Dynamic Equilibrium. O 2 Transport, Hg and equilibrium. Tro Chpt 14 Chemical Equilibrium

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1 Tro Chpt 14 Chemical Equilibrium The extent of chemical reactions The equilibrium constant, K Calculating equilibrium constants Predicting reaction directions Le Chatelier s Principle Suggested eoc problems for chapter 14:21, 27, 29, 31, 37, 41, 47, 51, 57, 61, 65 Tro 14.1 and 14.2 Dynamic Equilibrium Hemoglobin - a protein found in red blood cells that enhances the amount of O 2 that can be carried through the blood stream Hb + O 2 HbO 2 the concentrations of Hb, O 2, and HbO 2 are all interdependent the relative amounts of Hb, O 2, and HbO 2 at equilibrium are related to a constant called the equilibrium constant, K changing the concentration of any one of these necessitates changing the other concentrations to re-establish equilibrium O 2 Transport, Hg and equilibrium Tro 14.3 Dynamic Equilibrium and the equilibriuim constants K in the lungs, with high concentration of O 2, the equilibrium shifts to combine the Hb and O 2 together to make more HbO 2 N 2 O 4 2NO 2 (colorless) (brown) in the cells, with low concentration of O 2, the equilibrium shifts to break down the HbO 2 and increase the amount of free O 2 O 2 Hb + Oin 2 HbO 2 lungs cells

2 Dynamic Equilibrium continued The Significance of the Equilibrium constant K is small (e.g ) K is large (e.g. 50) cis-2-butene trans-2-butene K is intermediate (e.g. 1) Writing Equilibrium Constant Expressions: Relationships between K and the Chemical Equation N 2 + 3H 2 2NH 3 K expressions for individual reactions and overall reactions N 2 + O 2 2NO K c = 4.3 x NO + O 2 2NO 2 K c = 6.4 x 10 9 N 2 + 2O 2 2NO 2 K c =? NH 3 + O 2 2NO + H 2 O K c = 2.75 x 10-15

3 K and the direction of a chemical reaction 2 SO 2 + O 2 2 SO 3 vs. 2 SO 3 2 SO 2 + O 2 Tro 14.4 Expressing K in Terms of Pressure K p vs. K c Form of K and the coefficients of a chemical reaction SO 2 + 1/2O 2 SO 3 n(aa + bb cc + dd) N 2 (g) + 3H 2 (g) 2NH 3 (g) K c = 3.5 x 10 8 at 25 C K p = (3.5 x 10 8 )*(RT) (-2) = 5.8 x 10 5 Tro 14.5 Reactions involving Solids and Liquids Tro 14.6 and 14.8 Reaction tables, stoichiometry and equilibrium concentrations CaCO 3 (s) CaO (s) + CO 2 (g) Solids and pure liquids DO NOT appear in the reaction quotient or K expression In a study of the conversion of methane to other fuels, a chemical engineer mixes gaseous CH 4 and H 2 O in a 0.32 L flask at 1200 K. At equilibrium, the flask contains 0.26 mol of CO, mol of H 2 and mol of CH 4. What is [H 2 O] at equilibrium? K c = 0.26 for the reaction: CH 4 + H 2 O CO + 3H 2 Answer: [H 2 O] = 0.56 M

4 Reaction tables, stoichiometry and equilibrium concentrations 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 mol of CO and mol of H 2 O are placed in a 125 ml flask at 900 K, what is the composition of the equilibrium mixture? By the way, at this temperature, K c is 1.56 for the reaction: CO (g) + H 2 O (g) CO 2 (g) + H 2 (g) If we have 2.00 M CO and 1.00 M H 2 O, what is the composition of the equilibrium mixture? Remember, K c is 1.56 for the reaction: CO (g) + H 2 O (g) CO 2 (g) + H 2 (g) Answer: [CO] = [H 2 O] = 0.89 M [CO 2 ] = [H 2 ] = 1.11 M Answer: [CO] = 1.27 M, [H 2 O] = 0.27 M [CO 2 ] = [H 2 ] = 0.73 M Do we ALWAYS have to use the quadratic expression? Tro 14.7 The Reaction Quotient Q: Predicting the Direction of Change Phosgene is a potential warfare agent that is now outlawed by international agreement. It decomposes by the reaction: COCl 2 (g) CO (g) + Cl 2 (g) K c = 8.3 x 10-4 at 360 C Calculate [CO], [Cl 2 ] and [COCl 2 ] when 5.00 mol of COCl 2 decomposes in a 10.0 liter flask. The reaction quotient has the same algebraic format as the equilibrium constant K N 2 O 4 2NO 2 Answer: [CO] = [Cl 2 ] = 2.0 x 10-2 M [COCl 2 ] = 0.48 M

5 Comparing Q to K N 2 O 4 2NO 2 (K) Tro 14.9 Stressing out a happy system Le Chatelier s Principle Le Chatelier s principle: when a chemical system at equilibrium is disturbed, it re-attains equilibrium by undergoing a net reaction that reduces the effect of the disturbance

6 Predicting the effect of a change in concentration on the equilibrium position To improve air quality and obtain a useful product, sulfur is often removed from coal and natural gas by treating the fuel contaminant hydrogen sulfide with O 2 : 2H 2 S (g) + O 2 (g) 2S (s) + 2H 2 O (g) Predicting the effect of a change in concentration on the equilibrium position for reactions involving gases How could you change the volume of the reaction to increase the yield of products? CaCO 3 (s) Cl 2 (g) + I 2 (g) CaO (s) + CO 2 (g) 2ICl (g) What happens to: a) [H 2 O] is O 2 is added b) [H 2 S] if O 2 is added c) [O 2 ] if H 2 S is removed d) [H 2 S] if sulfur is added S (s) + 3F 2 (g) SF 6 (g) The effect of a change in temperature PCl 3 (g) + Cl 2 (g) PCl 5 (g) ΔH rxn = -111 kj PCl 3 (g) + Cl 2 (g) PCl 5 (g) + heat The van t Hoff Equation: Changes in K with changes in T:

7 N 2 (g) + 3H 2 (g) NH 3 (g) ΔΗ rxn = kj N 2 (g) + 3H 2 (g) NH 3 (g) ΔΗ rxn = kj N 2 (g) + 3H 2 (g) NH 3 (g) ΔΗ rxn = kj