U05 Notes Part4 Entropy Spontaneity
U05 Notes Part4 Entropy Spontaneity
U05 Notes Part4 Entropy Spontaneity
IB Topics 5 & 15
PART 4:
Entropy & Spontaneity
The time has come, the Walrus
said,
To speak of many things:
Of shoes--and ships--and sealing
wax
Of Entropy, Enthalpy, and Free
Energy
Is this your room?
In nature, things
tend to increase
in entropy, or
disorder.
So WHY do
things tend
toward
states of
higher
entropy???
Think of your room again..
If you were to throw everything up in the air and
then just wait for it all to land, there is a much
greater probability that things will end up
disordered. Its unlikely that things will land in an
orderly fashion, because there are fewer ordered
arrangements than there are disordered
arrangements.
There are more ways that gas
molecules can be mixed together than
there are ways they can be separated.
Once this valve is opened, it is more
likely that the gas molecules will spread
out than stay as is.
Daves Hand
2.6 million to
one
Johns Hand
2.6 million to
one
Johns hand is one of a very select
group of hands called a straight
flush. Out of the 2.6 million possible
hands, there are only 40 straight
flushes.
Dave has junk. There are over a
million hands that are junk.
Microstate
Macrostate
Johns Hand 7 8 9 10 J
Straight Flush
entropy of the products
and the
Srxn S (products ) - S (reactants )
reactants.
Example: Determine the entropy change for
the formation of ammonia from hydrogen and
nitrogen.
3H2(g) + N2(g) 2NH3(g)
Non-spontaneous: G = (+)
Reaction is at Equilibrium: G = 0
H2O(g)H2O(l)
- - spon. at
low temp.
H2O(s)H2O(l)
+ +
spon. at
high temp.
16CO2+18H2O2C8H18+25O2
+ -
never spon.
Calculating G values
1) Calculating Grxn from Gf
G
rxn G (
f products )- G (
f reactants )
Grxn
reactant product
s s
Gf (reactants ) Gf (products)
elements
G
rxn G (
f products )- G (
f reactants )
Example: Find the standard free energy of
combustion of methane given the standard free
energies of formation of methane, carbon dioxide,
water and oxygen.
Grxn
G
rxn G (
f products )- G (
f reactants )
Example: Find the standard free energy of
combustion of methane given the standard free
energies of formation of methane, carbon dioxide,
water and oxygen.
Gf (CH4) = -50
kJ/mol
Gf (CO2) = -394
kJ/mol
G
rxn
Gf (products
Gf (H2O) = -237
) - Gf (reactants )
kJ/mol 0
G rxn [G (CO 2 ) G (H2O)] - [G (CH4 ) G (O2 )]
f f f f
G rxn [(-394) 2(-237)] - [(-50) 2(0)]
-1
G rxn 818 kJ mol
Example: Determine whether or not the decomposition of calcium
carbonate is spontaneous at all temperatures. If not, provide
conditions of temperatures (if any) at which the reaction is
spontaneous.
At 25C (298K):
G = H - TS
G = 178 kJ/mol (298K)
(0.1653kJ/molK)
G = +129 kJ mol-1
Example: Determine whether or not the decomposition of calcium
carbonate is spontaneous at all temperatures. If not, provide
conditions of temperatures (if any) at which the reaction is
spontaneous.
G = H - TS
G = (+) (+)(+)
This rxn is spontaneous only at high temps.
Example: Determine whether or not the decomposition of calcium
carbonate is spontaneous at all temperatures. If not, provide
conditions of temperatures (if any) at which the reaction is
spontaneous.
TS = H when T = H/ S
T = 178/0.1653
T = 1077K (804 C)