Week 12/Th: Lecture Units 31 & 32

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1 Week 12/Th: Lecture Units 31 & 32 Unit 30: Chemical Spontaneity -- entropy, 2 nd Law of Thermo -- free energy -- spontaneity Unit 31: Phase Equilibria -- liquid / gas -- phase diagrams -- phase boundaries Unit 32: Solutions -- more concentration units Issues: Exam 3 next Monday!!! Homework Set 9 due on 08:00AM

2 Week 12/Th: Help Preparing for Exam Tuesday, November 13 th, 6-8:30pm in N100 BCC (LRC Mock Exam, reservation required!) Thursday, November 15 th, 6-8:30pm in N100 BCC (LRC Q&A Review Session) Sunday, November18 th, 4-6pm in 138 Chemistry (Dr. Pollock) Monday, November 19 th, EXAM day for everyone Tuesday, November 20 th, NO LECTURE Happy Thanksgiving! DJMorrissey, 2oo9

Week 12/Th: Where to Take the Exam CEM 141 - Fall Semester 2012 Exam 3 Rooms Monday,

3 Week 12/Th: Where to Take the Exam CEM Fall Semester 2012 Exam 3 Rooms Monday, November 19th 7:15pm-8:15pm Alternate Exam Monday, November 19, 6:45am-7:45am room 138 Chemistry

4 Week 12/Th: Phase Diagram.. Generic Last time we considered the equilibrium between the liquid and gas phases of a pure material. We can also look for equilibria between the other phases and we find a general pattern that we can display on a phase diagram for a pure material. Note: watch out for relative scales on the Temperature and Pressure axes Land marks: A triple point B critical point C Solid/Gas Equil. ΔS + ΔH + D Solid/Liquid Equil. Lines ΔP / ΔT

5 Week 12/Th: Phase Equilibria The boundaries between the phases of a pure material are points of equilibria between the phases. The energy content of the two phases is equal at equilibrium and the system does not change at the macroscopic level, ΔG = 0 ΔG = ΔH TΔS = 0 SULFUR ΔH ΔH / T = TΔS = ΔS for equilibria and for reversible change For example, NON-Hydrogen bonded liquids at their normal boiling points: ΔH VAP / T NB = ΔS VAP ~ 10.5 R

average value of 10.5 R and why? Q: What is ΔS VAP for water? Figure: http://serc.carleton.

6 Week 12/Th: Phase Diagram.. Water Q: Do you expect the ΔS VAP for water to be less than, about the same, or larger than the average value of 10.5 R and why? Q: What is ΔS VAP for water? Figure: research_education/equilibria/ phaserule.html A: Larger due to H-bonds in liquid not present in gas phase. ΔH VAP / T NB = (40.7 kj/mol) / 373 K = kj/mol-k ~ 13 * R

7 Week 12/Th: Solutions & Phase Change There are a number of practical applications that rely on understanding how the phase diagram of pure water changes for aqueous solutions. Recall that the definition of a solution is a mixture of matter that is random all the way down to the smallest components. For example, all mixtures of gases are solutions, alloys are solutions, most mixtures of solids are not solutions. The solubility of a substance in a solvent is often given in terms of the number of grams that can be dissolved in a given volume. Most of the time we are concerned with aqueous solutions and all of the solutions can be characterized by the amounts of the different components that are present. We have already introduced two units of measure for this

8 Week 12/Th: Concentration Units Name Symbol Definition Use Molarity M Moles solute per Liter of Solution molality m Moles solute per kg of Solvent Lab reactions Stoichiometry Physical properties (a number ratio) Unknown Quantity Moles of Solvent, Temperature dependent Volume of solution Mole fraction X Moles solute per Total moles Physical properties, gas mixtures (a number ratio) Volume of solution Weight percent Wt % (w/w) Mass of solute per Total mass Commercial composition (easy) Density and/or Volume of Solution (molar masses)

9 Week 12/Th: Example in Notes The density of a 19% solution of HCl is g/ml. (1) Calculate the molarity of the solution. (2) What is the mole fraction of HCl in this solution? 1L of solution = 1 L * 1000 ml/l * g/ml = g Mass of HCl in 1L = g * 19/100 = g Moles of HCl in 1L = 207.7g / g/mol = mol Thus moles in 1L à Molar

10 Week 12/Th: Example in Notes The density of a 19% solution of HCl is g/ml. (1) Calculate the molarity of the solution. (2) What is the mole fraction of HCl in this solution? 1L of solution = 1 L * 1000 ml/l * g/ml = g Mass of HCl in 1L = g * 19/100 = g Moles of HCl in 1L = 207.7g / g/mol = mol Mole Fraction of HCl = moles HCl / Total moles Moles water in 1L = ( )g / g/mol = mol water Χ(HCl) = / ( ) =

11 Week 12/Th: Another Concentration Example Concentrated nitric acid is listed as 70.7% (by mass) with a density of g/cm 3 ; calculate other concentration units. 1L of solution = 1 L * 1000 ml/l * g/ml = 1408 g Exam Review: HNO 3 -- Lewis Structure? -- # of lone pairs on N? -- bond order? -- electron arrangement? -- 3D structure? Mass of HNO 3 in 1L = 1408g * 70.7/100 = g a) Moles HNO 3 in 1L = 995.5g / 63.02g/mol = mol à 15.8 M molality = moles solute / kg of solvent Mass of water in 1 L = kg = kg b) molality = 15.8 moles HNO 3 / kg water = 38.3 m

12 Week 12/Th: Another Concentration Example Concentrated nitric acid is listed as 70.7% (by mass) with a density of g/cm 3 ; calculate other concentration units. 1L of solution = 1 L * 1000 ml/l * g/ml = 1408 g Mass of HNO 3 in 1L = 1408g * 70.7/100 = g a) Moles HNO 3 in 1L = 995.5g / 63.02g/mol = mol à 15.8 M Mass of water in 1 L = kg = kg b) molality = moles HNO 3 / kg water = 38.3 m 15.8M 38.3m X(HNO 3 ) = Mole fraction HNO 3 = moles HNO 3 / total moles Moles water = g / g/mol = mol c) X(HNO 3 ) = 15.8 moles / ( ) moles = 0.408

13 Week 12/Th: A Michigan Example Dow Chemical got started in Michigan extracting bromine from the brine water in the ground near Midland with NaBr at ~ 4.0x10-3 M. (a) How many grams of bromine per 1.0 m 3 of solution? (b) What is the volume of liquid bromine? The reaction is: 2NaBr (aq) + Cl 2 (g) à 2NaCl (aq) + Br 2 (l) 1m 3 of solution à 1 m 3 * 1000 L/m 3 * 4.0x10-3 mol/l = 4.0 mol NaBr a) Mass of Br 2 = 4.0mol NaBr * (1mol Br 2 /2 mol NaBr) * (79.9*2) g/mol Br 2 = 3.2x10 2 g b) Volume of Br 2 Volume = mass / density = 3.2x10 2 g / g/ml = 1.1x10 2 ml

First Law of Thermodynamics

First Law of Thermodynamics Remember: ΔE univ = 0 Total energy of the universe is constant. Energy can be transferred: ΔE = q + w q = heat w = work (F*D) = ΔPV 1 st Law, review For constant volume process: