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)