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Ch. 20: Entropy and Free Energy

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- Thermodynamics: the science of energy transfer
- Objective: To learn how chemists predict when reactions will be product-favored vs. when they will be reactant-favored

- ØThermodynamics tells us NOTHING about the rate of reaction.
- The study of rates and why some reactions are fast and others are slow is called kinetics (Ch. 15.)

- Entropy, S:Measure of dispersal or disorder.
- ØCan be measured with a calorimeter. Assumes in a perfect crystal at absolute zero, no disorder and S = 0.
- ØIf temperature change is very small, can calculate entropy change, DS = q/T (heat absorbed / T at which change occurs)
- ØSum of DS can give total entropy at any desired temperature. See Table 20.1

- In general, the final state is more probable than the initial one if:
- (1)energy can be dispersed over a greater number of atoms and molecules (hot cold)
- (2)the atoms and molecules can be more disordered (dissolving, diffusion of gas)

- More specifically,
- (1)if energy and matter are both more dispersed, it is definitely product-favored
- (2)if only energy or matter is dispersed, then quantitative information is necessary to decide which effects are greater
- (3)if neither matter nor energy is more dispersed, then the process will be reactant-favored

- Boiling water
- Melting ice
- Preparing solutions
- CaCO3 (s) CaO (s) + CO2 (g)

- Molecules of gas collecting
- Liquid converting to solid at room temp
- 2 CO (g) + O2 (g) 2 CO2 (g)
- Ag+ (aq) + Cl-(aq) AgCl (s)

- Sgas > S liquid > Ssolid
- Entropies of more complex molecules are larger than those of simpler molecules (Spropane > Sethane>Smethane)
- Entropies of ionic solids are higher when attraction between ions are weaker.
ØEntropy usually increases when a pure liquid or solid dissolves in a solvent.

Entropy increases when a dissolved gas escapes from a solution

- First law: Total energy of the universe is a constant.
- Second law: Total entropy of the universe is always increasing.
- Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.

- DSosystem = So (products) - So (reactants)
Can also relate surroundings to the system!

- DSosurroundings = q surroundings / T
= - DHsystem / T

- DSouniverse = DSosurroundings +DSosystem
- DSouniverse = - DHsystem / T +DSosystem
- Can use 2nd law to predict whether a reaction is product-favored or reactant-favored!
- The higher the temperature, the less important the enthalpy term is!

- Roald Hoffmann (1981 Nobel prize): “One amusing way to describe synthetic chemistry, the making of molecules that is at the intellectual and economic center of chemistry, is that it is the local defeat of entropy.”

- DG is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction!

- DGsystem = - T DSuniverse
= DHsystem - TDSsystem

- DGosystem = DHosystem - T DSosystem
- DGorxn = DHorxn - T DSorxn

- DGosystem or DGorxn If negative, then product-favored. If positive, then reactant-favored.
- DGoreaction = Gfo (products) - Gfo (reactants)

- DG is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction!
- Know the meaning of enthalpy-driven vs. entropy-driven reactions.
DGs are additive!

If not at standard conditions,

DG = DGo + RT ln Q

(Equilibrium is characterized by the inability to do work.)

At equilibrium, Q = K and DG = O

Therefore, substituting into previous equation gives 0 = DGo + RT ln K and

DGo = - RT ln K(can use Kp or Kc)

2020.5 Thermodynamics and Time

- First law: Total energy of the universe is a constant.
- Second law: Total entropy of the universe is always increasing.
- Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.
- Entropy : time’s arrow
- Absolutely MUST learn table in Chapter highlights!

- ØUnderstand relationship between DGo, K, and product-favored reactions!
- DGo<0 K>1product-favored
- DGo=0 K=1
- DGo>0 K<1reactant-favored