Lec 09 Jan 23 Fri - 8. If you want to understand the...

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Unformatted text preview: 8. If you want to understand the connection between equilibrium constant (K) and rate constant (k) – continue reading… • It is not because a reaction is fast or slow that you get lots of products • Think of the rate of product formation (related to k) separately from the extent (= how much) of product formation (related to K) • Large K means lots of products made eventually at equilibrium • Large k means the reaction reached its equilibrium position quickly • At equilibrium, rate (fwd) = rate (rev), species still being interconverted e.g. 2 NO2 (g) → 2 NO (g) + O2 (g) 1.26 [ products ]a [NO]2 [O 2 ] K= = [reactants ]b [NO 2 ]2 Concentrations (mol/L) If K is large (lots of product) Our rate laws valid only at the beginning of the reaction when very little product is present ! At equilibrium, conc. not changing NO O2 N O2 Time (s) If K is small (little product) Concentrations (mol/L) Smaller k Larger k NO2 NO O2 Time (s) 2.1 UNIT 2: Chemical Equilibrium (15-1) Chemical equilibrium : the state when the concentrations of all reactants and products remain constant with time. Dynamic, not static state. N2O4(g) ⇔ 2NO2(g) no colour brown (both rxns are elementary steps) Rate of forward reaction = rate of reverse reaction rate(fwd) = rate(rev) k(fwd) [N2O4]eq = k(rev) [NO2]eq2 k (fwd ) [ NO 2 ]2 = = const = K k (rev) [ N 2O 2 ] 1. Equilibrium Constants (15-2) For the reaction in solution: aA + bB ⇔ cC + dD Equilibrium Constant = K = [C] [D] a b [ A] [B] c d c d For the same reaction but in the gas phase, use pressure instead of concentrations: ! PP Equilibrium Constant = K = C a D b PA PB If pure solids (liquids), conc=1 ! K is dimensionless (even if concentrations do not seem to cancel) K is constant at a given temperature, but changes with temperature Why is K dimensionless? • each pressure value is actually divided by standard pressure (1 atm or 1 bar) or standard concentration (1 M) • so units cancel, which leaves K without units; we are really using acitivities Note difference from text: • text has Ka, Kb, Kw, Kp, Kc, Keq, Kf, Ksp for different reactions… • we will call all of these simply K; they are all forms of equilibrium constants ** for gases, calculate K by using gas pressures ** for solutions, calculate K by using solution concentrations 2.2 If you are not at equilibrium, (15-5) then your values of [products]/[reactants] will not be equal to the equilibrium value K. What is reaction quotient Q? -- Q tells you how far you are from equilibrium (determined by K, the equilibrium constant), and it tells you which way to go to get to K -- if Q is now, then K is the eventual equilibrium position. Q= [products]initial [reactants]initial NOW K= [products]eq [reactants]eq Eventually at equilibrium ! Comparing reaction quotient Q to equilibrium constant K will tell you ! which way the reaction will proceed to get to equilibrium: A. If Q < K: [products]eq [products]initial < [reactants]initial [reactants]eq Too small, will increase. Reaction goes to make more products, reactions goes to the right → ! B. If Q > K: [products]eq [products]initial > [reactants]initial [reactants]eq Too large, will decrease. Reaction goes to reduce products, reactions goes to the left ← ! C. If Q = K: Reaction does not shift, it is at equilibrium. Large K: equilibrium shifted to right (large conc. of products) Small K: equilibrium shifted to left (small conc. of products) ...
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This note was uploaded on 03/25/2009 for the course CHEM 102 taught by Professor Drk during the Spring '09 term at University of Alberta.

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