Chemical Thermodynamics Chapter Gibbs Free Energy and a Bit More About Entropy

Chemical Thermodynamics Spontaneous Processes Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

Chemical Thermodynamics Spontaneous Processes Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures. Above 0  C it is spontaneous for ice to melt. Below 0  C the reverse process is spontaneous.

Chemical Thermodynamics 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.

Chemical Thermodynamics 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.

Chemical Thermodynamics Eyeballing a Molecule for Entropy Larger and more complex molecules have greater entropies.

Chemical Thermodynamics Third Law of Thermodynamics The entropy of a pure crystalline substance at absolute zero is 0.

Chemical Thermodynamics Free Energy Changes Very key equation: This equation shows how  G  changes with temperature. (We assume  S &  H are independent of T.)

Chemical Thermodynamics 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.

Chemical Thermodynamics 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.

Chemical Thermodynamics Calculation of  G o 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  G o.

Chemical Thermodynamics Standard Free Energy Changes Standard free energies of formation,  G f  are analogous to standard enthalpies of formation,  H f .  G  can be looked up in tables, or calculated from  S°and  H .

Chemical Thermodynamics 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 of your textbook Gibbs Free Energy is strongly dependent on temperature

Chemical Thermodynamics Free Energy Pressure/Concentration All of the free energy calculations to this point have involved standard free energy change,  G o. 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.

Chemical Thermodynamics Free Energy Pressure/Concentration:Sample

Chemical Thermodynamics Free Energy and Equilibrium Remember from above: If  G is 0, the system is at equilibrium. Sample problem on page