Home > Chemistry > Chemistry 1A Part Deux: Lecture 25 Notes

Chemistry 1A Part Deux: Lecture 25 Notes


Today is a 50 minute lecture on entropy.


There is a natural tendency for energy and matter to disperse.

CH4 + 2O2  = CO2 + 2H2O     dHo = -891 kJ/mol

Bond energies = dHo = – Sum products + Sum reactants

2Fe + 3/2 O2 = Fe2O3 dHo = -824

Fe2O3 + 2Al = 2Fe + Al2O3 dHo =-852

The higher the enthalpy of reaction, the more favorable the reaction.

CaCl2 = Ca 2+ + 2Cl-    dHo = -81

NH4Cl = NH4+ + Cl-  dHo = 15

Gas expansion into a vacuum Br2 dHo = 0

Reaction enthalpies alone cannot predict reaction favorability.

There is a tendency for processes to happen in the direction of states with higher dispersion of matter and energy, greater disorder, and higher statistical probability.

Gases are more disordered than liquids, which are more entropic than solids.

Hot systems are more disordered, as are dissolved and more atoms and twisting/stretching modes.

A perfect crystal at 0K has zero entropy.  All standard molar entropies are positive, absolute quantities.  When you raise the temperature, the entropy of the system increases.

For a reversible process at constant pressure and temperature:

dS = Q rev/T   J/(K mol)

When temperature changes, add up very small increments:

dS = Cp dT/T

Heat packs are made of salts dissolving in water.

H2O s = H2O l

At constant T and P:

dS system= d H fusion/T system = 6.02 kJ./mol/273K

dS surroundings = -dH fusions/T surroundings = -6.02 kJ.mol/298 K

dS total = dS system + dS surroundings

Entropy of the Universe

processes proceed in the direction increasing entropy of the universe.

dS universe = dS sys + dS surroundings > 0

This is the direction that we are so familiar with.

Standard Entropies in J/mol K at 300 K

H2O  189

H2 131

O2 205

Co2 214

H2O 70

CH3OH 127

Br2 152

C6H6 173

C Si 6

Si 19

NaCl 72

SiOe 42

Gases have the highest entropy, then liquids and solids.  Complexity increases it as well.

S = Kb ln W

Entropy increases with the number of energetically equivalent microstates of a system

The Laws of Thermodynamics

1st Law

Energy is conserved

2nd Law

Entropy is not conserved

3rd Law

The entropy of ordered systems at OK is O.  Entropy is absolute.


PV = nRT

G = H – TS

S = Kb ln W

dS = Q rev/T

K = products/reactants

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