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ChemicalThermodynamics
Chapter 17.4 - 17.6Gibbs Free Energy
and a Bit More About Entropy
ChemicalThermodynamics
Spontaneous Processes
Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.
ChemicalThermodynamics
Spontaneous Processes
• Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures.
• Above 0C it is spontaneous for ice to melt.• Below 0C the reverse process is spontaneous.
ChemicalThermodynamics
Entropy on the Molecular Scale
• Ludwig Boltzmann described the concept of entropy on the molecular level.
• Temperature is a measure of the average kinetic energy of the molecules in a sample.
ChemicalThermodynamics
Entropy on the Molecular Scale• Molecules exhibit several types of motion:
Translational: Movement of the entire molecule from one place to another.
Vibrational: Periodic motion of atoms within a molecule.Rotational: Rotation of the molecule on about an axis or
rotation about bonds.
ChemicalThermodynamics
Eyeballing a Molecule for Entropy
Larger and more complex molecules have greater entropies.
ChemicalThermodynamics
Third Law of ThermodynamicsThe entropy of a pure crystalline substance at absolute zero is 0.
ChemicalThermodynamics
Free Energy Changes
Very key equation:
This equation shows how G changes with temperature.
(We assume S & H are independent of T.)
ChemicalThermodynamics
Gibbs Free Energy: Assessing
1. If G is negative, the forward reaction is spontaneous.
2. If G is 0, the system is at equilibrium.
3. If G is positive, the reaction is nonspontaneous in the forward direction.
ChemicalThermodynamics
Figuring out G without Calculations
By knowing the sign (+ or -) of S and H, we can get the sign of G and determine if a reaction is spontaneous.
ChemicalThermodynamics
Calculation of Go using Free Energies of Formation
Sample Problem on page 461 of your textbook
Once you have your final enthalpy and entropy values, you plug into Gibbs-Helmholtz equation along with temp. in K and solve for Go.
ChemicalThermodynamics
Standard Free Energy Changes
Standard free energies of formation, Gf are analogous to standard enthalpies of formation, Hf.
G can be looked up in tables, or calculated from S°and H.
ChemicalThermodynamics
Free Energy and Temperature
Very simple … just plug in new temperature to
We assume entropy and enthalpy are independent of temperature, so don’t change those values.
Sample problem on page 462-463 of your textbook
Gibbs Free Energy is strongly dependent on temperature
ChemicalThermodynamics
Free Energy Pressure/ConcentrationAll of the free energy calculations to this point have involved standard free energy change, Go. However, we can calculate free energy change, G, under any conditions.
Q is the reaction quotient we learned about in chapter 12, and is a similar construct to the equilibrium constant K.
ChemicalThermodynamics
Free Energy Pressure/Concentration:Sample
ChemicalThermodynamics
Free Energy and Equilibrium
Remember from above:If G is 0, the system is at equilibrium.
Sample problem on page 467-468