For equilibrium to occur: System must be closed. Temperature must be constant. Reactions must be reversible (do not go to completion). H 2 (g) + Cl 2 (g)  2HCl(g) + energy No visible change…… A dynamic equilibrium exists. The rate of forward rx. = the rate of the reverse rx. Homogeneous Equilibria: all gaseous or aqueous phases.

Equilibrium in N 2 O 4 (g) + q 2 NO 2 (g) Time concentration

Time Teq N 2 O 4 (g) + q 2 NO 2 (g) concentration

Predicting Changes 2H 2 (g) + O 2 (g)  2H 2 O(g) + energy

What would a catalyst do to the equilibrium position? What would happen if we added He gas ?

N 2 O 4 (g) + q 2 NO 2 (g) colorlessorange Describe what happens when your instructor removes the tube from the freezer, containing the system described above. Explain your observation(s) using LeChatelier’s Principle, and all of the appropriate terminology.

Quantitative Aspect of Equilibrium Measurements in the N 2 O 4 - NO  C If we divide Equilibrium product values by reactant, each factor raised to the correct power (explained in a moment)…. What do you notice????

Quantitative Aspect of Equilibrium (continue) N 2 O 4(g) + q → 2NO 2(g)

Mass Action Expression Given the general equation: aA (aq)* + bB (aq) cC (aq) + dD (aq) * if g = constant Temp. ( K eq can also be K p, K c, K sp, K a, K b, etc.)

What does the K value tell us? If K is > 1 If K is < 1 If K is = 1

Eg. Weak Acid: HC 2 H 3 O 2 ( + H 2 O) H + (aq ) + C 2 H 3 O 2 - (aq) K a = [H + ][C 2 H 3 O 2 - ] [HC 2 H 3 O 2 ] = 1.8 x What is there more of……reactant or product? Insoluble salts: PbI 2 (s) ( + H 2 O) Pb +2 (aq ) + 2 I - (aq) K sp = [Pb +2 ][I - ] 2 = 8.4 x What does this value tell us?

K eq is used to calculate concentrations of species at equilibrium. Given: N 2(g) + O 2(g) = 2NO (g). At 25°C the K c = 1.0 x [N 2 ] = & [O 2 ] = What is the concentration of NO?

For the system: CO 2 + H 2 = CO + H 2 O K c = 900°C. The initial concentrations of reactants are both 0.100mol/L. When the system reaches equilibrium what are the concentrations of reactants and products? Orig. Conc (mol/L) Change in Conc. Equilib. Conc. CO 2 H 2 CO H 2 O

Heterogeneous equilibrium Reactions in which one or more of the substances involved is a pure liquid or solid. CO 2 (g) + H 2 (g) CO(g) + H 2 O (l)

Solubility & K: (read pgs ) Calculating solubility from K sp Given, PbSO 4, K sp = 1.6 x Calculate its solubility, [Pb +2 ] & [SO 4 -2 ]. PbSO 4 (s) = Pb +2 (aq) + SO 4 -2 (aq)

Try this problem: Mg(OH) 2 (s) = Mg +2 (aq) + 2OH - 1 (aq) K sp = 1.5 x Calculate the [Mg +2 ], [OH -1 ], and the [Mg(OH) 2 ] at equilibrium. So… how do we set up the mass action expression?

More Problems: 1. Bromine(I) chloride gas is formed in an endothermic reaction. At 400°C, after the reaction reaches equilibrium, the mixture contained 0.82 M BrCl, 0.20 M Br 2 (g) and 0.48 M Cl 2 (g). a. write the equation for this reaction: b. Write the equilibrium Expression, the MAE c. Calculate the value for K eq : d. What direction is favored?

2. Will a precipitate form when the following two solutions are combined? (assume volumes are additive.) M Pb(NO 3 ) M NaI The K sp for PbI 2 = 8.4 x 10 -9