Unformatted text preview: me factor. b) If an eqn. is reversed,
sign of )H changes c) All substances NOT appearing in
desired eqn. MUST cancel 12 F) Enthalpy of Formation
Enthalpy change for the formation
of a compound from its elements )Hf 1) Standard enthalpy change
Enthalpy change when all reactants and
and products are in their standard states )H° 13 2) Standard State
Most stable state of a substance in its
pure form under standard pressure
(1 atm) & some specified temp. of
interest (usually 25 °C)
3) Thermochemical Standard States
A) solid or liquid
Pure substance at 1 atm
pressure of 1 atm
c) species in solution
Conc. of 1 M
14 4) Standard Enthalpy of Formation )H for the rxn in which 1 mole of a
cmpd. is formed from its elements
with ALL substances in their
standard states (in kJ/mol)
Note: )Hf° = 0 for an element in
its standard state
)Hf° 1/2 N2(g) + 3/2 H2(g) 6 NH3(g) ! 46.2 Na(s) + 1/2 Cl2(g) 6 NaCl(s) ! 411.0 C(graphite) 6 C(diamond) + 1.897
15 16 C) Determine )Hrxn from )Hf°
° )Hrxn = 3 n )Hf° ! 3 m )Hf°
prod. react. n = coef. in bal. eqn. for each product
m = coef. in bal. eqn. for each reactant 17 I) Spontaneous Processes
Process which proceeds on own
without outside assistance
- occurs in one direction only NONspontaneous Process
Can NOT proceed w/o outside assistance
- reverse of spont. process
Processes which are spont. in
one direction are nonspont. in
the reverse direction.
18 Mix 2 ideal gases Open stopcock
Reverse process is NOT spont. 19 Will all spont. rxns. be
exothermic or vice versa?
Will all nonspont. rxns. be
endothermic or vice versa?
Spontaneity can depend
on temp. & pressure
T > 0 °C : melting spont.
T < 0 °C : freezing spont.
T = 0 °C : equilibrium and neither
conversion occurs spont. 20 A) Reversible & IRreversible Processes
1) Reversible Process
Change made in a way that system &
surroundings can be restored to their
original states by exactly reversing chg
NO net chg. to system or its surr.
2) IRreversible Process
Can NOT simply be reversed to
restore system & surroundings
to their original states
A reversible change produces the max.
amt. of work which can be done by a
system on its surroundings.
21 3) Isothermal Expansion of a Gas
Expansion of ideal gas at
constant temperature. Remove partition - gas spont. expands
to fill the whole cylinder. No work is
done by the system.
Compressing the gas back to the
original state requires surroundings
to do work on the system.
Process is IRreversible
22 Can be done reversibly by doing the
expansion and compression infinitely
slowly so the external and internal
pressures are always in equilibrium.
- can not actually be
done in real processes
- ALL real processes are
a) A spont. process is an
b) Driving force is tendency to go
to a less ordered state or state
of higher probability.
23 nd II) Entropy, S & 2 Law of Thermodynamics
Measure of randomness or
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