Gibbs Free Energy

How can we use ΔH and ΔS to predict whether a given reaction occuring at constant temperature and pressure will be spontaneous?

Gibbs Free Energy (G) State Function ΔG is equivalent to -TΔSuniverse

Gibbs Free Energy If Suniv > 0 , G < 0 ( spontaneous rxn) If G > 0 (nonspontaneous rxn) If G = 0 (rxn at equilibrium )

Effect of Temperature on Free Energy

Free energy always decreases for any spontaneous process carried out at constant temperature and pressure

In terms of K and Q Q<K excess of reactants relative to products Reaction spontaneous in forward direction Q > K excess of products relative to reactants Reaction is spontaneous in reverse direction Q = K at equilibrium K = equilibrium constant, reactant vs. product favored Q = reaction quotient = plug in concentrations at a given time during reaction

Calculating Gibbs Free Energy 3 equations to use:

Gibb’s Free Energy Practice Problem 1 http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch21/problems/ex21_6s.html

Problem 1 Answer

Problem 1 Answer Cont.

Standard free energy of formation Δ Gof For a pure element Δ Gof = 0 For a solution assume 1 M concentration

Practice Problem 2 Use data from Appendix C to calculate the standard free-energy change for the reaction below. P4 (g) + 6 Cl2(g)  4 PCl3 (g) Answer = -1102.8 kJ

Free Energy and Equilibrium under non standard conditions G = G + RT lnQ (Under standard conditions, all concentrations are 1 M, so Q = 1 and lnQ = 0; the last term drops out.)

Practice Problem 3 Calculate Δ G at 298 K for a mixture of 1.0 atm N2, 3.0 atm H2, and 0.50 atm NH3 being used in the equation below: N2(g) + 3H2(g)  2NH3 ΔGo =-33.0 kj/mol ΔG = -44.9 kj/mol

Free Energy and Equilibrium Constant Relationship At equilibrium, Q = K, and G = 0. The equation becomes 0 = G + RT lnK Rearranging, this becomes G = RT lnK or, K = eG/RT

What does this mean? ΔGo < 0, K >1 ΔGo > 0, K < 1 More negative ΔGo is, the larger the K value ΔGo > 0, K < 1 ΔGo = O , K = 1

Online Practice Problems Visit the websites listed. Complete the practice problems on a separate sheet of paper. Check your answers

Practice Problem Websites Entropy http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch21/entropy.php Gibb’s Free Energy http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch21/gibbs.php Free Energy and Equilibrium http://www.mhhe.com/physsci/chemistry/chang7/ssg/chap18_5sg.html

Closure Practice Problems Chapter 19 in textbook (2015 Edition) Spontaneous Processes 19.11,19.17 (8th Edition – 19.1, 19.2) Entropy 19.25, 19.27, 19.41, 19.43, 19.47, 19.53, 19.54 (8th Edition – 19.25, 19.27, 19.29, 19.31, 19.33)

Closure Practice Problems Cont Gibb’s Free Energy 19.57, 19.59,19.60, 19.61, 19.69 8th edition – 19.49, 19.51, 19.53, 19.54, 19.56, 19.58, 19.63, 19.66) Free energy and equilibrium 19.79, 19.81, 19.83 8th edition – 19,71, 19.76, 19.79 Additional Exercises 8th edition – 19.82, 19.87, 19.88, 19.89, 19.90, 19.92