2Chemical EquilibriumThe state where the concentrations of all reactants and products remain constant with time.On the molecular level, there is frantic activity. Equilibrium is not static, but is a highly dynamic situation.(BDVD)
3Dynamic Equilibrium Reactions continue to take place. A + B C + D ( forward)C + D A + B (reverse)Initially there is only A and B so only the forward reaction is possibleAs C and D build up, the reverse reaction speeds up while the forward reaction slows down.Eventually the rates are equal.
5What is equal at Equilibrium? Rates are equal.Concentrations are not.Rates are determined by concentrations and activation energy.The concentrations do not change at equilibrium.
613.2 Law of Mass Action jA + kB lC + mD j, k, l, m are coefficients The law of mass action is represented by the equilibrium expression:K = [C]l[D]m PRODUCTSpower [A]j[B]k REACTANTSpowerK is called the equilibrium constant.is how we indicate a reversible reaction. [x] represents concentration.
7Playing with K If we write the reaction in reverse. lC + mD jA + kB Then the new equilibrium constant isK’ = [A]j[B]k = 1/K [C]l[D]m
8K’ = [A]nj[B]nk = ([A] j[B]k)n = Kn [C]nl[D]nm ([C]l[D]m)n Playing with KIf we multiply the equation by a constantnjA + nkB nlC + nmDThen the equilibrium constant isK’ = [A]nj[B]nk = ([A] j[B]k)n = Kn [C]nl[D]nm ([C]l[D]m)n
9K is CONSTANT At any temperature. Temperature affects rate. The equilibrium concentrations don’t have to be the same only K.Equilibrium position is a set of concentrations at equilibrium.One value at each temperature, but there are an unlimited number of possibilities.Usually written without units.
10Equilibrium Expression 4NH3(g) + 7O2(g) NO2(g) + 6H2O(g)What is the equilibrium expression for this reaction? #19aN2(g) + O2(g) NO(g)What is the equilibrium expression for this reaction? #19bN2O4 (g) 2NO2(g)
11Calculate K N2(g) + 3H2(g) 2NH3 (g) In the above reaction, at a given temperature. K =Calculate the value of K in the following reactions. #21ab1/2N2(g) + 3/2H2(g) NH3(g)2NH3(g) N2(g) + 3H2(g)
12Calculate K N2(g) + 3H2(g) 2NH3(g) Initial At Equilibrium [N2]0 =1.000 M [N2] = 0.921M[H2]0 =1.000 M [H2] = 0.763M[NH3]0 =0 M [NH3] = 0.157M
13Calculate K N2(g) + 3H2(g) 2NH3(g) Initial At Equilibrium [N2]0 = 0 M [N2] = M[H2]0 = 0 M [H2] = M[NH3]0 = M [NH3] = 0.203MK is the same no matter what the amount of starting materials (at the same temperature).
1413.3 Equilibrium and Pressure Some reactions are gaseousPV = nRTP = (n/V)RTP = CRTC is a concentration in moles/LiterC = P/RT
15Equilibrium and Pressure 2SO2(g) + O2 (g) 2SO3 (g)In term of partial pressures:Kp = (PSO3) (PSO2)2 (PO2)In terms of concentration:Kc = [SO3] [SO2]2 [O2]
16Practice N2(g) + 3H2(g) 2NH3(g) In the reaction above, the following equilibrium pressures were observed.PNH3 = 0.89 atmPN2 = 0.62 atmPH2 = atmCalculate the value for the equilibrium constant Kp at this temperature.
17K v. Kp For jA + kB lC + mD Kp = K(RT)n n = sum of coefficients of gaseous products minus sum of coefficients of gaseous reactants.
18Practice Kp = K(RT)n At 25°C, K = 2.01 x 104 for the reaction CO(g) + Cl2(g) COCl2(g)What is the value of Kp at this temperature?
19Practice #32For which reaction in #31 is Kp = K?
20Homogeneous Equilibria So far every example dealt with reactants and products where all were in the same phase.We can use K in terms of either concentration or pressure.Units depend on reaction.
2113.4 Heterogeneous Equilibria Are equilibria that involve more than one phase.If the reaction involves pure solids or pure liquids, the concentration of the solid or the liquid doesn’t change.The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present.As long as they are not used up we can leave them out of the equilibrium expression.
22For Example H2(g) + I2(s) 2HI(g) K = [HI]2 [H2][I2] K = [HI]2 [H2] But the concentration of I2 does not change, therefore:K = [HI] [H2]
2313.5 Applying the Equilibrium Constant Reactions with large K (>>1), essentially to completion. Large negative E.Reactions with small K (<<1) consist mostly of reactants.Time to reach equilibrium is related to rate and AE. It is not related to size of K.
24The Reaction QuotientTells you the directing the reaction will go to reach equilibriumCalculated the same as the equilibrium constant, but for a system not at equilibrium by using initial concentrations.Q = [Products]coefficient [Reactants] coefficientCompare value to equilibrium constant
25What Q tells us If Q<K Not enough products Shift to right If Q>K Too many productsShift to leftIf Q=K system is at equilibrium
26Using the Reaction Quotient For the reaction2NOCl(g) NO(g) + Cl2(g)K = 1.55 x 10-5 M at 35ºCIn an experiment mol NOCl, mol NO(g) and mol Cl2 are mixed in 2.0 L flask.Which direction will the reaction proceed to reach equilibrium?
27Using the Reaction Quotient For the synthesis of ammonia at 500°C, the equilibrium constant is 6.0x10-2. Predict the direction in which the system will shift to reach equilibrium in each of the following cases. Ex. 13.7[NH3]0 = 1.0 x 10-3 M; [N2]0 = 1.0 x 10-5 M; [H2]0 = 2.0 x 10-3 M[NH3]0 = 1.0 x 10-4 M; [N2]0 = 5.0 M;[H2]0 = 1.0 x 10-2 M
28Problems Involving Pressure For the reaction N2O4(g) NO2(g) KP = atm. At equilibrium, the pressure of N2O4 was found to be 2.71 atm? Calculate the equilibrium pressure of NO2(g) EX.13.8
29Problems Involving Pressure At a certain temperature a 1.00 L flask initially contained mol PCl3(g) and 8.70 x 10-3 mol PCl5(g). After the system reached equilibrium, 2.00 x 10-3 mol Cl2(g) was found in the flask. Calculate the equilibrium concentrations of all species and the value of K. EX.13.9PCl5(g) PCl3(g) + Cl2(g)
30No Equilibrium [x] ExCarbon monoxide reacts with steam to produce carbon dioxide and hydrogen. At 700K the equilibrium constant is Calculate the equilibrium concentrations of all species if 1.00 mol of each component is mixed in a 1.00 L flask.
31Intro to ICE 2 SO3(g) 2SO2 (g) + O2(g) At a certain temperature mol SO3 is placed into a 3.0 L rigid container. The SO3 dissociates during the reaction. At equilibrium, 3.0 mol SO2 is present. What is the value of K for this reaction?(#43)
32Intro to ICE2NH3 N2 + 3H2At a certain temperature. 4.0 mol NH3 is introduced into a 2.0 L container. The NH3 partially dissociates during the reaction. At equilibrium, 2.0 mol NH3 remains. What is the value of K for this reaction?(#44) WAssign
33What if you’re not given equilibrium concentration? H2 + F2 2HFThe Equilibrium constant for the above reaction is 115 at a certain temperature mol of each component was added to a L flask. Calculate the equilibrium concentrations of all species.(ex.13.11)
34Practical Example H2(g) + F2(g) 2HF(g) K = 1.15 x 102 at 25ºC Calculate the equilibrium concentrations if a 3.00 L container initially contains 3.00 mol of H2 and 6.00 mol F2 .[H2]0 = 3.00 mol/3.00 L = 1.00 M[F2]0 = 6.00 mol/3.00 L = 2.00 M[HF]0 = 0
35Q= 0<K so more product will be formed. Assumption since K is large reaction will go to completion.Stoichiometry tells us H2 is LR, it will be smallest at equilibrium let it be xSet up table of initial, change and equilibrium in concentrations.
36H2(g) F2(g) 2HF(g) Initial 1.00 M 2.00 M 0 M Change EquilibriumFor H2 and F2 the change must be -XUsing to stoichiometry HF must be +2XEquilibrium = initial + change
37H2(g) F2(g) 2HF(g) Initial 1.00 M 2.00 M 0 M Change -X -X +2X Equili 1.00 -X 2.00-X 2X Therefore, ice chart looks like this.Change in HF = twice change in H2
38H2(g) F2(g) 2HF(g) Initial 1.00 M 2.00 M 0 M Change -X -X +2X Equili 1.00 -X 2.00-X 2X Now plug these values into the equilibrium expressionK = 1.15 x 102 = (2X)2(1.00-x)(2.00-x)Solving this gives us a quadratic equation.Quadratic Calculator
39H2(g) F2(g) 2HF(g) Initial 1.00 M 2.00 M 0 M Change -X -X +2X Equili 1.00 -X 2.00-X 2X Now plug these values into the equilibrium expressionK = 1.5 x 102 = (2X)2(1.00-x)(2.00-x)Solving this gives us a quadratic equation.Quadratic gives us 2.14 mol/L and mol/L. Only is reasonable.
40[H2] = 1.00 M M = 3.2 x 10-2M[F2] = 2.00 M M = M[HF] = 2(0.968 M) = MIf substituted into the equilibrium expression we get 1.13 x 102 which is very close to given K.
41Practice H2O(g) + Cl2O(g) 2HClO(g) K = 0.090 In an experiment 1.0 g H2O(g) and 2.0 g Cl2O are mixed in a 1.00 L flask.Calculate the equilibrium concentrations.(#48)
4213.6 Solving Equilibrium Problems Balance the equation.Write the equilibrium expression.List the initial concentrations.Calculate Q and determine the shift to equilibrium.Define equilibrium concentrations.Substitute equilibrium concentrations into equilibrium expression and solve.Check calculated concentrations by calculating K.
44Process is the sameSet up table of initial, change, and equilibrium concentrations.Choose X to be small.For this case it will be a product.For a small K the product concentration is small.
45For example For the reaction 2NOCl 2NO +Cl2 K= 1.6 x 10-5 If 1.00 mol NOCl is put in a 2.0 L container, what are the equilibrium concentrations?K = [NO]2[Cl2] = 1.6 x [NOCl]2[NOCl]0= 0.50M, [NO]& [Cl2] =0
46K = [NO]2[Cl2] = (2x)2(x) = 1.6 x 10-5 [NOCl]2 (0.50 -2x)2 2NOCl 2NO + Cl2InitialChange -2X X +XEquil X X XK = [NO]2[Cl2] = (2x)2(x) = 1.6 x [NOCl] ( x)2Since K is so small, we we can make an approximation that x = 0.50This makes the math much easier. X = 1.0 x 10-2
475% RuleMany of the systems we will deal with have very small equilibrium constants.When this is the case, there will be very little shift to the right to reach equilibrium. Since x is so small, we will ignore it. However, the final value must be checked against the initial concentration. If the difference is less than 5%, then our assumption is valid.In the previous problem X = 1.0 x 10-2x = (1.0 x 10-2) = 0.48Is 1.0 x 10-2 five percent or less than 0.50?0.02/0.50 x 100 = 4%
48Practice Problem For the reaction N2O4(g) 2NO2(g) K = 4.0 x 10-7 1.0 mol N2O4(g) is placed in a 10.0L vessel. Calculate the concentrations of all species at equilibrium.(#52)
49Practice Problem For the reaction COCl2(g) CO(g) + Cl2(g) Kp = 6.8 x 10-9If COCl2(g) at an initial pressure of 1.00 atm decomposes, calculate the equilibrium pressures of all species?(#54)
50Practice Problem At 25°C, Kp = 2.9 x 10-3 NH4OCONH2(s) 2NH3g) + CO2(g) In an experiment, a certain amount of NH4OCONH2(s) is placed in an evacuated rigid container and allowed to come to equilibrium. Calculated the total pressure in the container at equilibrium.(#55)
51Practice Problem #56WA 2AsH3(g) 2As(s) + 3H2(g) In an experiment pure AsH3 was placed in a rigid, sealed flask at a pressure of torr. After 48 hours the pressure was observed to be constant at torr.Calculate the Equilibrium pressure of H2(g).Calculate Kp for this reaction. (#56)
52Le Chatelier’s Principle If a change is applied to a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce the change.3 Types of change: Concentration (adding or reducing reactant or product), Pressure, Temperature.
53Change amounts of reactants and/or products Adding product makes Q>KRemoving reactant makes Q>KAdding reactant makes Q<KRemoving product makes Q<KDetermine the effect on Q, will tell you the direction of shift.The system will shift away from the added component.
54Change Pressure By changing volume. When volume is reduced, the system will move in the direction that has the least moles of gas.COCl2(g) CO(g) + Cl2(g)When volume is increased, the system will move in the direction that has the greatest moles of gas.Because partial pressures (and conc.) change a new equilibrium must be reached.System tries to minimize the moles of gas.
55Change in Pressure By adding an inert gas. Partial pressures of reactants and product are not changed.No effect on equilibrium position.
56Change in TemperatureAffects the rates of both the forward and reverse reactions.Doesn’t just change the equilibrium position, changes the equilibrium constant.The direction of the shift depends on whether it is exo - or endothermic.
57Exothermic H < 0 Releases heat. Think of heat as a product. N2(g) + 3H2(g) 2NH3(g) + HeatRaising temperature push toward reactants.Shifts to left.
58Endothermic H > 0 Absorbs heat. Think of heat as a reactant. Heat + COCl2(g) CO(g) + Cl2(g)Raising temperature push toward products.Shifts to right.
63Free Energy and Equilibrium G tells us spontaneity at current conditions. When will it stop?It will go to the lowest possible free energy which may be an equilibrium.At equilibrium G = 0, Q = KGº = -RTlnK from [G = Gº + RTlnK]Gº= 0< 0> 0K= 1> 1< 1
64Free Energy and Equilibrium The overall reaction for the corrosion of iron by oxygen is4Fe(s) + 3O2(g) 2Fe2O3(s)Use the following data, calculate the equilibrium constant for this reaction at 25°C.Substance H°f (kj/mol) S°(J/K•mol)Fe2O3(s)Fe(s)O2(g) ExGº = -RTlnK
65Practice Calculate K for the following reaction (at 298 K) H2(g) + Cl2(g) 2HClH°f (kj/mol) = -184 S°(J/K•mol) = 20.If each gas is placed in a flask such that the pressure of each gas is 1 atm, in which direction will the system shift to reach equilibrium at 25°C?Gº = -RTlnK