Transcript Review - University of Illinois at Urbana–Champaign

Phase Transitions
H = Cice x T x mass Cice=2.087 J/goC
Hfusion= 6.02 kJ/mol x mass
H = Cwater x T x mass Cwater=4.184 J/goC
boiling
point
H
=
40.7
kJ/mol
x mass
vap
T
H = Csteam x T x mass Csteam=1.996 J/goC
100oC
melting point
water
0oC
-20oC
ice
heat added
steam
Thermite reaction
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
H is an extensive, State function
Hess’ Law
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
2Al(s) + 3/2 O2(g)  Al2O3(s)
Fe
O3(s) +3/22Fe(s)
2 2Fe(s)
O2(g)+3/2FeO22O(g)
_______________________________
3(s)
H = -1676 kJ/mol
H = +-__________
822 kJ/mol
2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(s) -854 kJ/mol
2 (+15 kJ/mol )
2
2
Fe(s)

Fe(l)
_______________________________
__________
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
Hrxn = -824 kJ/mol
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
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
Hof
Write the equation for which
Hrxn = Hof for NH2CH2COOH
1 mol product, from elements
in their standard states
1/2 N2 + 5/2 H2 + 2 Cgr+
O2 NH2CH2COOH
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
reactants
2 Al(s)
products
elements
2 Al(s)
Hof
Al2O3(s)
2 Fe(s)
Fe2O3
3/2 O2(g)
Hof Al2O3(s) + 2 Hof Fe (l)
2 Fe (l)
Fe2O3
Al(s)
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
reactants
2 Al(s)
products
elements
2 Al(s)
Hof
Al2O3(s)
2 Fe(s)
Fe2O3
3/2 O2(g)
2 Fe (l)
Hof Al2O3(s) + 2 Hof Fe (l) - Hof Fe2O3 - Hof Al(s)
Hrxn = nHof products - nHof reactants
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) + 2 Hof Al(s)]
Hrxn = [(-1676) + 2 (15)]- [(-822) + 0]kJ
= -824 kJ
Bond Energies
chemical reactions = bond breakage and
bond formation
bond energies positive
energy required to break bond
bond breakage a) endothermic (raise P.E.)
exothermic (lower P.E.)
bond formation b)
exothermic
Bond energies
CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g)
C-H
O=O
C=O
O-H
413 kJ
495 kJ
799 kJ
467 kJ
Hrxn = bonds broken
- bonds formed
Hrxn =[ 4 (C-H)+ 2 (O=O)]
- [ 2 (C=O)+ 4 (O-H)]
= -824 kJ
Hrxn= Hof products- Hof reactants =- 802 kJ