Chemical Systems & Equilibrium

1. Chemical Systems & Equilibrium Unit 4

2. Calculating the Equilibrium Constant • The equilbrium constant, Keq, is the ratio of equilibrium concentrations at a particular temp • Kc for solution-phase systems or Kp for gas-phase systems • Keq= [C]c[D]d for the eqn [A]a[B]baA+bBcC+dD Note: The equilibrium constant depends ONLY on the concentration of gases (not liquids/solids)

3. [H2(g)][I2(g) ] [N2(g)][H2(g) ]3 [0.11][1.91]3 [N2O4(g)] K = K = K = K = [HI(g) ]2 [NH3(g) ]2 [0.25]2 [NO2(g) ]2 Questions: Equilibrium Law Expression 1. Write the equilibrium law expression for the following: a) 2NO2(g) ↔ N2O4(g) b) 2HI(g) ↔ H2(g) + I2(g) 2. A reaction vessel contains NH3, N2 and H2 gas at equilibrium at a certain temperature. The equilibrium concentrations are [NH3] = 0.25mol/L, [N2] = 0.11mol/L and [H2] = 1.91 mol/L. Calculate the equilibrium constant for the decomposition of ammonia. 2NH3(g) ↔ N2(g) + 3H2(g) K = 12.3

4. [NO2(g)]2 K = [NO (g) ]2 [O2 (g) ] Questions: Equilibrium Law Expression 3. Nitryl chloride gas, NO2Cl, is in equilibrium at a certain temperature in a closed container with NO2 and Cl2 gases. At equilibrium, [NO2Cl] = 0.00106mol/L and [NO2] = 0.0108mol/L. If K = 0.558, what is the equilibrium concentration of Cl2? 4. Write a balanced equation for the reaction with the following equilibrium law expression:

5. Heterogeneous Equilibria • homogeneous equilibria = equilibria in which all entities are in the same phase • Reactants and products are all gas or all aqueous • heterogeneous equilibria= equilibria in which reactants and products are in more than one phase • Reactants and products are in different phases

6. N2O4(g)2NO2(g) Kc = Kp = P2 [NO2]2 NO2 [N2O4] P N2O4 Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.

7. CaCO3(s) CaO(s) + CO2(g) Kc = [CaO(s) ][CO2(g)] [CaO(s)] Kc [CaCO3(s)] [CaCO3(s)] Heterogenous equilibrium applies to reactions in which reactants and products are in different phases. [CaCO3(s)] = constant [CaO(s) ] = constant Kc= [CO2(g)] = [CO2(g)] The concentration of solids and pure liquids are considered to be constant and are not included in the expression for the equilibrium constant.

8. CaCO3(s)CaO(s) + CO2(g) PCO 2 does not depend on the amount of CaCO3 or CaO PCO2 = Kp

9. equilibrium equilibrium equilibrium N2O4(g) 2NO2(g) Start with NO2 Start with N2O4 Start with NO2 & N2O4 Equilibrium favors the reactant side

10. CHECKPOINT The reaction at 200C between ethanol and ethanoic acid produces ___________________ and __________________. Write the equation for this reaction Determine the equilibrium constant expression for the reaction

11. Calculating Equilibrium Concentrations (when given one concentration) Sample Problem: When ammonia is heated it decomposes: 2NH3(g)↔N2(g)+ 3H2(g) When 4.0 mol of ammonia is introduced in a 2.0L container and heated. The equilibrium amount of ammonia is 2 0 mol. Determine the equilibrium concentrations of the other two entities. STEP 1: Determine the concentration (initial and equilibrium) for known values STEP 2: Setup an ICE Table STEP 3: Determine the value of X STEP 4: Use x value to determine the other quantities

12. Determine the concentrations [NH3]initial = 4.0mol/2.0L = 2.0mol/L [NH3]equilibrium = 2.0mol/2.0L = 1.0mol/L Setup ICE Table

13. Determine the value of X [NH3](g)equil= 2 0mol / L - 2x [NH3](g)equil= 1.0mol/L (from calculations in Step 1) 2.0mol/L – 2x = 1.0mol/L -2x = - 1.0mol/L x = 0.5mol/L Use X to determine other quantities

14. constant

15. Reversible Reactions • For a given overall system composition, the same equilibrium concentrations are reached whether equilibrium is approached in the forward or the reverse direction • What about Keq will it be the same in fwd/rev?