For PC this week due Feb 16 do the following EOCs 2 4 8 12 14 16 20 24 26 28 32 38 44 48 53 62 64 Exam 1 gets released Friday Feb 13 at 8 pm Please rework problems you didn t get correct since you may see them again 1 16 4 Using Equilibrium Constants in Calculations 0 50 M of CO and H2O each put into a container and allowed to reach equilibrium what will be their eq conc Calc Q CO H2O CO2 H2 Kc 1 845 2 The equilibrium constant is 49 for the following reaction at 450oC If 1 00 mole of HI is put into an evacuated 1 00 liter container and allowed to reach equilibrium what will be the equilibrium concentration of each substance Enter eq H2 concentration into clicker H2 I2 2HI 3 H2 g CO2 g H2O g CO g At eq 0 11 mol each of H2O and CO and 0 087 mol of H2 and CO2 in a 1 0 L container Kc 4 Calculations Where the Solution Involves a Quadratic Expression Quadratic equations Appendix A for review ax2 bx c 0 Always gives 2 roots but only one will make sense 5 When do you need to use the quadratic equation Page 736 blue box at top of page problem solving tip 16 1 Quantity x that you add or subtract can be ignored if x is less than 5 of the quantity of reactant initially present Tip ignore X first because calculations are quicker then see if what you get divided by initial x 100 5 if so then you are fine if not then you have to use quadratic equation 6 N2 C2H2 2HCN Kc 2 3x 10 4 at 300 C 3 5 M 2 5M initial conc what are the eq conc 7 16 5 More about Balanced Equations and Equilibrium Constants The value of Kc depends upon how the balanced equation is written Kc PCl5 PCl3 Cl2 Equil s 0 028 M 0 172 M 0 086 M 8 PCl3 Cl2 PCl5 Equil s 0 172 M 0 086 M 0 028 M 9 2SO2 O2 2SO3 Kc 0 15 2SO3 2SO2 O2 Kc SO3 SO2 O2 Kc 10 If an equation for a reaction is multiplied by any factor n then the original value of Kc is raised to the nth power Thus we must always write the balanced chemical equation to accompany the value of Kc for a chemical reaction H2 Br2 2 HBr Kc 7 9 x 1011 at 500 K 1 2H2 1 2Br2 HBr Kc 11 AgCl s Ag aq Cl aq Kc 1 8 x 10 10 Kc Ag aq 2NH3 aq Ag NH3 2 aq Kc 1 1x107 Net equation AgCl s 2NH3 aq Ag NH3 2 aq Cl aq Knet K1 x K2 2 0 x 10 3 12 16 6 Disturbing a Chemical Equilibrium Equilibrium may be disturbed in 3 ways 1 Changing concentration of a reactant or product 2 Changing the volume for systems involving gases 3 Changing the T Le Chatelier s Principle A change in any of the factors that determine the equilibrium conditions of system will cause the system to change in such a manner as to reduce or counteract the effect of the change If a change of conditions stress is applied to a system in equilibrium the system responds in the way that best tends to reduce the stress in reaching a new state of equilibrium 13 Effect of the Addition or Removal of a Reactant or Product Also true for changes in pressure for reactions involving gases Look at the following system at equilibrium at 450oC H I 2 HI 2 g g 2 g HI Kc H 2 I2 2 49 If some H 2 is added Q K c This favors the forward reaction Equilbrium will shift to the right or product side 14 H 2 g I 2 g 2 HI g HI Kc H 2 I2 2 49 If we remove some H 2 Q K c This favors the reverse reaction Equilbrium will shift to the left or reactant side 15 Adding or removing reactants or products does not change Kc It only changes Q Equilibrium is then reestablished after some time to change Q back to Kc A B C D stress Q Direction of shift A B C D of A or B of C or D of A or B of C or D Q K Q K Q K Q K to right to left to left to right 16 Effect of Volume Change on Gas Phase Equilibria and pressure for reactions involving gases Predict what will happen if the volume of this system at equilibrium is changed by changing the pressure at constant temperature NO 2 NO2 g 2 4 g N 2O 4 Kc 2 NO2 17 Total P is increased by reducing the V the system shifts in the direction that occupies the smaller V in the direction with the fewest gas molecules 18 Stress Q V P Q Kc V P Q Kc Direction of shift of A 2D Toward smaller of moles of gas Toward larger of moles of gas If P changes are due to V changes then the above apply Adding a nonreacting gas to a system does not change the partial P of the reacting gas If you pump He into a reaction mixture but the He does not take part in the reaction then the partial P of the gases already present do not change system remains at equilibrium 19 N O 2 NO2 g 2 4 g N 2O 4 Kc NO2 2 If the volume is decreased which increases the pressure Q K c This favors product formation or the forward reaction The forward reaction produces fewer moles of gas 20 NO 2 NO2 g 2 4 g N 2O 4 Kc 2 NO2 If the volume is increased which decreases the pressure Q K c This favors the reactants or the reverse reaction More moles of gas are produced 21 N2O4 g 2NO2 g If add more N2O4 eq shifts which way Remove NO2 eq shifts which way 22 Effect of Temperature Changes on Equilibrium Composition Heat is added to a system by increasing T equilibrium shifts to undo effect 23 Consider the following reaction at equilibrium o 2 SO 2 g O 2 g 2 SO3 g H rxn 198 kJ mol Is heat a reactant or product in this reaction 2 SO 198 kJ 2 SO 2 g O 2 g 3 g Heat is a product of this reaction Increasing the reaction t emperature stresses the products 24 This favors the reverse or reactant reaction 25 p 678 N2O4 g 2NO2 g H 58 kJ mole If increase heat which way does the eq shift If decrease heat which way does the eq shift 26 How will an increase in temperature affect each of the following reactions Reaction Effect on Equilibrium a 2 NO 2 g N 2O 4 g H orxn 0 b H Cl 2 HCl 92 kJ 2 …