# Wright CHM 1220 - CHM 1220 SI_Chemical Equilibrium_Rev 2_ANSWERS (2 pages)

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**View the full content.**## CHM 1220 SI_Chemical Equilibrium_Rev 2_ANSWERS

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## CHM 1220 SI_Chemical Equilibrium_Rev 2_ANSWERS

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- Chm 1220 - General Chemistry II

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Questions Comments Suggestions Email me at stevens 161 wright edu Chapter 14 SI Activity Chemical Equilibrium ANSWERS CHM 1220 Problems 1 The equilibrium constant Kp for the reaction PCl5 g PCl3 g Cl2 g is 3 81 102 at 600 K and 2 69 103 at 700 K Without doing any calculations predict whether this reaction is endothermic or exothermic Since the reaction becomes increasingly product favored as the temperature increases the reaction must be endothermic by Le Ch tlier s principle We can imagine heat as a reactant for endothermic reactions so heating the reaction vessel would be like adding reactant which forces the reaction toward the right Verify your answer in part by calculating the change in enthalpy Need to find H for rxn given two Ks and two Ts K H 1 1 ln 2 R T2 T1 K1 1 1 K R ln 2 H T2 T1 K1 1 H 1 K 1 1 2 69 103 1 1 R ln 2 8 31447 molJ K ln 68 3 2 3 81 10 700 K 600 K K1 T2 T1 kJ mol Predict how the equilibrium would shift under the following stresses i an increase in volume ii addition of an inert gas and iii addition of a catalyst i Rxn shifts toward side with more molecules of gas to fill the available space ii Adding inert gas i iii Shifts Right Products No effect increases the pressure which causes the rxn to shift toward the side with fewer moles of gas iii Catalysts affect the rates of the forward and reverse rxns equally so there is no net effect on the equilibrium ii Shifts Left Reactants Calculate Kc for the reaction at 600 K Use relationship between Kc and Kp K p K c RT n K c K p RT n 3 81 102 0 082057 L atm mol K 600 K 2 1 7 73824 7 74 If the initial concentrations of the species are PCl5 0 500 M PCl3 0 150 M and Cl2 0 600 at 600 K in which direction does the reaction proceed toward equilibrium What are the equilibrium Use values from ICE table in equilibrium expression concentrations PCl3 Cl2 0 600 x 0 150 x Need Q before determining rxn direction Kc PCl5 0 500 x PCl3 Cl2 0 600 M 0 150 M 0 180 Q Rewrite expression as a standard form quadratic eqn PCl5 0 500 M K c 0 500 x 0 600 x 0 150 x Comparing Q to K 3 869 7 738 x 0 0900 0 750 x x 2 Since Q 0 180 is smaller than K 7 74 the rxn will proceed to the right products Then need ICE table to find concentrations I C E PCl5 g PCl3 g Cl2 g 0 500 M 0 150 M 0 600 M x x x 0 500 x 0 150 x 0 600 x 0 3 779 8 488 x x 2 Solve using the quadratic formula work not shown x 8 912 or x 0 4240 Reject because would cause a negative concentration Calculate equilibrium concentrations PCl x 0 500 0 4240 0 0759 M 5 0 500 0 150 x 0 150 0 4240 0 574 M PCl 3 0 600 x 0 600 0 4240 1 02 M Cl 2 2 The equilibrium concentrations in a gas mixture at a 298 K for the reaction H2 I2 2 HI are H2 0 13 M I2 0 70 M and HI 2 1 What equilibrium concentrations are obtained at the same temperature when 0 20 mol of HI is injected into an empty 500 0 mL container You may assume all three gases behave ideally Calculate Kc from data given HI 2 1 48 46154 Kc H 2 I2 0 13 0 70 2 2 Calculate HI injected n 0 20 mol 0 40 M HI V 1L 500 mL 1000 mL Use ICE table to find new concentrations I C E H2 g 0 x x I2 g 0 x x 2 HI g 0 40 M 2x 0 40 2 x Rewrite expression as a standard form quadratic eqn K c x 0 16 1 6 x 4 x 2 2 48 46154 x 2 0 16 1 6 x 4 x 2 0 0 16 1 6 x 44 46154 Solve using the quadratic formula work not shown x 0 08062 or x 0 04464 Reject because would cause a negative concentration Calculate equilibrium concentrations H 2 I2 x 0 045 M HI 0 40 2 x 0 40 2 0 04464 0 31 M Use values from ICE table in equilibrium expression HI 0 40 2 x Kc H 2 I2 x x 2 2 3 For a particular biochemical reaction of the form A 2 B at 37 C normal body temperature the enthalpy change is 20 kJ mol and the entropy change is 34 J mol K Assuming that these quantities are independent of temperature calculate the temperature conditions under which the reaction is product favored Then calculate the equilibrium concentration of the product if the reaction were to take place at 37 C with an initial reactant concentration of 2 00 M Next set up an ICE table Product favored when K 1 so when ln K 0 A 2B H S 0 ln K I 2 00 M 0 00 M RT R C x 2x S H E 2 00 x 2x R RT Use values from ICE table in equilibrium expression T S H 2 2 B 2 x R R K A 2 00 x 2 0 104 moJol H T Rewrite expression as a standard form quadratic eqn S 34 molJ K T 590 K Use the given data to find K H S ln K RT R J J 2 0 104 mol 34 mol ln K J 8 31447 mol K 335 K 8 31447 molJ K ln K 3 09118 K e 3 09118 K 0 045448 Rev 2 Fall 2017 J Stevens K 2 00 x 2 x 2 0 090897 0 045448 x 4 x 2 0 Solve using the quadratic formula work not shown x 0 156 or x 0 1452 Reject because would cause a negative concentration Calculate equilibrium concentration B 2 x 2 0 1452 0 29 M

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