Lesson # 4 The Reaction Quotient & More Equilibrium Calculations Chemical Equilibrium Lesson # 4 The Reaction Quotient & More Equilibrium Calculations

K vs. Q The reaction quotient, Q, is like K when the system is not at equilibrium. It is again the product of the concentration of the products divided by the concentration of the reactants, with molar coefficients as powers. Q = [A]a [B]b [C]c [D]d for aA + bB = cC + dD

K vs. Q If the system is at equilibrium and you calculate Q, it will be the same value as K. If Q is less than K, that means that the system is not yet at equilibrium, so will need to shift to the right, towards the products. If Q is greater than K, that means the system is beyond the equilibrium, and so will need to shift to the left towards the reactants.

Example 1 For the synthesis of ammonia gas at 500°C in a closed vessel from nitrogen and hydrogen gases, the equilibrium constant, K, is 6.01x10-2. For each of the following initial conditions, determine if the given concentrations represent an equilibrium. If not, predict the direction in which the reaction will proceed to reach equilibrium. [NH3] = 1.00x10-3 mol/L [N2] = 1.00x10-5 mol/L [H2] = 2.00x10-3 mol/L [NH3] = 2.00x10-4 mol/L [N2] = 1.50x10-5 mol/L [H2] = 3.54x10-1 mol/L c) [NH3] = 1.00x10-4 mol/L [N2] = 5.00 mol/L [H2] = 1.00x10-2 mol/L

Example 2 Phosphorus pentachloride decomposes into phosphorus trichloride and chlorine. A 2.00 L sealed container at 30°C initially contains 0.298 mol of phosphorus trichloride gas, 8.70x10-3 mol of phosphorus pentachloride gas, and no chlorine gas. At equilibrium, the flask contains 2.00x10-3 mol of chlorine gas. Calculate the equilibrium concentrations of all entities and the value of K.

Example 3 Carbon monoxide reacts with steam to produce carbon dioxide and hydrogen. At 700 K, the equilibrium constant is 5.10. Calculate all equilibrium concentrations if 1.250 mol of each entity in initially placed in a 500.0 mL sealed flask.

Example 4 Carbon dioxide decomposes into carbon monoxide and oxygen gas. At 2000°C, K = 6.40x10-7. Calculate the concentrations of all entities at equilibrium if 0.250 mol of CO2 is placed in a 1.000 L closed container and heated.

Example 5 If 0.500 mol of dinitrogen tetroxide is placed in a 1.00 L closed container at 150°C, what will be the concentrations of dinitrogen tetroxide gas and nitrogen gas at equilibrium? N2O4 (g) = 2 NO2 (g). K = 4.50 under these conditions.