1. Hess’ Law Thermite reaction 2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
Exothermic
H is an
extensive,
State function
Hess’ Law

2. 2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
+ 3/2 O2(g)
 Al2O3(s)
H = kJ/mol
_______________________________ __________
Fe2O3(s)  2Fe(s) + 3/2 O2(g)
2 Fe(s) + 3/2 O2(g)  Fe2O3(s)
H = kJ/mol +
2Al(s)
+ Fe2O3(s)
 Al2O3(s)
+ 2Fe(s)
-854 kJ/mol
2 ( )
+15 kJ/mol
Fe(s)  Fe(l)
_______________________________ __________
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
Hrxn = -824 kJ/mol

3. Hess’ Law Always end up with exactly the same reactants and products
If you reverse a reaction, reverse the sign of H
If you change the stoichiometry, change H

4. Heats of formation, Hof
H = heat lost or gained by a reaction
“o” = standard conditions:
all solutes 1M
all gases 1 atm
“f” = formation reaction:
1mol product
from elements
in standard states
for elements in standard states,
Hof = 0

5. Hof  NH2CH2COOH N2 + H2 + Cgr 2 + O2 Write the equation for which
Hrxn = Hof
for NH2CH2COOH
1 mol product,
from elements
in their standard states
 NH2CH2COOH N2
+ H2
+ Cgr 2
+ O2
1/2
5/2

6. 2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
reactants
elements
products
Hof
2 Al(s)
2 Al(s)
Al2O3(s)
2 Fe(s)
Fe2O3
2 Fe (l)
3/2 O2(g)
Hof Al2O3(s)
+ 2 Hof Fe (l)
Fe2O3
Al(s)

7. 2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
reactants
elements
products
Hof
2 Al(s)
2 Al(s)
Al2O3(s)
2 Fe(s)
Fe2O3
2 Fe (l)
3/2 O2(g)
Hof Al2O3(s)
+ 2 Hof Fe (l) -
Hof
Fe2O3 -
Hof
Al(s)
Hrxn =
nHof products -
nHof reactants

8. 2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
Hrxn =
nHof products
- nHof reactants
Hrxn =
[Hof Al2O3(s) + 2
Hof Fe(l)] -
[Hof Fe2O3(s)
+ Hof Al(s)] 2
[(-1676)
+ (15)] 2 -
[(-822)
+ 0]kJ
Hrxn =
= -824 kJ

9. Bond Energies chemical reactions = bond breakage and bond formation
positive
energy required to break bond
bond breakage
(raise P.E.)
bond formation
(lower P.E.)

10. Bond energies CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g) Hrxn =
bonds broken
C-H kJ
O=O 495 kJ
C=O 799 kJ
O-H kJ
- bonds formed
Hrxn =
[ (C-H) 4
+ (O=O)] 2 -
[ (C=O) 2
+ (O-H)] 4
= -824 kJ
Hrxn=
Hof products
- Hof reactants
=- 802 kJ