04 Equilibrium Lec

04 Equilibrium Lec - Equilibrium Chem rxns reach a state of...

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Equilibrium • Chem rxns reach a state of dynamic equilibrium * rate forward = rate reverse * no net change in composition • If a rxn mix starts out or is forced away from eq., the composition will change until eq. is established Reversible Reactions Where do we start? Start with reactants, form products Start with products, form reactants N 2 (g) + 3 H 2 (g) 2 NH 3 (g) Time Æ Concentration Ammonia Ni trogen Hydrogen 1
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What is Equilibrium • Forward and reverse processes occur at same rate. Why? * number of ammonias dissociating in unit time is same as number of N 2 + H 2 collisions leading to ammonia * if too much N 2 + H 2 present, more collisions and thus more ammonia will form N 2 (g) + 3 H 2 (g) 2 NH 3 (g) Law of Mass Action • No matter how we start, the ratio: [NH 3 (g)] 2 = [N 2 (g)] 1 •[H 2 (g)] 3 N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 1 Generalized • For a general rxn: a A + b B c C + d D [C] c •[D ] d = [A] a •[B ] b Use concentrations, but units not included. In actuality, they do not have units because we SHOULD use What is an ACTIVITY? ACTIVITIES 2
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ACTIVITY of a SUBSTANCE: A dimensionless quantity defined……. . α = effective concentration of a substance in the system effective concentration of a substance in a standard reference state System? The reaction we are interested in. Activities have no units!!! Hence K has no units! WHAT ARE THE STANDARD REFERENCE STATES?
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04 Equilibrium Lec - Equilibrium Chem rxns reach a state of...

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