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Chemistry 102(01) Spring 2014 Instructor: Dr. Upali Siriwardane e-mail: [email protected] Office: CTH 311 Phone 257-4941 Office Hours: M,W 8:00-9:00 & 11:00-12:00 am; Tu,Th,F 9:30 - 11:30 am. or by appointment. Test Dates: 9:30-10:45 am., CTH 328 March 31, April 23, May 19, May 21, 2014 (Test 1): Chapter 13 2014 (Test 2): Chapter 14 &15 2014 (Test 3) Chapter 16 &17 2014 (Make-up test) comprehensive: Chapters 13-17 CHEM 102, Fall 2014 LA TECH 17-1 Chapter 6. Thermochemistry 6.1 Chemical Hand Warmers 231 6.2 The Nature of Energy: Key Definitions 232 6.3 The First Law of Thermodynamics: There Is No Free Lunch 234 6.4 Quantifying Heat and Work 240 6.5 Measuring for Chemical Reactions: Constant-Volume Calorimetry246 6.6 Enthalpy: The Heat Evolved in a Chemical Reaction at Constant Pressure 249 6.7 Constant-Pressure Calorimetry: Measuring 253 6.8 Relationships Involving 255 6.9 Determining Enthalpies of Reaction from Standard Enthalpies of Formation 257 6.1 0 Energy Use and the Environment 263 CHEM 102, Fall 2014 LA TECH 17-2 Chapter 17. Free Energy and Thermodynamics 17.1 Nature’s Heat Tax: You Can’t Win and You Can’t Break Even 769 17.2 Spontaneous and Nonspontaneous Processes 771 17.3 Entropy and the Second Law of Thermodynamics 773 17.4 Heat Transfer and Changes in the Entropy of the Surroundings 780 17.5 Gibbs Free Energy 784 17.6 Entropy Changes in Chemical Reactions: Calculating 788 17.7 Free Energy Changes in Chemical Reactions: Calculating 792 17.8 Free Energy Changes for Nonstandard States: The Relationship between and 798 17.9 Free Energy and Equilibrium: Relating to the Equilibrium Constant (K) CHEM 102, Fall 2014 LA TECH 17-3 What forms of energy are found in the Universe? mechanical thermal electrical nuclear mass: E = mc2 others yet to discover CHEM 102, Fall 2014 LA TECH 17-4 What is 1st Law of Thermodynamics Eenergy is conserved in the Universe All forms of energy are inter-convertible and conserved Energy is neither created nor destroyed. CHEM 102, Fall 2014 LA TECH 17-5 What exactly is DH? Heat measured at constant pressure qp Chemical reactions exposed to atmosphere and are held at a constant pressure. Volume of materials or gases produced can change. CHEM 102, Fall 2014 LA TECH 17-6 What is the internal energy change (DU) of a system? DU is part of energy associated with changes in atoms, molecules and subatomic particles Etotal = Eke + E pe + DU DU = heat (q) + w (work) DU = q + w DU = q -P DV; w =- P DV CHEM 102, Fall 2014 LA TECH 17-7 How is Internal Energy, DU measured? Heat measured at constant volume qv Chemical reactions take place inside a closed chamber like a bomb calorimeter. Volume of materials or gases produced can not change. ie: work = -PDV= 0 DU = qv + w qv = DU + o; w = 0 DU = qv = DU(internal energy ) CHEM 102, Fall 2014 LA TECH 17-8 Enthalpy Heat changes at constant pressure during chemical reactions Thermochemical equation. eg. H2 (g) + O2 (g) ---> 2H2O(l) DH =- 256 kJ; DH is called the enthalpy of reaction. if DH is + reaction is called endothermic if DH is - reaction is called exothermic CHEM 102, Fall 2014 LA TECH 17-9 Entropy, S The thermodynamic property related to randomness is ENTROPY, S. Product-favored processes: final state is more DISORDERED or RANDOM than the original. Spontaneity is related to an increase in randomness. CHEM 102, Fall 2014 LA TECH Reaction of K with water 17-10 Physical Process” S[H2O(l)] > S[H2O(s)] at 0 C. CHEM 102, Fall 2014 LA TECH 17-11 Gibbs Free Energy, G o DG = o DH - T o DS 1. If DH is negative it helps product to be favored 2. If DS is positive it helps product to be favored 3. If DG is negative reaction is product favored Gibbs free energy change = difference between the enthalpy of a system and the product of its absolute temperature and entropy predictor of spontaneity Total energy change of the system energy lost in disordering the system CHEM 102, Fall 2014 LA TECH 17-12 Thermodynamics Standard States The thermodynamic standard state of a substance is its most stable pure form under standard pressure (1 atm) and at some specific temperature (25 ºC or 298 K) superscript circle is used to denote a thermodynamic quantity that is under standard state conditions: ΔH = ΔH° ΔS = ΔS° ΔG = ΔG° CHEM 102, Fall 2014 LA TECH 17-13 Standard Thermodynamic Data o ΔH f o ΔG f S o - Standard Enthalpy of Formation - Standard Free Energy of Formation - Standard Free Energy of Formation Hydrogen ΔHof (kJ/mol) ΔGof (kJ/mol) So (J/mol K) H2 (g) H (g) H2O (l) H2O (g) 0 218.0 -285.8 -241.8 0 203.2 -237.1 -228.6 130.7 114.7 69.9 188.8 H2O2 (l) -187.8 -120.4 109.6 CHEM 102, Fall 2014 LA TECH 17-14 Standard Molar Entropy Values CHEM 102, Fall 2014 LA TECH 17-15 Chemical Thermodynamics spontaneous reaction – reaction which proceed without external assistance once started chemical thermodynamics helps predict which reactions are spontaneous CHEM 102, Fall 2014 LA TECH 17-16 Thermodynamics Will the rearrangement of a system decrease its energy? If yes, system is favored to react — a product-favored system. Most product-favored reactions are exothermic. Often referred to as spontaneous reactions. “Spontaneous” does not imply anything about time for reaction to occur. Kinetic factors are more important for certain reactions. CHEM 102, Fall 2014 LA TECH 17-17 1) Give the definitions of the following: a) Enthalpy (H): b) Enthalpy change of a thermo-chemical reaction (DH): c) Entropy of a substance (S): d) Entropy change of a chemical reaction(DS): e) Thermodynamic Standard State(0): CHEM 102, Fall 2014 LA TECH 17-18 Laws of Thermodynamics Zeroth: Thermal equilibrium and temperature First : The total energy of the universe is constant Second : The total entropy (S) of the universe is always increasing Third : The entropy(S) of a pure, perfectly formed crystalline substance at absolute zero is zero CHEM 102, Fall 2014 LA TECH 17-19 2) Give the definitions of the following: a) Zeroth Law of thermodynamics: b) First Law of thermodynamics: c) Second Law of thermodynamics: d) Third Law of thermodynamics: CHEM 102, Fall 2014 LA TECH 17-20 Why is it necessary to divide Universe into System and Surrounding Universe = System + Surrounding universe system surroundings Boundary? CHEM 102, Fall 2014 LA TECH 17-21 Types of Systems Isolated system no mass or energy exchange Closed system only energy exchange Open system both mass and energy exchange CHEM 102, Fall 2014 LA TECH 17-22 Why is it necessary to divide Universe into System and Surrounding Universe = System + Surrounding CHEM 102, Fall 2014 LA TECH 17-23 3) Why we need to divide universe into surroundings and system for thermodynamic calculations? Give the signs of the DH (heat) and DS (disorder) and DG ( free energy) when system lose or gain them. Loss Gain DH (heat) DS (disorder) DG ( free energy) CHEM 102, Fall 2014 LA TECH 17-24 Second Law of Thermodynamics In the universe the ENTROPY cannot decrease for any spontaneous process The entropy of the universe strives for a maximum in any spontaneous process, the entropy of the universe increases for product-favored process DSuniverse = ( Ssys + Ssurr) > 0 DSuniv = entropy of the Universe DSsys = entropy of the System DSsurr = entropy of the Surrounding DSuniv = DSsys + DSsurr CHEM 102, Fall 2014 LA TECH 17-25 Entropy of the Universe DSuniv = Dsuniv DSsys DSsurr + + + + +(DSsys>DSsurr) - + + (DSsurr>DSsys) CHEM 102, Fall 2014 LA TECH DSsys + DSsurr 17-26 4) Explain the ways that DS of the universe, DSuniv could be +. DSuniv = DSsys + DSsurr + + + CHEM 102, Fall 2014 LA TECH 17-27 Entropy and Dissolving CHEM 102, Fall 2014 LA TECH 17-28 5) Assign a sign to the entropy change for the following systems. a) mixing aqueous solutions of NaCl and KNO3 together: b) spreading grass seed on a lawn: c) raking and bagging leaves in the fall: shuffling a deck of cards: d) e) raking and burning leaves in the fall: CHEM 102, Fall 2014 LA TECH 17-29 Expansion of a Gas The positional probability is higher when particles are dispersed over a larger volume Matter tends to expand unless it is restricted CHEM 102, Fall 2014 LA TECH 17-30 Gas Expansion and Probability CHEM 102, Fall 2014 LA TECH 17-31 Entropies of Solid, Liquid and Gas Phases S (gases) > CHEM 102, Fall 2014 LA TECH S (liquids) > S (solids) 17-32 6) Taking following examples explain how disorder is related to a measuring positional probability) or dispersion among the allowed energy states? a) Expansion of gases: Two gas molecules trapped in two vessels with a tube with a stop cock. CHEM 102, Fall 2014 LA TECH 17-33 6) Taking following examples explain how disorder is related to a measuring positional probability) or dispersion among the allowed energy states. b) Distribution of Kinetic energy at 0, 25 and 100°C for O2 CHEM 102, Fall 2014 LA TECH 17-34 Entropy and Molecular Structure CHEM 102, Fall 2014 LA TECH 17-35 Entropy, S Entropies of ionic solids depend on coulombic attractions. o S (J/K•mol) CHEM 102, Fall 2014 LA TECH MgO 26.9 NaF 51.5 17-36 Qualitative Guidelines for Entropy Changes Entropies of gases higher than liquids higher than solids Entropies are higher for more complex structures than simpler structures Entropies of ionic solids are inversely related to the strength of ionic forces Entropy increases when making solutions of pure solids or pure liquids in a liquid solvent Entropy decrease when making solutions of gases in a liquid CHEM 102, Fall 2014 LA TECH 17-37 Entropy of a Solution of a Gas CHEM 102, Fall 2014 LA TECH 17-38 7) Arrange following in the order of increasing entropy? • a) C(s) (diamond) • b) C(s) (graphite) • c) O2(g) • d) CO2(g) • e) CO(g) • f) Hg(l) CHEM 102, Fall 2014 LA TECH 17-39 Entropy Change Entropy (DS) normally increase (+) for the following changes: i) Solid ---> liquid (melting) + ii) Liquid ---> gas + iii) Solid ----> gas most + iv) Increase in temperature + v) Increasing in pressure(constant volume, and temperature) + vi) Increase in volume + CHEM 102, Fall 2014 LA TECH 17-40 Qualitative prediction of DS of Chemical Reactions Look for (l) or (s) --> (g) If all are gases: calculate Dn Dn = Sn (gaseous prod.) - S n(gaseous reac.) N2 (g) + 3 H2 (g) --------> 2 NH3 (g) Dn = 2 - 4 = -2 If Dn is - DS is negative (decrease in S) If Dn is + DS is positive (increase in S) CHEM 102, Fall 2014 LA TECH 17-41 Predict DS! 2 C2H6(g) + 7 O2(g)--> 4 CO2(g) + 6H2O(g) 2 CO(g) + O2(g)-->2 CO2(g) HCl(g) + NH3(g)-->NH4Cl(s) H2(g) + Br2(l) --> 2 HBr(g) CHEM 102, Fall 2014 LA TECH 17-42 8) Taking following physical and chemical changes qualitatively predict the sign of DS. a) 2H2O (g) ------> 2 H2O (l) b) 2H2O (g) ------> 2 H2 (g) + O2 (g) c) N2 (g) + 3 H2 (g) ------> 2 NH3 (g) CHEM 102, Fall 2014 LA TECH 17-43 Entropy Changes for Phase Changes For a phase change, DS = q /T (q = heat transferred) Boiling Water H2O (liq) H2O(g) DH = q = +40,700 J/mol SYS SYS q 40,700 J/mol DS = = = + 109 J/K • mol T 373.15 K CHEM 102, Fall 2014 LA TECH 17-44 9) How is entropy related to the heat and temperature? CHEM 102, Fall 2014 LA TECH 17-45 Phase Transitions Heat of Fusion energy associated with phase transition solid-toliquid or liquid-to-solid DGfusion = 0 = DHfusion - T DSfusion 0 = DHfusion - T DSfusion DHfusion = T DSfusion Heat of Vaporization energy associated with phase transition gas-toliquid or liquid-to-gas DHvaporization = T DSvaporization CHEM 102, Fall 2014 LA TECH 17-46 10) The normal boiling point of benzene is 80.1°C and heat of evaporation (∆H°vap)is 30.7 kJ/mol. Calculate the ∆Ssurr (in J/K mol) for the evaporation of benzene. CHEM 102, Fall 2014 LA TECH 17-47 2nd Law of Thermodynamics 2 H2(g) + O2(g) 2 H2O(liq) DSosys = -326.9 J/K Entropy Changes in the Surroundings q surr - DH system DS surroundings = = T T o o o Can calc. that DH rxn = DH system = -571.7 kJ DS o surroundings = - (-571.7 kJ)(1000 J/kJ) 298.15 K = +1917 J/K CHEM 102, Fall 2014 LA TECH 17-48 2nd Law of Thermodynamics 2 H2(g) + O2(g) 2 H2O(liq) DSosys = -326.9 J/K DSosurr = +1917 J/K DSouni = +1590. J/K The entropy of the universe is increasing, so the reaction is product-favored. CHEM 102, Fall 2014 LA TECH 17-49 Gibbs Free Energy, G DSuniv = DSsurr + DSsys D S univ = -D H sys T + D S sys Multiply through by (-T) -TDSuniv = DHsys - TDSsys -TDSuniv = DGsystem Under standard conditions — DGo = DHo - TDSo CHEM 102, Fall 2014 LA TECH 17-50 Gibbs Free Energy, G DGo = DHo - T DSo Gibbs free energy change = difference between the enthalpy of a system and the product of its absolute temperature and entropy predictor of spontaneity Total energy change for system energy lost in disordering the system CHEM 102, Fall 2014 LA TECH 17-51 11) Define the following: a) Gibbs Free Energy (G): b) Gibbs Free Energy change for a reaction (DG): c) How is DGsys is related to DSuni and temperature? CHEM 102, Fall 2014 LA TECH 17-52 Free energy, DG The sign of DG indicates whether a reaction will occur spontaneously. + Not spontaneous 0 At equilibrium Spontaneous The fact that the effect of DS will vary as a function of temperature is important. This can result in changing the sign of DG. CHEM 102, Fall 2014 LA TECH 17-53 DG and Ecell The sign of DG indicates whether a reaction will occur spontaneously. Therefore Ecell value have to be + (positive) for spontaneous redox reaction DG = -nFEcell n = number of electrons transferred F = Faraday constant ((96500 C/mol) Ecell = E½(cathode)- E½(anode) CHEM 102, Fall 2014 LA TECH 17-54 How do you calculate DG at different T and P Nerst Equation, since DG = -nFE ell c DG = DGo + RT ln Q Q = reaction quotient at equilibrium DG = 0 0 = DGo + RT ln K DGo = - RT ln K If you know DGo you could calculate K or vice versa. CHEM 102, Fall 2014 LA TECH 17-55 11) Define the following: d) How you decided from the sign of DG whether and chemical reaction is? i) Spontaneous ii) Never take place iii) Equilibrium e) How is Gibbs Free Energy change (DG°) related to Ecell: f) How is non standard (DG) related to (DG°) and Q (reaction quotient) CHEM 102, Fall 2014 LA TECH 17-56 11) Define the following: g) How is standard (DG°) related to Keq (equilibrium constant)? CHEM 102, Fall 2014 LA TECH 17-57 Gibbs Free Energy, G o DG = o DH o TDS - DHo DSo DGo Reaction exo(-) increase(+) - Prod-favored endo(+) decrease(-) + React-favored exo(-) decrease(-) ? T dependent endo(+) increase(+) ? T dependent CHEM 102, Fall 2014 LA TECH 17-58 12) Predict the DGsys changes for different signs of DHsys and DSsys at low/high temperatures for the equation: DGsys = DHsys TDSsys DGsys DHsys + DTDSsys a) b) c) d) CHEM 102, Fall 2014 LA TECH 17-59