Bài giảng hoá đại cương Equilibrium

General Equi  librium Chapter 15 Thermondyanamics, Kinetics, and now Equilibrium • Thermodynamics Is a reaction spontaneous? Why? ✓ What are the driving forces that push reactions? ✓ • Kinetics What is the speed or rate of a reaction? ✓ What factors affect reaction rate? ✓ • Equilibrium What is the extent of a reaction? ✓ What are the concentrations of reactant and product once the quantites are no longer changing ✓ 2 What is Equilibrium? • • • • • • The state in which the rate of the forward reaction equals the rate of the reverse reaction The state in which the concentrations of all the reactants and products remain constant with time However, don’t be fooled, although it may appear as if everything has stopped, at the molecular level there is frantic activity It is a dynamic equilibrium Very few chemical reactions proceed only in one direction, in fact most reactions are reversible In a closed vessel most reactions will reach equilibrium, however some reactions so favor the products that the reaction appears to have gone to completion - far to the right A system that appears to not even started may actually be at equilibrium far to the left 3 So What’s Equal when “Equalibrium is reached?” • Usually the quantities of reactants and products are not equal It is possible that they could be equal, but far more likely that they will not be equal • The rate of the forward and reverse reactions are equal • Any pressure, concentrations, or quantities remain constant (equal) 4 The Equilibrium Konstant • • • The equilibrium constant is a number equal to a particular ratio of equilbrium concentrations of the products to reactants at a particular temperature The magnitude of K is an indication of how far a reaction proceeds toward product at a given temperature Some reactions so favor the products that the reaction appears to have “gone to completion” − Equilibrium lies far to the right ✓ • A system that appears that there is “no reaction” may actually be at equilibrium far to the left ✓ • K will be large, perhaps x 1025 K will be small, perhaps as small as x 10−30 When significant quantities of both reactant and product are present, K will have an intermediate value ✓ K will be close to 1, perhaps or 0.10 5 Chemical and Physical • Many different types of chemical reactions can reach equilibrium ✓ weak acid equilbrium: HC2H3O2(aq) ✓ gas phase equilbrium: 2NO2(g) H+ + C2H3O2− N2O4(g) • Demo 6 2NO2 N2O4 Equilibrium 7 Name the brown gas, NO2 and name the colorless gas, N2O4 • • NO2 = N2O4 = 8 Name the brown gas, NO2 and name the colorless gas, N2O4 • • NO2 = nitrogen dioxide N2O4 = dinitrogen tetraoxide 9 From the observations of the tubes containing the equilibrium, select the true statement(s) NO2 N2O4 is exothermic N2O4 NO2 ∆H is negative N2O4 + Energy NO2 NO2 N2O4 removing energy favors the forward reaction adding energy increases the speed of both the forward and reverse reactions 10 10 PCl5(g)  PCl3(g) + Cl2(g) ∆H = +87.9 kJ For the reaction above, in which direction will the reaction shift when the volume of the system is increased? shift left shift right • The volume increase will cause an immediate decrease in pressure and the attempt to make up this pressure change will cause a shift toward more molecules = right no change • 76 76 PCl5(g)  PCl3(g) + Cl2(g) ∆H = +87.9 kJ • For the reaction above, in which direction will the reaction shift when PCl3(g) is added in a sealed rigid container shift left shift right no change 77 77 PCl5(g)  PCl3(g) + Cl2(g) ∆H = +87.9 kJ • For the reaction above, in which direction will the reaction shift when PCl3(g) is added shift left shift right no change 78 78 PCl5(g)  PCl3(g) + Cl2(g) ∆H = +87.9 kJ • For the reaction above, in which direction will the reaction shift when the total pressure is increased by adding an inert gas such neon into the reaction vessel? shift left shift right no change 79 79 PCl5(g)  PCl3(g) + Cl2(g) ∆H = +87.9 kJ • For the reaction above, in which direction will the reaction shift when the total pressure is increased by adding an inert gas such neon into the reaction vessel? shift left shift right no change • the individual partial pressures of the gases not change, so there is no shift in the equilibrium position 80 80 PCl5(g)  PCl3(g) + Cl2(g) ∆H = +87.9 kJ • For the reaction above at equilibrium, in which direction will the reaction shift when the temperature is decreased shift left shift right no change 81 81 PCl5(g)  PCl3(g) + Cl2(g) ∆H = +87.9 kJ • For the reaction above, in which direction will the reaction shift when the temperature is decreased? shift left • You can assume that the thermodynamics play a larger role than the pressure effects, however I don’t think AP will put you in this situation shift right no change 82 82 A(s) + 2B(g)  3C(g) • Which direction will the reaction shift when the pressure is increased by reducing the volume? shift left shift right no change 83 83 A(s) + 2B(g)  3C(g) • Which direction will the reaction shift when the pressure is increased? shift left shift right no change • You might think it would stay the same because the amounts of moles are the same on both sides, but only gases respond to pressure changes, and thus the reaction will shift left 84 84 A(s) + 2B(g)  3C(g) • If Keq is 3.5 at 600ºC and Keq is 45 at 100ºC, is this reaction exothermic or endothermic? exothermic endothermic 85 85 A(s) + 2B(g)  3C(g) • If Keq is 3.5 at 600ºC and 45 at 100ºC, is this reaction exothermic or endothermic? exothermic endothermic 86 86 A(s) + 2B(g)  3C(g) • Which direction will the reaction shift if a bit more A is added? shift left shift right no change 87 87 A(s) + 2B(g)  3C(g) • Which direction will the reaction shift if more A is added? shift left shift right no change • If there is no change in volume of the space available to the gases due to the increase in A added, there will be no effect on equilibrium AP will expect you to ignore any volume change, unless they make it clear 88 88 A(s) + 2B(g)  3C(g) • If the system is at equilibrium, what will happen to the mass of A if more B is added to the reaction vessel? increase decrease stay the same 89 89 A(s) + 2B(g)  3C(g) • What will happen to the mass of A if more B is added to the reaction vessel? increase decrease stay the same? 90 90 [...]... reactants or products in the equilibrium expression) • The equilibrium condition can be reached from either the forward or reverse directions 21 21 Distinguish Equilibrium Position from Equilibrium Constant • For a particular temperature, there is only one equilibrium constant • But there are an infinite number of sets of various equilibrium concentrations, each one called an equilibrium position 22 22... in the equilibrium expression 23 23 Keq and Condensed Phases • Equilibrium that involve more than one phase is called heterogeneous equilibrium • Remember that if a pure solid or a pure liquid is in an equilibrium reaction, they do not show up in the equilibrium expression ✓ the AP exam will try to trick you into using them 24 24 What determines the position of the Equilibrium Position? • The equilibrium. .. previous equilibrium state) and a new equilibrium position is established with a different K value 12 2NO2 12 Chemical and Physical • Many chemical reactions can reach equilibrium ✓ weak acid equilbrium: HC2H3O2(aq) ‣ ✓ H+ + C2H3O2− which will be studied intensively in unit G gas phase equilbrium: NO2(g) N2O4(g) • Some physical processes can also reach equilibrium ✓ vapor equilbrium: H2O(L) H2O(g) 13 13 Equilibrium. .. no relationship between these equilibrium concentrations, however 18 18 Mass Action Expression, Q • For any reaction: ✓ aA + bB  cC + dD • A mass action expression can be written: ✓ [C]c [D]d The reaction quotient, Q = [A]a [B]b • If the reaction is at equilibrium, Q is equal to K ✓ [C]c [D]d and K (aka Keq) = [A]a [B]b the equilibrium expression • Unlike rate laws, the equilibrium expression depends... only on the stoichiometry not on the mechanism 19 19 Calculating the Equilibrium Constant • Apply the Law of Mass Action [H2] [I2] • and an Equilibrium Constant Keq = [HI]2 will emerge 2HI ⇄ H2 + I2 Exp 1 [H2] 0.107 [I2] 0.107 [HI] 0.786 Keq 0.0185 2 0.213 0.213 1.573 0.0183 3 0.320 0.320 2.36 0.0184 20 20 Just How Constant is the Equilibrium Constant? • The constant does vary with temperature • It... exist in equilibrium at a particular temp Take a brief look at the Phet Demo 16 16 2HI ⇄ H2 + I2 Consider three separate trials with different starting quantities 1 Exp 1: 1 M of HI, none of H2 and I2 2 Exp 2: 1 M each of H2 and I2 , none of HI 3 Exp 3: 1 M of all three, H2 , I2 and HI 17 17 After Reaching Equilibrium • All three trials will have different quantities when the system reaches equilibrium. .. already know and love • Vapor above a liquid in a closed container will reach equilibrium • The rate of the condensation and rate of evaporation will be equal • The amount of gas that can be evaporated at a particular temp is known as equilibrium vapor pressure 14 14 Chemical and Physical • Many chemical reactions can reach equilibrium ✓ weak acid equilbrium: HC2H3O2(aq) ‣ ✓ H+ + C2H3O2− which will be... equilbrium: HC2H3O2(aq) ‣ ✓ H+ + C2H3O2− which will be studied intensively in unit G gas phase equilbrium: NO2(g) N2O4(g) • Some physical processes can also reach equilibrium ✓ vapor equilbrium: H2O(L) H2O(g) ✓ solubility equibrium: HgBr2(s) Hg2+ + Br− 15 15 Equilibrium Situations that you already know and love • • • Saturated solution ✓ A solution that contains as much dissolved solid as possible (at a given... between.” • The equilibrium position is determined by ✓ ✓ ✓ the initial concentrations relative bond energies of reactants and products relative degree of “organization” of the reactants and products • Energy and organization (entropy) are important because molecules try to achieve minimum energy and maximum disorder • Recall from Unit C that there is a relationship between thermodynamics and equilibrium. .. The equilibrium expression for a reaction is the reciprocal for that reaction in reverse • For Rx fwd: 2 A + B  3 C ✓ Keq Rxfwd [C]3 = [A]2 [B] • For Rx rvs: 3 C  2 A + B ✓ Keq Rxrvs [A]2 [B] = [C]3 • Thus Keq Rxrvs = 1 / Keq Rxfwd • So when you reverse a reaction, inverse the Keq value 26 26 Keq and Stoichiometry • When the balanced equation for a reaction is multiplied by some factor n, the equilibrium