Presentation on theme: "Reactions Don’t Just Stop, They find Balance"— Presentation transcript:
1 Reactions Don’t Just Stop, They find Balance Chemical EquilibriumReactions Don’t Just Stop, They find Balance
2 Equilibrium is Attained When the Rates of the Forward and Reverse Reactions are the Same.Forward Rate = kf [A]Reverse Rate = kr [B]kf [A] = kr [B]
3 Equilibrium : The Haber Process and Nitrogen FixationN2(g) + 3H2(g) 2NH3(g)Chemistry at work, p. 521
4 Equilibrium : The Haber Process and Nitrogen FixationNote that equilibriumcan be reached fromeither the forwardor reverse direction
5 jA + kB pR + q S [R]p [S]q K = [A]j [B]k The LAW OF MASS ACTION allows us to express the relative concentrations of reactants and products at equilibrium in terms of a quantity called the equilibrium constant, ‘K’ such that:[R]p [S]qK =[A]j [B]k
6 The Haber Process and the Law of Mass Action N2(g) + 3H2(g) 2NH3(g)[NH3]2K =[N2][H2]3The equilibrium constant expression depends only on the stoichiometry of the reaction, not on its mechanism.
7 N2O4 2 NO2 [0.0172]2 [NO2]2 Kc = = = 0.221 [N2O4] [0.00140] Suppose we discover that the equilibrium concen-trations of NO2 and N2O4 are M andM, respectivelyN2O4 2 NO2[NO2]2[0.0172]2Kc === 0.221[N2O4][ ]
8 Heterogeneous Equilibria a reaction which may possess reactants orproducts which are in different phases.CaCO3(s) CaO(s) + CO2(g)
9 The density of a pure liquid or solid is a constant at any given temperature and changes very little with temperature. Thus the effective concentration of a pure liquid or solid is constant regardless of how much pure liquid or solid is present.Given: CaCO3(s) CaO(s) + CO2(g)[CaO(s)][CO2]K =so K =[CO2][CaCO3(s)]Even though they do not appear in the equilibrium constant expression,pure solids and liquids must be present for equilibrium to be established
10 The Magnitude of Equilibrium Constants What does it mean when the constant equalsK = 1 x 1040orK = 1 x 10-40
11 N2(g) + 3H2(g) 2NH3(g) (.00272)2 = 104.7 (.0402)(.01207)3 In one of their experiments, Haber and co-workers introduced a mixture of hydrogen and nitrogen into a reaction vessel and allowed the system to attain chemical equilibrium at 472 °C. The equilibrium mixture of gases was analyzed and found to contain M H2, M N2, and M NH3. From these data, calculate the equilibrium constant for:N2(g) + 3H2(g) 2NH3(g)(.00272)2(.0402)(.01207)3= 104.7
12 Converting from Kc to Kp Solutions are Understood in terms of Molarities Gas Pressure is Understood in Terms of AtmospheresKp = Kc(RT)n, n = (mol. of prod – mol. of react)Using the value of Kc = for the reaction:N2(g) + 3H2(g) 2NH3at 472 °C Convert to KpKp = ( L-atm/mol-K)(745 K)-20.105Kp=( L-atm/mol-K)(745 K)2Kp= 2.81 x 10-5 atm
13 Predicting the Direction of Equilibrium Le Châteliers principle : If a system at Equilibrium is disturbed by a change in temperature, pressure or the concentration of a component, the system will shift its equilibrium position so as to counteract the effect of the disturbance.
14 Effects of Pressure and Volume Changes If a system is at equilibrium and the total pressure is increased by the application of an external pressure by a change in volume, the system will respond by a shift in equilibrium in the direction that reduces the pressure by shifting to the side with less moles.2NO2(g)N2O2(g)
15 What would happen if 1 atm of argon gas were added to the following reaction already at equilibrium. N H2 2 NH3
16 Effects of Temperature Changes When heat is added to a system, the equilibrium shifts in the direction that absorbs heat. In an endothermic reaction reactants are converted to products, and K increases. In an exothermic reaction, the opposite occurs.Room temperatureheatingcoolingCo(H2O)62+(aq)+ 4Cl-(aq)CoCl42-(aq)+6H2O(l) H > 0
17 Predicting the Direction of Equilibrium Given,N2(g) + 3H2(g) 2NH3(g)[NH3]2K =and Kc = 0.105[N2][H2]3If the equilibrium concentrations were to start at:[2.00]2Q == 0.500[1.00][2,00]3What must happen in order for the value, lets now call it the reaction quotient ‘Q’, to return to Kc= 0.105?
18 Predicting the Direction of Equilibrium [NH3]2Q == and Kc = 0.105[N2][H2]3Equilibrium will re-emerge if the concentration of NH3 decreases and the concentrations of N2 and H2 increase. In a closed system, this would require that the reaction favor the formation of reactantsN2(g) + 3H2(g) NH3(g)If Q > K, the reaction will shift to the reactantsIf Q = K, the reaction is at equilibriumIf Q < K, the reaction will shift to the products
19 ICEBOX A mixture of 5. 00 x 10-3 mol of H2 and 1 ICEBOX A mixture of 5.00 x mol of H2 and 1.00 x mol of I2 is placed in a 5.00 L container at 448°C and allowed to come to equilibrium. Analysis of the equilibrium mixture shows that the concentration of HI is 1.87 x 10-3 M. Calculate Kc at 448 °C for the reaction: H2(g) + I2(g) 2HIH2(g) I2(g) HIInitial1.00 x 10-3 M2.00 x 10-3 MO MChange-.935 x 10-3-.935 x 10-3+1.87 x 10-3 MEquilib..065 x10-31.065 x 10-31.87 x 10-3 M[HI]2(1.87 x 10-3 )2(1.065 x 10-3)(.065 x10-3)Kc ==[H2] [ I2]
20 A 1. 000-L flask is filled with 1. 000 mol of H2 and 2 A L flask is filled with mol of H2 and mol I2 at 448°C. The value of the equilibrium constant for the following reaction is What are the equilibrium concentrations of H2, I2, and HI? H2(g) + I2(g) 2HIH2(g) I2(g) HIInitial1.000 M2.000 M0 MChangeEquilibrium
21 H2(g) + I2(g) 2HI Initial 1.000 M 0 M 2.000 M Change - x M - x M Equilibrium( x)( x)2x[HI]250.5 =(2x)2=[H2][I]( x)( x)4x2 = 50.5(x x )46.5x x = 0x =-(-151.1) (-151.4)2 - 4(46.5)(101.0)2(46.5)[H2] = M[I2 ] = M[HI] = Mx= or