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Chemical Equilibrium Chemical Equilibrium Rate of forward and reverse reactions are equal Reactant and product concentrations no longer change Represent a reaction at dynamic equilibrium with a double arrow A B Haber Process: N2(g) + 3H2(g) 2NH3(g) Equilibrium Equilibrium Chemical Clock Demo http://lecturedemo.ph.unimelb.edu.au/Heat -Thermodynamics/Non-equilibriumThermodynamics/Hg-1-Chemical-Clock-AB Equilibrium Constant (Keq) Law of mass action expresses relationship between concentrations of reactants and products at equilibrium Equilibrium Constant Expressions aA + bB cC + dD Keq = (PC)c(PD)d / (PA)a(PB)b for gasses Keq = [C]c[D]d / [A]a[B]b for solutions Exercise 1 Write equilibrium constant expressions for the following reversible reactions: a) H2(g) + I2(g) 2 HI(g) b) 2Cl2(g) + 2H2O(g) 4HCl(g) + O2(g) a) Kc = [HI]2 / [H2][I2] or Kp = P2HI / PH2PI2 b) Kc= [HCl]4[O2] / [Cl2]2[H2O]2 or Kp = P4HClPO2 / P2Cl2P2H2O Equilibrium Constant (Keq) Equilibrium constant expression depends only on stoichiometry of reaction, not the mechanism Value of Keq varies only with temperature Keq > 1: reaction favors products Keq < 1: reaction favors reactants Need to specify temperature as well as direction when writing an equilibrium constant Keq is a unitless value Summary Points Equilibrium constant of a reaction in the reverse direction is the inverse of the equilibrium constant of the forward reaction The equilibrium constant of a reaction that has been multiplied by a number is the equilibrium constant raised to a power equal to that number The equilibrium constant for a net reaction of two or more steps is the product of the constants of the individual steps Heterogeneous Equilibria Contains substances at equilibrium in different phases Solids and liquids do not appear in the equilibrium expression because they do not have varying concentrations Calculating Equilibrium Constants Tabulate the known initial and equilibrium concentrations of all species in the equilibrium constant expression For the species where both initial and equilibrium concentrations are known, calculate the change in concentration that occurs Use the stoichiometry of the reaction to calculate the changes in all species Calculate equilibrium concentration from initial and changes in concentration Summary of Steps to Calculate Equilibrium Concentrations 1. Write a balanced equation for the reaction 2. Identify the knowns and unknowns 3. Assign the variable X to the unknown (usually a change in concentration) 4. Prepare a table of concentrations 1. Row 1: chemicals involved Row 2: initial concentrations Row 3 Changes in concentrations (X appears) Row 4: Equilibrium concentrations (rows 2 + 3) Summary of Steps to Calculate Equilibrium Concentrations 5. Write equilibrium expression 6. Solve for unknown variable using K and data from row 4. Determine of approximations can be made. 7. Answer the question 8. Check for validity of approximations. For expressions of C+x or C-x, x can be omitted if (x/C)100 < 5% Example: Solution Equilibrium NH3(aq) + H2O(l) NH4+(aq) + OH- (aq) Enough ammonia is dissolved in 50 liters of water at 25oC to produce a solution that is 0.0124 M in ammonia. The solution is then allowed to come to equilibrium. Analysis of the equilibrium mixture shows that the concentration of OH- is 4.64 x 10-4 M. Calculate Keq at 25oC for the reaction. Initial NH3(aq) M H2O(l) NH4+(aq) M OH-(aq) M .0124 0 0 Change Equilibrium 4.64 x 10-4 After stoichiometry NH3(aq) M H2O(l) NH4+(aq) M OH-(aq) M Initial .0124 0 0 Change - 4.64 x 10-4 + 4.64 x 10-4 + 4.64 x 10-4 Equilibrium 0.0119 4.64 x 10-4 4.64 x 10-4 Answer Keq = [NH4+][OH-] / [NH3] = (4.64 x 10-4)(4.64 x 10-4) / 0.0119 = 1.81 x 10-5 Applications of Equilibrium Constants Predicting Direction of Reaction Compare the starting position to the equilibrium position in order to determine how the reaction must progress in order to achieve equilibrium When we substitute reactant and product concentration or partial pressure into a Keq expression, we achieve the reaction quotient (Q) If Q = Keq then the system is at equilibrium If Q > Keq then reaction moves to the left If Q < Keq then reaction moves to the right Applications of Equilibrium Constants Calculating Equilibrium Concentrations Use the ICE process described earlier Example: A 1.0 L container initially holds 0.015 mol of H2 and 0.020 mol of I2 at 721 K. What are the concentrations of H2, I2 and HI after the system has achieved a state of equilibrium? The value of Keq is 50.5 for the reaction: H2(g) + I2(g) 2HI(g) Gas Equilibrium ICE H2(g) I2(g) HI(g) Initial (M) .015 .020 0 Change (M) -x -x 2x Equilibrium (M) 0.015 –x 0.020 –x 2x Gas Equilibrium Keq = [HI]2 / [H2][I2] = (2x)2 / (0.015 –x)(0.020-x) = 50.5 46.5x2 – 1.77x + 0.0152 = 0 Solve quadratic equation X = 0.025 M and 0.0131M [H2] = 0.002 M, [I2] = 0.007 M, [HI] = 0.026 M Le Chatelier’s Principle If a system at equilibrium is disturbed by a change in temperature, pressure, or concentration the system will shift its equilibrium to counteract the disturbance. Le Chatelier : Change in Reactant or Product Concentration Shifts away from an increase in a substance and toward a decrease in a substance. N2(g) + 3H2(g) 2NH3(g) Addition of N2 shifts to right Addition of NH3 shifts to left Haber process removes NH3 as it is produced, shifts right to continually produce more NH3 Le Chatelier: Effect of Volume or Pressure A change in volume, shifts to reduce the pressure (fewer moles of gas) N2O4(g) 2NO2(g) Volume decreases, shifts to left Pressure decreases, shifts to right Does not change the value of Keq at constant temperature Le Chatelier: Effect of Temperature Changes Keq changes as temperature changes Treat temperature as a reactant or product and determine shift accordingly Increase temperature of endothermic increase Keq Increase temperature of exothermic decrease in Keq Le Chatelier: Effect of Catalysts Catalyst lowers activation energy of both the forward and reverse reactions by the same amount The rates of both forward and reverse reactions increases by the same amount Value of Keq does not change, but equilibrium is reached sooner Exercise 8 What changes in the equilibrium composition of the reaction N2(g) + O2(g) 2NO(g) will occur at constant temperature if a) The partial pressure of N2(g) is increase b) The pressure of the NO(g) is decreased c) the total pressure of the system is increased d) The total volume of the system is increased Exercise 8 answer a) b) c) d) Shifts to right Shifts to right Unchanged No change