LG 3.2 The Equilibrium Constant and Reaction Quotient PDF

Chemical Equilibrium THE EQUILIBRIUM CONSTANT K, & REACTION QUOTIENT, Q  DESIRED LEARNING COMPETENCIES ▪ Write expressions for the equilibrium constant, K, and reaction quotient, Q, of heterogeneous equilibria and different forms of reversible reactions; and ▪ Explain wh...

Chemical Equilibrium THE EQUILIBRIUM CONSTANT K, & REACTION QUOTIENT, Q  DESIRED LEARNING COMPETENCIES ▪ Write expressions for the equilibrium constant, K, and reaction quotient, Q, of heterogeneous equilibria and different forms of reversible reactions; and ▪ Explain what the magnitude of the equilibrium constant means.  LAW OF MASS ACTION Cato Maximilian Gudberg & Peter Waage expresses the relationship between the reactant and product concentrations present at equilibrium. Explain what the magnitude of the equilibrium constant means. At a given temperature, the equilibrium constant, K has a constant value, regardless of how the reaction is run.  REACTION QUOTIENT, Q ratio of the concentration of products over the concentrations of reactants at any point or time, t, in a reaction each raised to the power of their corresponding stoichiometric coefficients. [𝐷]𝑑𝑡 [𝐸]𝑒𝑡 𝑄= [𝐴]𝑎𝑡 [𝐵]𝑎𝑡 Q can be expressed in terms of molar concentration (Qc) or partial pressure (Qp) for gases. Just like K, Q is also unitless or dimensionless.  EQUILIBRIUM CONSTANT, K Once the system reaches equilibrium, the concentrations no longer change, thus, Q equals K. Since Q = K at equilibrium, the equilibrium constant expression is written similar to the reaction quotient. [𝐷]𝑑𝑒𝑞 [𝐸]𝑒𝑒𝑞 K= [𝐴]𝑎𝑒𝑞 [𝐵]𝑎𝑒𝑞 The expression depends only on the stoichiometry of the reaction and not on its mechanism.  LEARNING CHECK!! Write the reaction quotient and equilibrium constant expressions in terms of molar concentration for the following reactions: CO(g) + 3H2(g) ⇌ CH4(g) + H2O(g)  LEARNING CHECK ANSWER!! Write the reaction quotient and equilibrium constant expressions in terms of molar concentration for the following reactions: CO(g) + 3H2(g) ⇌ CH4(g) + H2O(g) [CH4] [H2O] [CH4] [H2O] 𝑄= K= [𝐶𝑂][H2]3 [𝐶𝑂][H2]3  LEARNING CHECK!! Write the reaction quotient and equilibrium constant expressions in terms of molar concentration for the following reactions: HF(aq) ⇌ H+ - (aq) + F (aq)  LEARNING CHECK ANSWER!! Write the reaction quotient and equilibrium constant expressions in terms of molar concentration for the following reactions: HF(aq) ⇌ H+(aq) + F-(aq) [H+] [F−] [H+] [F−] 𝑄= K= [𝐻𝐹] [𝐻𝐹]  HOMOGENOUS EQUILIBRIA CO(g) + 3H2(g) ⇌ CH4(g) + H2O(g) HF(aq) ⇌ H+(aq) + F-(aq) Where the reactants and products involved are in a single phase or in the same phase. Q expression are values at any time, t, including initial values while those used in the K expression are only equilibrium values.  HETEROGENOUS EQUILIBRIA 3Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g) [H2]4 [H2]4 𝑄= K= [H2O]4 [H2O]4 R and Q for pure solids and liquids are omitted because their concentrations are constant at a given temperature.  WORKED EXAMPLES Consider the synthesis of phosgene (COCl 2), a toxic gas used in the polymer and insecticide industries at 100oC where Kc = 4.56 x 109. What does this tell us? CO(g) + Cl2(g) ⇌ COCl2(g) [COCl2] 𝐾= = 4.56 𝑥 109 𝐶𝑂 [Cl2] -- The numerator of the expression must be larger than the denominator -- The concentration of COCl2 must be greater than that of CO or Cl 2. -- The equilibrium favors the formation of products  WORKED EXAMPLES EXPERIMENT INITIAL INITIAL EQUILIBRIUM EQUILIBRIUM Kc [N2O4](M) [NO2](M) [N2O4](M) [NO2](M) 1 0.001 0.020 0.0014 0.0172 0.211 2 0.00 0.023 0.0028 0.0253 0.211 3 0.00 0.040 0.0045 0.0310 0.213 4 0.020 0.00 0.0045 0.0310 0.213 N2O4(g) ⇌ 2NO2(g) [NO2]2 [0.0172]2 K= = = 0.211 N2O4 0.0014 -- If Kc will be calculated the same way it will yield the same value. -- Same is true if we reverse the reaction. Kc will still be constant in all experiments for that direction  LEARNING CHECK!! Write the reaction quotient and equilibrium constant expressions in terms of molar concentration for the following reactions: 1. 2O3(g) ⇌ 3O2(g) 2. CaCO3(s) ⇌ CaO(s) + CO2(g) 3. Ag+(aq) + 2NH3(aq) ⇌Ag(NH3)2+(aq) Tell whether each reaction favors product-formation or reactant-formation. 1. 2NO(g) + O2(g) ⇌ 2NO2(g) Kp = 5.0 x 1012 2. 2HBr(g) ⇌ H2(g) + Br2(g) Kc = 5.8 x 10-18  REFERENCES Albarico, J.M. (2013). THINK Framework. Based on Ramos, E.G. and N. Apolinario. (n.d.) Science LINKS. Quezon City: Rex Bookstore Inc. 2. Brown, T. L., Bursten, B., Lemay, H. E., Murphy, C. J., & Woodward, P. M. (2012). Chemistry: The Central Science (12th ed.). Glenview, Illinois: Pearson Education, Inc. 3. Burdge, J., & Overby, J. (2012). Chemistry: Atoms First. New York, New York: McGraw-Hill Companies, Inc. 4. Ebbing, D. & Gammon, S. (2009). General Chemistry (9th ed.). Boston, New York: Houghton Mifflin Company. 5. Silberberg, M. S. (2010). Chemistry: The Molecular Nature of Matter and Change (5th ed.). New York, New York: McGraw-Hill Companies, Inc.