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