Test 3 Content List Outline a Forward and reverse processes occur at the same rate but there is no chemical change i Water Vapor with liquid water the only change is from liquid to vapor a Rates of forward and reverse reactions are equal Chemical change occurring Bonds 1 Physical Equilibrium 2 Chemical Equilibrium are broken formed 3 Nonreversible Reaction a Reactants completely consumed products are formed and it goes to completion i Such as Metabolizing glucose 4 Reversible Reaction a Involves forward and reverse reactions The rate for the reverse reaction increases as the amount of product increases b Chemical equilibrium is when the forward and reverse reaction rates are equal c Concentrations of reactions and products do not change d Consider A B i ii Over time the curve levels out and the concentrations are no longer changing 1 This is the chemical equilibrium iii A catalyst can get them there faster but the concentrations will be the same at e No matter what concentration you start with at some point equilibrium i ii iii K equilibrium constant 5 For a general reversible reaction such as a aA bB cC dD b The Law of Mass Action states that if the system is at equilibrium at a given temperature then the following is constant i ii Example 1 N2O4 g 2 NO2 g 2 Keq NO2 2 N2O4 c The larger the equilibrium constant the more it proceeds to completion i If K is large products are favored ii If K is less than 1 then the reactants are favored 1 Kfwd Krev 1 Kfw Krev 6 Equilibrium Constants can be expressed using Kc or Kp a Kc uses the concentration of the reactants and products b Kp uses pressure of the gaseous reactants and products c Example i 2 NO g O2 g 2NO2 g Concentrations of the reaction species at equilibrium are found to be NO 0 0542 M O2 0 127 M NO2 15 5 M Calculate the equilibrium constant Kc Kc NO2 2 NO 2 O2 Kc 15 5 2 0 0542 2 0 127 Kc 6 44 x 10 5 1 If you were given pressures you could use Kp d N2O4 g 2NO2 g i In terms of Kc 1 Kc NO2 2 N2O4 ii In terms of Kp 1 Kp PNO2 2 PN2O6 iii Kc does not equal Kp 7 Converting Kp to Kc a b Delta n moles of gaseous products moles of gaseous reactants i i i ii Delta n c d a b c Example i CO g 2H2 g CH3OH g Equilibrium constant Kc for the reaction is 10 5 at 220 degrees C What is the value of Kp at this temperature R 0 0821 L atm K mol T 273 220 493 K Delta n 1 3 2 Kp 10 5 0 0821 x 493 2 Kp 6 41 x 10 3 8 Homogeneous equilibrium a All species are in the same phase i Gas gas ii Liquid liquid iii Liquid aqueous i If it says l leave it out c N2O4 g 2NO2 g 9 Heterogeneous equilibrium a Reacting species are in different phases i Gas liquid ii Liquid solid iii Solid gas b Concentration of pure liquids not included in the expression for equilibrium constant Ignore solids and liquids for delta n b Concentration of solids are not included in the expression c d CaCO3 s CaO s CO2 g e Example i Write the expressions for the following reversible reactions at equilibrium HF aq H2O l H3O aq F aq Kc F H3O HF 10 Remember when writing equilibrium constant expressions a The concentrations of the reacting species in the condensed phase are expressed in M In the gaseous phase the concentrations can be expressed in M or in atm b The concentrations of pure solids pure liquids and solvents do not appear in the equilibrium constant expressions c The equilibrium constant is a dimensionless quantity d In quoting a value for the equilibrium constant you must specify the balanced equation and the temperature If the reaction can be expressed as a sum of two or more reactions then the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions e 11 Multiple Equilibria a A reaction can be an individual reaction step or a multistep reaction b If the overall reaction is the sum of two or more reactions the overall reaction of the Equilibrium Constant is the product of the Equilibrium Constants for the steps i ii iii Example iv Important 1 1 If the equation is REVERSED the equilibrium constant is inverted a A 2B 3C b 3C A 2B i Kfwd C 3 A B 2 i Krev A B 2 C 3 12 Reaction Quotient chemical reaction a A function of the concentrations or pressures of the chemical species involved in a b The reaction quotient Q has the same form as the equilibrium constant K c The major difference between Q and K is that the concentrations used in Q are not necessarily equilibrium values d e Q Kc f Q Kc g Q Kc i The system is at equilibrium ii Concentrations of reactants and products stays the same i The reaction proceeds to the right ii Alligator eats to the right iii Generates more products consumes more reactants i The reaction proceeds to the left ii Alligator eats to the left iii Consume more products generate more reactants h Q helps us predict how the equilibrium will respond to an applied stress 13 Predicting the Shift without calculations a CO g 2H2 g CH3OH g b What would happen if methanol CH3OH is added i Equilibrium will be disrupted and the increase in products mean that Q K ii In order to reestablish equilibrium the reaction will progress to the left towards the reactants c What would happen if more reactants were added i Q K because the value for the amount of reactants or the denominator of the Q expression has increased ii Reaction will favor product so reaction will progress to the right d These demonstrate Le Chatelier s Principle a If we are given the concentrations in a reaction mixture and Kc then we can predict which direction the reaction will proceed AND we can also calculate the concentration of each species when it reaches equilibrium b Steps of the ICE Method 14 Using the ICE Method i Write the balanced equation ii Write the reaction quotient Q iii Convert all amounts into the correct units M or atm iv When reaction diagram is not known compare Q with K v Construct a reaction table 1 Check the sign of x the change in the concentration or pressure vi Substitute the quantities into K equation vii To simplify the math assume that x is negligible viii Solve for x 1 Check that assumption is justified 5 error If not solve quadratic equation for x ix Find the equilibrium quantities c Example i Cis stilbene trans stilbene 1 Check to see that calculated values give the known K The equilibrium constant Kc for …
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