Transcript Lecture 7
Outline: CAPA TA’s 1/24/07 – Wed & Sunday evening Today: Chapter 14 Thermodynamics: Enthalpy (Heat) Entropy Free Energy • • Thermo = heat dynamics = movement Want to test your knowledge? (and those keypads once again?) What’s the functional group here? 20% 20% 20% 20% 20% 1 2 3 Aldehyde Alcohol Ketone Acid Ester 4 5 O OH What provides the polymeric backbone to DNA’s double helix? 20% 20% 20% 20% 20% 1 2 3 Ribose sugars Phosphates Amino acids 1&2 1&3 4 5 Why are we so interested in heat? Chemical reactions that produce heat Chemical reactions that absorb heat “Exothermic” “Endothermic” Heat is related to whether the reaction will happen! The first law…. DE = q + w and w = - PDV DE = q - PDV q = DE + PDV For constant pressure: PDV = D(PV) q = D(E + PV) Make a new state variable: E + PV = H “Constant pressure heat” = Enthalpy (H) Another definition: Enthalpy of formation (DHfo) : The constant-pressure heat required to form a chemical compound from the elements (in their most stable form). DHfo = 0 for elements Look up tables of DHfo (Appendix D) Define Examples: H2(g) + O2 (g) H20 (l) 0 kJ/mol 0 kJ/mol - 285 kJ/mol DH for formation reaction: DHfo DHrxn = S DHproducts - S DHreactants Example of DHfo usefulness: CH4(g) + O2(g) H2O (g) + CO2 (g) How much heat is generated if you burn 1.0 mole of methane gas? Example of DHfo usefulness: CH4(g) + 2O2(g) 2 H2O (g) + CO2 (g) 1. Balance equation... 2. Info from Appendix D: DHfo (CH4(g)) = -74.6 kJ/mol 1 DHfo (O2(g)) = 0.0 kJ/mol 2 DHfo (H2O(g)) = -241.8 kJ/mol 2 DHfo (CO2(g)) = -393.5 kJ/mol 1 Example of DHfo usefulness: CH4(g) + 2O2(g) 2 H2O (g) + CO2 (g) 1. Balance equation... 2. Info from Appendix D… 3. Products – Reactants… -802.5 kJ/mol Practice !!! Remember? A new topic! Entropy Start with definition: Spontaneity: Every chemical process has a spontaneous direction. e. g. 2 H2 + O2 2 H2O apple brown apple (fast) (slow) What governs spontaneity? DH (heat of reaction) ? DE (energy of reaction) ? Both exothermic and endothermic reactions can be spontaneous… T (temperature) ….. Something else ? ? Answer: Entropy Definition: Entropy = “randomness” = “disorder” = S Second Law of thermodynamics: Entropy always increases in spontaneous reactions (entropy of the universe that is...) DS = q/T > 0 (spontaneous) At a molecular level, entropy (order/disorder) is easy to visualize: (s) () (g) Argon example Entropy is a “state function”: DS = Change in entropy (randomness) = related to the flow of heat (at constant temperature) TDS = qT where qT is the heat of reaction at constant T Don’t forget (T in Kelvin) Examples: Phase changes () (g) constant temp bath Macroscopic H2O example: (1) 36 g water freezes into ice cube in a freezer at -10°C spontaneously (2) 36 g ice cube melts to water at 5°C spontaneously How can both spontaneous reactions of ice/water have increasing entropy? Ice cube example (cont’d): 2 mols H2O(s) 2 mols H2O() (@ 5oC) What is DS for this reaction? q(H2O) = n DHfus = 2 mol (6.0 kJ/mol) = (heat absorbed by water) = +12 kJ DS(H2O) = q(H2O) / T = +12 kJ/273K = +44 J/K (more entropy / more disordered) Consider the universe? q(H2O) = -q(freezer) = -12 kJ DS(freezer) = q(freezer) / T = -12kJ/278K = -43.2 J/K (less entropy / less disordered) Overall entropy (of universe): DS(universe) = DS(H2O) + DS(freezer) = +44.0 + -43.2 = +0.8 J/K (more entropy / more disordered) What about freezing water? 2 mols H2O(l) 2 mols H2O(s) (freezer @ -10oC) What is DS for this reaction? q(H2O) = n DHfus = 2 mol (-6.0 kJ/mol) = (heat lost by water) = -12 kJ DS(H2O) = q(H2O) / T = -12 kJ/273K = -44 J/K (less entropy / less disordered) But why is it spontaneous? q(H2O) = -q(freezer) = +12 kJ DS(freezer) = q(freezer) / T = +12kJ/263K = +45.6 J/K (more entropy / more disordered) Overall entropy (of universe): DS(universe) = DS(H2O) + DS(freezer) = -44.0 + 45.6 = +1.6 J/K (more entropy / more disordered) On what does entropy depend? Temperature obvious (s) () (g) Molar mass less obvious Concentration Phase Generally, as molar mass , intermolecular disorder Table 14-2: H2 = 130.7 J/K mol (page 580) F2 = 202.8 J/K mol Cl2 = 223.1 J/K mol On what does entropy depend? Temperature obvious (s) () (g) Molar mass less obvious Concentration Phase p.126 Let’s test those minds… Quiz #2 Please put away all books, papers, etc. Quiz #2 You may leave when you are done… hand them into me on your way out.