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CHEMICAL EQUILIBRIUM condensed version Slide 2 At equilibrium, the rate at which NO 2 forms in the forward reaction equals the rate at which N 2 O 4 forms in the reverse reaction: N 2 O 4 2 NO 2 colorless brown Forward Reaction: N 2 O 4 2 NO 2 rate f = k f [N 2 O 4 ] Reverse Reaction: 2NO 2 N 2 O 4 rate r = k r [NO 2 ] 2 Slide 3 Products raised to stoichiometric powers divided by reactants raised to their stoichiometric powers The EQUILIBRIUM CONSTANT EXPRESSION For a general equilibrium equation: a A + b B d D + e E Slide 4 For a general equilibrium equation: When the reactants and products in a chemical reaction are all gases, we can formulate the equilibrium-constant expression in terms of partial pressures. Where p is pressure Relating K p and K c : a A + b B d D + e E Slide 5 For a heterogeneous equilibria: (substances are in different phases) Whenever a pure solid or a pure liquid is involved in a heterogeneous equilibrium, its concentration is not included in the equilibrium-constant expression. PbCl 2 (s) Pb 2+ (aq) + Cl - (aq) CaCO 3 (s) CaO(s) + CO 2 (g) H 2 O(l) + CO 3 2- (aq ) OH - (aq) + HCO 3 - (aq) Slide 6 Slide 7 Magnitude of the equilibrium constant: K c >> 1 (large K c ) Reaction is strongly product favored. very little reactant remains often written as a forward reaction only. assume reaction goes to completion. K c If Q = K c, the system is at equilibrium. increase forward If Q < K c, Q must increase to reach equilibrium, the system shifts forward. decrease backwards If Q > K c, Q must decrease to reach equilibrium, the system shifts backwards. Reaction quotient a A + b B c C + d D Slide 14 Slide 15 Slide 16 1.Fuel engineers use the extent of the change from CO and H 2 O to CO 2 and H 2 to regulate the proportions of synthetic fuel mixtures. If 0.250 mol of CO and 0.250 mol of H 2 O are placed in a 125-mL flask at 900 K. What is the composition of the equililbrium mixture? At this temperature, K c is 1.56 for the equation: CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) 2. Phosgene is a potent chemical warfare agent that is now outlawed by internation agreement. It decomposes by the reaction: COCl 2 (g) CO(g) + Cl 2 (g) K c = 8.3 x 10 -4 (at 360 o C) Calculate [CO], [Cl 2 ] and [COCl 2 ] when each of the following amounts of phosgene decomposes and reaches equilibrium in a 10.0 L flask: a) 5.00 mol of CoCl 2 b) 0.100 mol of COCl 2 Slide 17 Slide 18 Slide 19 Consider the reaction: N 2 O 4 2 NO 2 H o = 58 kJ ChangeSHIFT Addition of N 2 O 4 (g)right Addition of NO 2 (g)left Removal of N 2 O 4 (g)left Removal of NO 2 (g)right Addition of He(g)none Decrease in container volumeleft Increase in container volumeright Increase in Tempright Decrease in Templeft