Describes Thermodynamics
how energy is transferred or converted in chemical processes
Energy conservation: the 1st law keeps track of energy transfer
E=q+w
E = total energy of a system q = heat flow w = work
q > 0 when heat flows into system q < 0 when heat flows out of system w > 0 when work is done on system w < 0 when work is done by system
2nd law tells conditions needed for a change to be spontaneous
Entropy (S) new state function
Indicates the direction of a spontaneous change
Spontaneity
Spontaneous process = change in system occurs without impetus Nonspontaneous process = change will not occur without doing something to make it happen
1
Chemical Thermodynamics
Difference between work and heat
Heat
Work
Energy transferred into random motion
Chemical Thermodynamics
Energy transferred into orderly motion
2
Spontaneity & the 2nd law
Universal observation that some things occur spontaneously & others do not
Ball rolls downhill Tennis balls stop moving Beakers break
NOTE a nonspontaneous change can happen if the system is changed. (a ball will roll uphill if it is pushed)
What do spontaneous events have in common?
Energy becomes randomized
2nd law:
Energy will spontaneously move toward a more random distribution
3
Chemical Thermodynamics
Entropy (S)
Measure of how randomly energy is distributed
S is larger when energy is more randomly distributed
ice 0o C water 20o C
Energy is not randomly distributed
spontaneous
Entropy increases
Energy is randomly distributed
An increase in entropy is often associated with disorder
E.g. ice cube contains "ordered" molecules become disordered when melts
Entropy depends upon both the system and the surroundings
System = region of interest (e.g. ice cube) Surroundings = everything else (e.g. water, glass, table, etc...) Stot = Ssys + Ssur
4
Chemical Thermodynamics
Effect of Ssys & Ssur
Water becomes more ordered
Ssur < 0
Salt becomes less ordered Ssys > 0 Dissolution is spontaneous:
Stot > 0 Ssys > | Ssur|
Entropy plays an important role in solubility
Chemical Thermodynamics 5
2nd law of thermodynamics
Stot > 0 for any spontaneous process
Stot = Ssys + Ssur Ssys does not need to be positive for a spontaneous change (so long as Stot > 0)
Spontaneous processes that create order:
Freezing, protein folding, cell walls, etc... Occur spontaneously even though Ssys < 0 Because Stot > 0 due to entropy change of surroundings ( Ssur > 0)
Stot > 0 when forward change is spontaneous Stot = 0 when no change is spontaneous Stot< 0 when reverse change is spontaneous
Direction of spontaneity
S = Sf Si (state function)
S is related to heat and temperature
Ssys = qrev/T system depends upon a "reversible" change Ssur = -qsys/T surroundings depends upon real heat that left system
Chemical Thermodynamics
Actual heat flow during the change
6
Molecular organization in a cell wall
Molecular assembly is spontaneous because more disorder is created in the surroundings than order in the system
Membrane forming molecule
Imbedded protein
Chemical Thermodynamics 7
Reversibility
reversible process
ice 0o C
Add heat so slowly so that equilibrium is not disturbed
water 0o C
water 0o C
equilibrium
any step in melting can be exactly reversed
irreversible process
ice 0o C
water 20o C
water ~20o C
Chemical Thermodynamics
not in equilibrium
cannot be exactly reversed since is never in equilibrium
8
Reversibility & entropy
Reversible change
Equilibrium exists throughout the change
Process can be exactly reversed at any point during the change
Ice/water at 0o C
Irreversible change
A change that removes system from equilibrium Cannot be reversed along same path
A change can only be reversible if equilibrium exists throughout the change
Entropy is defined as the change for a reversible path
Even if the process is irreversible! energy Maximum change of any process is when the change is reversible qrev = maximum amount of energy to be randomized qrev qrev the actual amount of heat that is observed in the change Ssys = T
9
Chemical Thermodynamics
Expansion of a gas into a vacuum
Keep gas at constant T during expansion Open stopcock volume becomes 2x bigger Gas expands
Stot > 0 E=q+w E = 0 (E only depends upon T) w = 0 (no work since doesn't push on surroundings) Ssur = 0 Ssys > 0 expansion is driven by increased disorder in gas (i.e. the system)
q=0
So, qrev q since Stot > 0
Chemical Thermodynamics
10
Effect of temperature on entropy
S increases with T
Entropy of a pure substance
For a pure substance: S (g) > S (l) > S (s)
S of phase transitions
At Tf: (s) & (l) in equilibrium (i.e. melting is reversible at Tf)
qrev = Hf
Sfus = Hfus/Tf
At Tb: (l) & (g) in equilibrium
qrev = Hvap
Svap = Hvap/Tb
Increasing disorder
11
Chemical Thermodynamics
Standard molar entropies (So)
Value of S per mole of substance in standard state at 1 atm.
Bottom of entropy scale: (3rd law)
So = 0 for substances at 0 K in a perfectly crystalline state
Standard state state (s, l, g) of pure substance at 1 atm
Trends:
So (g) > So (l) > So (s) (with some exceptions) So generally increases with number of atoms in a molecule and molar mass
Chemical Thermodynamics
12
Entropy changes in chemical reactions
Standard entropy change of a reaction ( So)
So =
nSoproducts
mSoreactants
n, m = reaction stoichiometry
2 CH3OH (g) + 3 O2 (g) 2 CO2 (g) + 4 H2O (g)
So = 2 SoCO2 + 4 SoH2O 2 SoCH3OH 3 SoO2
= 2(213.6 J mol-1K-1) + 4(188.83 J mol-1K-1) 2(237.6 J mol-1K-1) 3(205.0 J mol-1K-1) = 92.3 J mol-1K-1
Chemical Thermodynamics
13
Gibbs free energy (G = H TS)
Predicts spontaneity of a chemical process, like Stot Because G is just another way of writing Stot
Stot = Ssys + Ssur Ssur = -qsys - Hsys (because Hp = q) = T T
Stot = Ssys
Hsys T
G = -T Stot = Hsys T Ssys
constant P only For process: i f i f is spontaneous: Stot > 0 & G < 0
H is heat that leaves sysem
f i is spontaneous: Stot < 0 & G > 0
Chemical Thermodynamics
i
f is at equilibrium: Stot = 0 & G = 0
14
Gibbs free energy
Better than entropy don't need to calculate Ssys and Ssur G is determined from system's properties ( H, Ssys, T) Chemical reactions
Will be spontaneous in the direction that minimizes Gibbs energy
Gibbs energy determines position of equilibrium (K comes from G)
Chemical Thermodynamics
15
Standard free energy of formation
Standard free energy of formation ( Gfo)
Go for the reaction that forms a compound from its elements with all species in their standard states Gfo = 0 for an element in its standard state
Chemical reactions:
Calculate Grxno from tables of Gfo Go = nGfoproducts mGforeactants
2 CH3OH (g) + 3 O2 (g) 2 CO2 (g) + 4 H2O (g)
Grxno = 2 GfoCO2 + 4 GfoH2O 2 GfoCH3OH 3 GfoO2
= 2(-394.4 kJ/mol) + 4(-228.6 kJ/mol) 2(-161.9 kJ/mol) 3(0 kJ/mol)
= -1380 kJ/mol
Chemical Thermodynamics
Rxn will spontaneously move to products since Grnxo < 0
16
Effect of temperature on spontaneity
G = H T Ssys
depends on T
Increased T makes G smaller or more negative if Ssys > 0
i.e. increased T increases spontaneity
e.g. solubility increases with T
Whether G < 0 depends upon values of H, Ssys & T
H
+ +
Ssys
+ +
T Ssys
+ +
G
+ or + or +
exothermic
endothermic
spon./non-spon. always spontaneous spontaneous at low T either either spontaneous at high T always nonspontaneous
17
Chemical Thermodynamics
Free energy and equilibrium
Gibbs free energy is related to equilibrium
G = Go + RT lnQ
for rxn in any state for rxn in standard state
At equilibrium, Q = K & G = 0
= Go + RT lnK Go = -RT lnK
Equilibrium constant can be calculated from standard free energy
K = exp(- Go/RT)
Chemical Thermodynamics
18