Chemical Reactions Equilibrium Part 4 Prepare for Final Exam Lecture 43 December 7th Lecture 44 December 9th Read Ch 15 6 15 7 Suggested Problems BLB 15 51 53 70 Packet 15 11 13 FINAL EXAM Monday Dec 12th 2 30 PM CH110 FA11 SAS 1 Chemical Reactions Equilibrium Part 4 KEY QUESTIONS THE BIG PICTURE Do real reactions always proceed to completion What forces drive the reaction process How do the properties of heterogeneous reactions differ from homogeneous Remember macroscopic properties come from microscopic origins What is Le Ch tlier s Principle When a system is disturbed why does it attempt to return to equilibrium How fast do systems reach equilibrium By the end of Today s Lecture you should know Calculations involving Keq Predicting the direction of a reaction using Q Le Ch tlier s Principle Catalysis CH110 FA11 SAS 2 Le Ch tlier s Principle and Reactions Consider the Following Reactions 2CrO42 aq 2H aq Cr2O72 aq H2O l Cr2O72 aq 2OH aq 2CrO42 aq H2O l Are these examples of Redox Reactions Are these Neutralization Reactions Cr2O72 is orange and CrO42 yellow What will happen to the solution if I add acid What will happen to the solution if I add base CH110 FA11 SAS 3 Le Ch tlier s Principle and the Real World Example the Haber Bosch Process Catalytic synthesis of ammonia from nitrogen and hydrogen gases cat N2 g 3 H2 g 2 NH3 g Why is this important Commodity chemical that is important to economy 13 6 million metric tons consumed in the U S in 2006 U S Geological Survey most recent data available Starting material for many other chemicals from fertilizers to pharmaceuticals CH110 FA11 SAS 4 Practice Problem N2 g 3H2 g 2NH3 g In the Haber Bosch process above the following equilibrium concentrations of NH3 N2 and H2 were determined at 472oC What is Kp for these conditions A B C D E 2 79x10 5 1 71x10 3 1 05x10 1 6 40 3 91x102 H2 0 1207 M N2 0 0402 M NH3 0 00272 M CH110 FA11 SAS 5 Practice Problem N2 g 3H2 g 2NH3 g In the Haber Bosch process above what is the estimated Hrxn based on the bond enthalpies shown below A B C D E 890 kJ mol 632 kJ mol 109 kJ mol 892 kJ mol 1064 kJ mol H H N H N N N N N N CH110 FA11 SAS 432 kJ mol 391 kJ mol 160 kJ mol 418 kJ mol 941 kJ mol 6 Le Ch tlier and Haber Bosch How can we increase NH3 production N2 g 3 H2 g cat 2 NH3 g In this reaction there are Less moles of product gas than reactant Remove product from reaction mixture Exothermic reaction heat is product The Problem it takes a LONG time for the reaction to reach equilibrium at low temp The Solution Use a catalyst Fe and metal oxides optimum temp is 400 500 oC P 200 600 atm CH110 FA11 SAS 7 How Fast is Equilibrium Reached The equilibrium position Keq tells us about how fast equilibrium will be reached from a given set of conditions N2 g O2 g 2NO g K 10 30 So 2NO g N2 g O2 g K 1030 BUT NO g at room temperature What DOES affect equilibrium concentration volume if gases are involved temperature changes the value of Kc What DOES affect the rate of a reaction CH110 FA11 SAS 8 Catalysts Equilibrium and Rates Catalysts consumed during the reaction Increases the at which equilibrium is achieved change Keq CH110 FA11 SAS 9 Demo Peroxide Decomposition 2 H2O2 l 2 H2O l O2 g Hrxn 196 kJ mol Keq 3 0 x 1020 25oC We have seen this reaction in multiple demonstrations and Keq favors Bottles of aqueous hydrogen peroxide solution are commonly used as a disinfectant so it must be relatively stable Metals in your blood catalyze the decomposition of H2O2 in your wound in Chem 110 we can catalyze the decomposition reaction with MnO2 CH110 FA11 SAS 10 Thermite Revisited 2AI s Fe2O3 s 2Fe s Al2O3 s Name the type of reaction What is Ho for the thermite reaction shown above Hof 822 2 kJ mol for Fe2O3 s Hof 1669 8 kJ mol for Al2O3 s A B C D E 847 6 kJ mol 847 6 kJ mol 2492 kJ mol 2492 kJ mol Can t be determined CH110 FA11 SAS 11 Phosphorus Which of the following reactions involving phosphorus is a redox reaction A P4 s 5O2 g P4O10 s B P4O10 s 6H2O l 4H3PO4 aq C 3Ca2 aq 2PO43 aq Ca3 PO4 2 s D CaO s H3PO4 aq CaHPO4 aq H2O l E 3Ca3 PO4 2 s 6H2SO4 aq 6CaSO4 aq 3Ca H2PO4 2 aq CH110 FA11 SAS 12 Dietary Calcium Cows milk contains 300 mg of calcium in the form of Ca3 PO4 2 per 240 mL serving The FDA recommends 1000 mg calcium day for adults If the solubility of tricalcium phosphate in water is about 0 0020 g 100 mL at room temperature how much Ca2 aq is available in a glass of milk A B C D E 0 78 mg 1 9 mg 4 8 mg 12 mg 190 mg CH110 FA11 SAS 13 Dietary Calcium Tricalcium phosphate Ca3 PO4 2 from cow s milk reacts with hydrochloric acid in the stomach yielding bio available calcium Ca3 PO4 2 s 2HCl aq Ca H2PO4 2 aq 2CaCl aq What is the equilibrium constant expression for this reaction CH110 FA11 SAS 14 What You Should Know Le Chatlier s Principle How does Q help us to predict the direction in which a reaction will proceed What drives a system to return to equilibrium when it has been disturbed What happens to a system when the concentration of reactants or products is increased or decreased What happens to a system when the volume is changed What happens to a system when the temperature is changed Catalysts Do catalysts change the position of equilibrium Why or why not Is equilibrium generally reached quickly How do catalysts change the rate at which equilibrium is achieved CH110 FA11 SAS 15 Final Concepts You Should Know Chapter 3 Chemical Equations How do I balance a chemical equation What is the percent yield of a reaction Chapter 4 Chemical Reactions What are precipitation reactions What are metathesis reactions What are redox reactions in particular and what are oxidation numbers What are net ionic equations and spectator ions Chapter 5 Thermochemistry What is enthalpy and the enthalpy of a reaction What is heat of formation How do bond enthalpies relate to both of these What is Hess s Law Chapter 15 Chemical Equilibrium What is an equilibrium constant How do you use an ICE table to calculate equilibrium constants or concentrations What is LeCh tlier s principle CH110 FA11 SAS 16
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