → 2CO2(g) ΔH = ?
2(ΔHfCO2) – (2(ΔHfCO) + ΔHfO2) =
2(-393.5) – (2(-110.5) + 0)
ΔH = kJ/mol

12. B. Standard Heats of Formation
CaCO3(s) → CaO(s) + CO2 (g) ΔH = ?
(ΔHfCaO + ΔHfCO2) – (ΔHfCaCO3) =
( ) – (-1207)
ΔH =179 kJ/mol

13. B. Standard Heats of Formation
What is the standard heat of reaction for the combustion of methane?
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH = ?
(2(ΔHfH2O) + ΔHfCO2) – (ΔHfCH4 + 2(ΔHfO2)) =
(2(-242) ) – ( (0))
ΔH =-803 kJ/mol