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Transcript
A quote of the week
(or camel of the week):
Minds are like parachutes – they
only function when open
Thomas Dewar
Physical Chemistry EPM/04
1
Entropy and reaction
spontaneity
Back to the II law ot thermodynamics
A spontaneous change is accompanied by an increase in the
total entropy of the system and its surroundings.
∆stot = ∆ssys + ∆ssur = ∆ssys −
∆h
Tsur
∆stot > 0
∆stot = 0
∆stot < 0
spontaneous
equilibrium
externally driven
Physical Chemistry EPM/04
2
Gibbs free energy
∆Stot = ∆S −
∆H
T
− T∆Stot = ∆H − T∆S
∆G = ∆H − T ∆S
∆G < 0
∆G = 0
∆G > 0
spontaneous
equilibrium
externally driven
forced
spontaneous in reversed
direction
Physical Chemistry EPM/04
3
Gibbs free energy (2)
reaction
∆S
spontaneous
exothermic
>0
always
exothermic
≤0
|T∆S|<|∆H|
endothermic
>0
|T∆S|>|∆H|
endothermic
≤0
never
∆G = ∆H − T ∆S
Physical Chemistry EPM/04
4
Gibbs free energy (3)
The standard reaction free energy ∆G0 is the difference between
the free energies of formation of the products and the reactants
(all in their standard states).
n
n
i =1
i =1
∆Gr0, 298 = ∑ ni ∆Gi0, f , pr , 298 − ∑ ni ∆Gi0, f ,re , 298
The standard free energy of formation, ∆G0f , of a compound is
the standard reaction free energy per mole for its synthesis
from elements in their most stable forms. Standard free
energies of elements in their most stable forms are equal to
zero at 298K.
Physical Chemistry EPM/04
5
Gibbs free energy (4)
Physical Chemistry EPM/04
6
Gibbs free energy (5)
Physical Chemistry EPM/04
7
Gibbs free energy (6)
Physical Chemistry EPM/04
8
Chemical equilibrium
∆G = ∆G 0 + RT ln Q
For reaction: aA + bB + ... = mM + nN + ...
reaction quotient Q is defined as:
Q=
aMm ⋅ aNn
a Aa ⋅ aBb
where: a means activity of the reactant or product indicated.
Physical Chemistry EPM/04
9
Chemical equilibrium (2)
Q=
aMm ⋅ aNn
a Aa ⋅ aBb
Activity:
• for liquids and solids in pure form a=1
• for gases ai=Pi/P0
• for solutions ai=fi·ci, where c is concentration and f is activity
coefficient. For perfect solutions f=1. For real solutions f→1
when c→0. Standard state for compounds in solutions is when
a=1.
Physical Chemistry EPM/04
10
Chemical equilibrium (3)
∆G = ∆G 0 + RT ln Q
At equilibrium:
∆G 0 = − RT ln K
∆G = 0
K = Qeq =
m
n
aeqM
⋅ aeqN
a
b
aeqA
⋅ aeqB
K is known as the reaction equilibrium constant. It is reaction
specific and depends on temperature only. (Guldberg–Waage law).
Physical Chemistry EPM/04
11
Chemical equilibrium (4)
∆G = ∆G 0 + RT ln Q
At standard conditions:
Q = 1;
ln Q = 0
∆G = ∆G 0
One can say, that the reaction quotient term in the
top equation represents the deviation in Gibbs free
energy from the standard state. Standard Gibbs
free energy is reaction characteristic, while the
second term depends on composition of the
reaction mixture.
Physical Chemistry EPM/04
12
Conclusions from the
Guldberg-Waage law
K = Qeq =
m
n
aeqM
⋅ aeqN
a
b
aeqA
⋅ aeqB
If, in the reaction mixture of a reaction at equilibrium, one increases activities (concentrations, pressures) of the reactants, the reaction will move to the right (toward products), reducing the denominator and increasing the numerator to maintain the constancy of K;
If one increases activities of the products of the reaction being at
equilibrium, the reaction will move to the left.
If one reduces activities of the reactants ...
If one reduces activities of the products ...
Physical Chemistry EPM/04
13
Le Chatelier principle
If a system, containing a chemical reaction at equilibrium, is subject to
an external factor (P, T) disrupting this equilibrium,
the system reacts in a way leading to minimalization of this external
factor.
Temperature: increase in temperature shifts the equilibrium of
exothermic reaction to the left, while that of endothermic reactions – to
the right.
Pressure can influence state of equlibrium only in reactions
leading to change in number of moles of gaseous substances.
Physical Chemistry EPM/04
14
Le Chatelier principle (2)
Influence of pressure upon a dissociation in the gas phase:
A
2B
The principle says: in reactions leding to increase in number of moles of
gaseous substances, increase in pressure shifts the equilibrium to the
left, while decrease – to the right.
initial state, Pi
external change, P2>Pi
Physical Chemistry EPM/04
system response,
P2>Pf>Pi
15