Chemistry Lecture Recap: Thermodynamics and Gibbs Free Energy

Chem. 1B 10/18 Lecture

Announcements I Exam 2: Next Week on Thursday (10/27) Will Cover Titrations, Solubility, Complex Ions (from Ch. 16) + Chapter 17 (Thermodynamics) SacCT/Website Updated bonus points + quiz keys (1 to 5 posted) Lab: Lab Midterm and Exp 3 report on Wed./Thurs. Note: Syllabus says Lab midterm is on Exp 1, 2, 5, and 7, but will also have questions related to pre-lab on Experiment 3

Announcements II Today s Lecture Thermodynamics Gibb s Free Energy Reversible Reactions Relating Gibb s Free Energy Changes to Equilibrium Constants

Chem 1B Thermodynamics Chapter 17 Gibbs Free Energy Meaning of Gibbs Free Energy G < 0 means a spontaneous process G > 0 means a non-spontaneous process Does this match our previous knowledge? H < 0 is generally spontaneous (and gives rise to G < 0 as G = H T S) S > 0 is generally spontaneous (and gives rise to G < 0 as G = H T S)

Chem 1B Thermodynamics Chapter 17 Gibbs Free Energy How Temperature Affects Reaction Direction We have 4 combinations of signs of H and S: H < 0 and S > 0 ( G always < 0) H < 0 and S < 0 ( G < 0 at low T only) H > 0 and S > 0 ( G < 0 at high T only) H > 0 and S < 0 ( G always > 0) When H and S have the same sign, process depends on T

Chem 1B Thermodynamics Chapter 17 Gibbs Free Energy Some Example Problems Under what temperature regimes will these reactions be spontaneous? N2(g) + O2(g) 2NO(g) H = 91.3 kJ/mol N2(g) + 3H2(g) 2NH3(g) H = -91.8 kJ/mol NH4NO3(s) NH4+(aq) + NO3-(aq) H = 25.7 kJ/mol N2(g) + 2H2(g) N2H4(g) H = 95.4 kJ/mol 2C2H6(g) + 7O2(g) H = -2,855 kJ/mol 4CO2(g) + 6H2O(g)

Chem 1B Thermodynamics Chapter 17 Gibbs Free Energy Some Example Problems From Standard Tables, Calculate G for the following reaction: N2(g) + 3H2(g) 2NH3(g) N2(g) 191.6 H2(g) 130.7 0 NH3(g) 192.8 -45.9 Species S (J/mol K) H (kJ/mol) 0 At what temperature is G = 0 for the above reaction? Note: can also calculate G from standard values

Chem 1B Thermodynamics Chapter 17 Reversibility Reactions can be reversible or irreversible For a process to use 100% of available energy (Gibbs free energy) for work, process must be reversible Example of reversible process: H2O(s) H2O(l) at 0 C at T > 0 C, this process is spontaneous and non- reversible When chemical reactions are used to generate energy (e.g. electrochemical reactions in a battery), some energy becomes heat

Chem 1B Thermodynamics Chapter 17 Reversibility Battery Example cont. The greater the power draw on the battery (current), the less efficient (or reversible) the energy use will be

Chem 1B Thermodynamics Chapter 17 Gibbs Free Energy Free Energy Under Nonstandard Conditions G = G + RTlnQ where Q = reaction quotient Example AgCl(s) Ag+(aq) + Cl-(aq) Standard conditions G = 72.9 kJ/mol - would require [Ag+] = [Cl-] = 1 M - Q = [Ag+][Cl-]

Chem 1B Thermodynamics Chapter 17 Gibbs Free Energy and Equilibrium Under Nonstandard Conditions: Grxn= G rxn+ RTlnQ Example Reaction: CaSO4(s) Ca2+(aq) + SO42-(aq) G rxn= 23.7 kJ/mol and Q = [Ca2+][SO42-] If we put CaSO4(s) in water Q = 0 (before reaction) lnQ is undefined but negative, making Grxnnegative even though G rxnis positive As CaSO4(s) dissolves, [Ca2+] and [SO42-] increase, and Grxnincreases (e.g. when [Ca2+] = 0.001 M, Grxn= -10.5 kJ/mol)

Chem 1B Thermodynamics Chapter 17 Gibbs Free Energy and Equilibrium Example Reaction cont. CaSO4(s) Ca2+(aq) + SO42-(aq) We also could start by mixing high concentrations of Ca2+(aq) + SO42-(aq) (e.g. 2 M each) In this case Grxn> G rxnand reaction goes even stronger to reactants Q > 1 At Equilibrium Grxn= 0 = G rxn+ RTlnK or G rxn= -RTlnK which allows us to determine K from G or visa versa

Chem 1B Thermodynamics Chapter 17 Example Problems If for the reaction: 2H2(g) + N2(g) H > 0 and S < 0, how can N2H4(g) ever be formed? Not from 2H2(g) + N2(g) (in more than small quantities), but can t rule out other sources For example: N2O(g) + 3H2(g) calculate G to show What caused G < 0? Favorability in forming H2O(g) (Note: this reaction is energetically favorable, but may not occur) N2H4(g) has N2H4(g) + H2O(g)

Chem 1B Thermodynamics Chapter 17 Example Problems Catalysts can help energetically favorable reactions occur, but can not allow products to form if G of products is higher than reactants. Which of the following hydrocarbons can be produced by syngas (CO + H2) assume H2O forms if needed? CH4 C2H6 CH3OH How do we solve? Make balanced reactions and calculate G (or H and S ) What are the best conditions for these reactions? C2H2

Chem 1B Thermodynamics Chapter 17 Example Problems HI has a Gf = 1.7 kJ/mol at 298 K Can it be formed from H2(g) + I2(s)? What is K for the reaction: H2(g) + I2(g) if Gf (I2(g)) = 19.3 kJ/mol Does increasing T favor reactants or products? Water is sprayed into a reaction flask at equilibrium and absorbs 99% of the HI but little of the other gases. Explain what this will do to G. 2HI(g)

Chem 1B Thermodynamics Chapter 17 Equilibrium and Temperature We know G changes with temperature according to: G = H T S (note: H and S may change with T but generally not a lot) We also know that G = -RTlnK -RTlnK = H T S or lnK = - H /RT + S /R A Plot of lnK vs. 1/T would give m (slope) = - H /R and b (y-intercept) = + S /R What would a positive slope in the above plot mean? What would a positive y-intercept mean?

Chem 1B Thermodynamics Chapter 17 Equilibrium and Temperature A chemist has designed a catalyst allowing ethanol to be made from CO + H2. The catalyst will only work at T > 150 C. At that temperature will the product still be favored? Determine the K at that temperature.