phase transitions
Post on 31-Dec-2015
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Phase Transitions
T
heat added
0oC
100oC
ice
H = Cice x T x mass
melting point
Hfusion= 6.02 kJ/mol x mass
water
H = Cwater x T x mass boiling pointHvap = 40.7 kJ/mol x mass
steamH = Csteam x T x mass
Cice=2.087 J/goC
Cwater=4.184 J/goC
Csteam=1.996 J/goC
-20oC
Thermite reaction
H is an
2Al(s) +Fe2O3(s)
Hess’ Law
extensive, State function
Al2O3(s) + 2Fe(l)
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
2Al(s) H = -1676 kJ/mol
H = - 822 kJ/mol
2Al(s) -854 kJ/mol
Fe(s) Fe(l) +15 kJ/mol_______________________________ __________
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
Hrxn = -824 kJ/mol
2 ( )2 2
2 Fe(s) + 3/2 O2(g) Fe2O3(s)Fe2O3(s) 2Fe(s) + 3/2 O2(g)_______________________________ __________+
+ 3/2 O2(g)
+ Fe2O3(s) Al2O3(s) + 2Fe(s)
Al2O3(s)
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
NH2CH2COOH
from elementsin their standard states
N2 + H2 + Cgr+ O21/2 5/2 2
for NH2CH2COOH
1 mol product,
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
reactants productselements
2 Al(s) 2 Al(s) Al2O3(s)
Fe2O3
2 Fe(s)
3/2 O2(g)2 Fe (l)
Hof
Hof Al2O3(s)
+ 2 Hof Fe (l) Fe2O3 Al(s)
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
reactants productselements
2 Al(s) 2 Al(s) Al2O3(s)
Fe2O3
2 Fe(s)
3/2 O2(g)2 Fe (l)
Fe2O3 Hof Al2O3(s)
+ 2 Hof Fe (l)
Hrxn = nHof products -
- Hof
nHof reactants
Hof
- Hof Al(s)
2Al(s) +Fe2O3(s) Al2O3(s) + 2Fe(l)
Hrxn =
Hrxn =
Hrxn =
= -824 kJ
nHof products - nHo
f reactants
[Hof Al2O3(s) +
-
[(-1676) -
Hof Fe(l)]2
[Hof Fe2O3(s) + Ho
f Al(s)]2
+ (15)]2 [(-822)+ 0]kJ
Bond Energies
chemical reactions = bond breakage andbond formation
bond energiesenergy required to break bondbond breakage a) endothermic
b) exothermic(raise P.E.)
bond formation (lower P.E.)exothermic
positive
Bond energies
CH4 (g)
Hrxn=
C-H 413 kJO=O 495 kJC=O 799 kJO-H 467 kJ
Hrxn =
Hrxn =
bonds broken - bonds formed
[ (C-H)+ (O=O)] [ (C=O)+ (O-H)] = -824 kJ
Hof products- Ho
f reactants =- 802 kJ
-4 22 4
+ 2O2 (g) CO2 (g) + 2H2O (g)
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