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

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 [C]c x [D]d [A]a x [B]b Keq = a constant =

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:

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 [H2] x [CO2] [H2O] x [CO] Keq = [PCl3] x [Cl2] [PCl5] Keq = [HF]2 [H2] x [F2] Keq =

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 [Ca2+] x [F-]2 40.7 Keq = 8.4 x 10-13 = Keq = 3.4 x 10-11 = [Ca2+] x [F-]2

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

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 [HBr]2 [H2] Keq = [CH3COCH2Cl][HCl] [CH3COCH3][Cl2] Keq = 1 [Cl2] Keq =

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.

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.

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.

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