Chemical Reactions Equilibrium Part 1 Prepare for Recitation December 8th ALEKS Objective 14 due SUNDAY December 4th ANGEL Quiz 14 December 8th ALEKS FINAL ASSESSMENT Deadline December 9th Lecture 40 November 30th Lecture 41 December 2nd Lecture 42 December 5th Read Ch 15 1 15 5 HW Preparation BLB 15 21 28 29 32 35 46 Packet 15 1 10 CH110 FA11 SAS 1 Chemical Reactions Equilibrium Part 1 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 the meaning of chemical equilibrium How does the equilibrium constant help explain the extent of a reaction What is the relationship between various equilibrium constant expressions By the end of Today s Lecture you should know Chemical Equilibrium Equilibrium Constant Keq Equilibrium constant expression Relationship between Kp and Kc CH110 FA11 SAS 2 Chemical Equilibrium Chemical Equilibrium Reactants Products At Equilibrium 1 2 Equilibrium is not always achieved quickly Examples we ve already seen vapor pressure above liquid saturated solution phase equilibrium CH110 FA11 SAS 3 Equilibrium Graphically CO 3 H2 CH4 H2O 1200 K start 1 mol CO 3 mol H2 in 10 0 L volume moles forward reverse reactions proceed 3 2 1 H2 CO CH4 or H2O time reaching equilibrium What affect do starting concentrations have on equilibrium concentrations At equilibrium 1 839 mol H2 0 613 mol CO 0 387 mol H2O CH4 CH110 FA11 SAS 4 Equilibrium Constant aA bB cC dD If units are given in Keq Kc If units are given in Keq Kp Keq provides the products to reactants at equilibrium It is a KEQ is a for a given temperature It is a function of the temperature and for gases not the CH110 FA11 SAS 5 How are Kc and Kp related CO 3 H2 Kc CH4 H2O n P V Kp Kp Kp T 1200K RT Kp n change in number of moles going from reactants to products CH110 FA11 SAS 6 Practice Problem Acetic acid is a weak acid that dissociates as follows in aqueous solution CH3COOH aq CH3COO aq H aq At equilibrium at 25oC a 0 1000 M solution of acetic acid has the following concentrations CH3COOH 0 0990 M CH3COO 1 33x10 3 M and H 1 33x10 3 M What is the equilibrium constant Kc for the ionization of acetic acid at 25oC A B C D E 5 71x104 1 00x10 1 1 75x10 7 1 79x10 5 5 71x106 CH110 FA11 SAS 7 Practice Problem A 2 00 L contains A g and B g which follow the equilibrium shown below At 0oC the equilibrium partial pressures are A g 0 360 atm B g 0 780 atm What is the value of Kp A g 2B g A 1 14 B 2 17 C 0 845 D 1 69 E 7 65 CH110 FA11 SAS 8 Equilibrium Demo 2NO2 N2O4 Keq is a function of temperature only the volume of the tube is constant As I change the temperature of the sealed tube I shift the equilibrium and you see a color change CH110 FA11 SAS 9 Practice Problem Which of the following is the correct equilibrium constant expression for the equilibrium between dinitrogen tetroxide and nitrogen dioxide N2O4 g A NO2 N2O4 B NO2 2 N2O4 C NO2 N2O4 2 D NO2 N2O4 E NO2 2 N2O4 2NO2 g CH110 FA11 SAS 10 Practice Problem 2NO g N2 g O2 g In the reaction above the following equilibrium concentrations of NO N2 and O2 were determined at 800oC What are Kc and Kp for these conditions NO 0 0120 M N2 0 508 M O2 0 410 M CH110 FA11 SAS 11 Keq for Related Reactions Rule 1 The same equilibrium is reached regardless of the direction it is approached from Keq for a reaction in the reverse direction is the of Keq in the forward direction 2 NO g N2 g O2 g N2 g O2 g Keq 2 NO g Keq CH110 FA11 SAS 12 Keq for Related Reactions Rule 2 Stoichiometry If the coefficients in a reaction are multiplied or divided by an integer their Keq are related by the exponential of that integer Sometimes it is useful to manipulate the overall stoichiometry of a reaction for thermochemical calculations 2 NO g N2 g O2 g Keq NO g N2 g O2 g Keq CH110 FA11 SAS 13 Keq for Related Reactions Rule 3 The Keq for a net reaction is the product of the Keq for the individual reactions It is often necessary to break a reaction into two or more steps for Hess s Law calculations N2 g O2 g 2 NO g Keq 2NO g O2 g 2 NO2 g Keq N2 g 2O2 g 2 NO2 g Keq CH110 FA11 SAS 14 Practice Problem The following reaction reaches equilibrium at a certain temperature and Keq is determined H2 I2 2 HI Keq 16 At the same Temperature what is Keq for the reaction HI A B C D E 1 2 H2 1 2 I2 16 4 1 4 1 16 There is not enough information to answer this problem CH110 FA11 SAS 15 What You Should Know Chemical Equilibrium and Keq What is happening in a system that has reached chemical equilibrium What is the meaning of the equilibrium constant Keq and how does it help predict the outcome of a reaction What is the relationship between Kc and Kp and what does each tell you If you know Keq for a given reaction how does it help you understand what will happen in a related reaction CH110 FA11 SAS 16
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