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The Equilibrium Expression and the Equilibrium Constant (p.57m)

The Equilibrium Expression and the Equilibrium Constant (p.57m). [C] c x [D] d. [A] a x [B] b. Eqm Constant & Expression (p.57). Remember questions 6 & 7 from earlier in this unit where we graphed [A] and [B]?

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The Equilibrium Expression and the Equilibrium Constant (p.57m)

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  1. The Equilibrium Expression and the Equilibrium Constant (p.57m)

  2. [C]c x [D]d [A]a x [B]b Eqm Constant & Expression (p.57) • Remember questions 6 & 7 from earlier in this unit where we graphed [A] and [B]? • The ratio of [B]/[A] was 5.0 and after we added some B the eqm shifted but the ratio remained equal to 5.0 • What one finds is that for the general rxn: a A + b B  c C + d D is that • The expression is called the eqm expression while the value of Keq is called the eqm constant Keq = a constant =

  3. Characteristics of Eqm (p.40m) • Let’s look at the eqm A  B where we start with lots of A and no B. • Plotting the [A] and [B] on p. 40 you get:

  4. [PCl3] x [Cl2] [HF]2 [PCl5] [H2] x [F2] [H2] x [CO2] [H2O] x [CO] Examples (p.57m) • H2O(g) + CO(g) H2(g) + CO2(g) • PCl5(g) PCl3(g) + Cl2(g) • H2(g) + F2(g) 2 HF(g) • Note that 2 HF is the same as HF + HF Keq = Keq = Keq =

  5. [Ca2+] x [F-]2 40.7 One More Thing… (p.58m) • Consider CaF2(s) Ca2+(aq) + 2 F-(aq) • You might expect to include [CaF2] but • CaF2 has a constant density of 3.18 x 103 g/L • We can convert this to a molar concentration • [CaF2] = 40.7 M (this is a constant) • The Keq value is 8.4 x 10-13 (also a constant) • Rather than have 2 constants, we can combine them Keq = 8.4 x 10-13 = [Ca2+] x [F-]2 Keq = 3.4 x 10-11 =

  6. General Rule (p.59t) • Things that have a constant concentration are not included in the eqm expression (their molarities are already included in the constant’s value) • What things have a constant [ ] ? • Solids • Pure liquids (a liquid is pure if it is the only liquid on either side of the eqn) • Everything else must be included: • Gases, aqueous ions, mixtures of liquids

  7. [HBr]2 1 [H2] [Cl2] [CH3COCH2Cl][HCl] [CH3COCH3][Cl2] Examples (p.59m) • Br2(l) + H2(g) 2 HBr(g) • CH3COCH3(l) + Cl2(g) CH3COCH2Cl(l) + HCl(g) • Cl2(g) + 8 H2O(l) Cl2•8H2O(s) • Note that solids and pure liquids have no effect on shifting eqm when added b/c [ ] = constant Keq = Keq = Keq =

  8. LeChat’s Principle and Keq (p.61t) • When conc. or pressure is changed, the system will readjust to the same Keq. • The only factor that changes Keq is temperature. A change in temp ALWAYS changes the value of Keq. • If you look at the graphs from the last lesson, you should be able to see that this can make sense.

  9. Example (p.61m) • For the eqm: 2 NO(g) + Cl2(g) 2 NOCl(g) + 76 kJ a decrease in temp shifts eqm right, therefore, [NOCl] goes up while [NO] and [Cl2] go down. • Since Keq is [products]/[reactants], the value of Keq increases. • If a rxn shifts to reactants, Keq decreases.

  10. Meaning of the Size of Keq (p.61b) • If Keq = 1 then the ratio of [products] / [reactants] is equal. • If Keq is large (> 1) then there must be a large amount of products present at eqm. • If Keq is small (< 1) there must be a small amount of products present at eqm.

  11. Homework • Hebden #31 odds, 32, 34, 35 odds, 36-38, 40, 44-46. • Quiz on Friday.

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