END OF SEASON_CH15_CH16 comb_F2006_DRAFT

END OF SEASON_CH15_CH16 comb_F2006_DRAFT - CHAPTER 15...

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CHAPTER 15: PRINCIPLES OF CHEMICAL EQUILIBRIUM with additional features from CHAPTER 16: ACIDS AND BASES The issue: When we did stoichiometry, we assumed that reactions go to completion. Many do. Many others do not. Herewith, the story of reactions that do not go to completion, told in the context of common materials … acids and bases. The concept: CHEMICAL EQUILIBRIUM when concentrations of all reactants and products are constant over time REVERSIBLE REACTIONS … reactions that go forward and backward at the same time Observations: STUDIO #12 and STUDIO #13 ENTER: THE EQUILIBRIUM CONSTANT and the MASS ACTION EXPRESSION EX: NaCl (s) Na + (aq) + Cl - (aq) K eq = [Na + ][Cl - ] or CH 3 COOH(aq) CH 3 COO - (aq) + H + (aq) K eq = [CH 3 COO - ][H + ] [CH 3 COOH] CONCEPT: dynamic equilibrium WHERE / WHEN does equilibrium occur … one illustration:
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OBSERVATIONS ABOUT EQUILIBRIA Example: the water-gas shift reaction == used to make hydrogen on an industrial scale H 2 O (g) + CO (g) <===> H 2 (g) + CO 2 (g) recall: for molecules to react, they must collide rates of reaction: f(concentration, temperature) ... recall Boltzmann distriburion rate ===> equals rate <=== :: EQUILIBRIUM What would you do to make more hydrogen?
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Law of Mass Action: for water-gas shift reaction: for production of ammonia: N 2 (g) + 3 H 2 (g) <===> 2 NH 3 (g) WHY? fertilizer … direct injection into soil Mass action expression: (note: K values are usually written without units)
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Some data: at 500 o C at equilibrium: [N 2 ] = 0.399 M [H 2 ] = 1.197 M [ NH 3 ] = 0.203 M What's K for the reaction as written? What's K for the reverse reaction? LE CHATELIER'S PRINCIPLE the position of equilibrium ... the role of stress "if a stress is imposed on a system at equilibrium, the position of equilibrium will shift in a direction that reduces that stress" What's stress? Effect of a change in pressure on: N 2 (g) + 3 H 2 (g) <===> 2 NH 3 (g) (Aside: how would you change the pressure?) Effect of a change in temperature: Temperature o C 300 atm 400 atm 500 atm 400 48% NH 3 55% NH 3 61% NH 3 500 26% NH 3 32% NH 3 38% NH 3 600 13% NH 3 17% NH 3 21% NH 3
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Note: is heat a reactant or product?
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