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 1 The mass action expression or reaction quotient has the symbol Q It is a measure of the progress of the reaction Q has the same form as Kc The major difference between Q and Kc is that the concentrations used in Q are not necessarily equilibrium values For this general reaction cC dD aA bB C D Q a b A B c d 2 Why do we need another equilibrium constant that does not use equilibrium concentrations Q will help us predict how the equilibrium will respond to an applied stress To make this prediction we compare Q with Kc Q is a measure of the progress of the reaction When Q Kc then the reaction is at equilibrium 3 When Q K c The system is at equilibrium Q K c The reaction occurs to the left to a greater extent Q K c The reaction occurs to the right to a greater extent To help understand this think of Q and K c as fractions 4 The equilibrium constant for the following reaction is 49 at 450oC If 0 22 mole of I2 0 22 mole of H2 and 0 66 mole of HI were put into an evacuated 1 00 liter container would the system be at equilibrium If not what must occur to establish equilibrium H2 I2 2HI 5 Kc 19 9 for the reaction Cl2 g F2 g 2 ClF g What will happen in a reaction mixture originally containing Cl2 0 5 M F2 0 2 M and ClF 7 3 M 6 The concentration eq constant for the gas phase reaction H2CO H2 CO has the numerical value 0 50 at a given T A mixture of H2CO H2 and CO is introduced into flask After a short time analysis of a small sample of the rxn mixture shows H2CO 0 50 M H2 0 80 M and CO 0 25 M 7 16 3 Determining an Equilibrium Constant One liter of equilibrium mixture from the following system at a high temperature was found to contain 0 172 mole of PCl3 0 086 mole of Cl2 and 0 028 mole of PCl5 Calculate Kc for the reaction PCl5 PCl3 Cl2 Equil s M 0 028 M 0 172 M 0 086 8 At a given temperature 0 80 mole of N2 and 0 90 mole of H2 were placed in an evacuated 1 00 liter container At equilibrium 0 20 mole of NH3 was present Calculate Kc for the reaction N2 3H2 2NH3 9 The decomposition of PCl5 was studied at another temperature One mole of PCl5 was introduced into an evacuated 1 00 liter container The system was allowed to reach equilibrium at the new temperature At equilibrium 0 60 mole of PCl3 was present in the container Calculate the equilibrium constant at this temperature PCl5 PCl3 Cl 2 10 A mixture of CO and Cl2 is placed in a reaction flask CO 0 0102 M Cl2 0 00609 M when the reaction CO g Cl2 g COCl2 g has come to equilibrium at 600 K Cl2 0 00301 M What is Kc 11 The equilibrium constant for the butane isobutane isomerization rxn is 2 5 at 25 C If 1 75 mol of butane and 1 25 mol of isobutane are mixed is the system at equilibrium 12 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 13 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 14 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 15 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 16 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 17 N2 C2H2 2HCN Kc 2 3x 10 4 at 300 C 3 5 M 2 5M initial conc what are the eq conc 18