Equilibrium

For equilibrium to occur: System must be closed. Temperature must be constant. Reactions must be reversible (do not go to completion). H2(g) + Cl2(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 N2O4(g) + q 2 NO2 (g) concentration Time

N2O4(g) + q 2 NO2 (g) concentration Teq Time 1st change: conc of N2O4 > ; 2nd change: temp < Time Teq

Predicting Changes 2H2(g) + O2(g) 2H2O(g) + energy

2H2(g) + O2(g) 2H2O(g) + energy What would a catalyst do to the equilibrium position? What would happen if we added He gas ?

N2O4(g) + q 2 NO2 (g) colorless orange 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.

Mass Action Expression Given the general equation: aA(aq)* + bB(aq) cC(aq) + dD(aq) * if g = gas @ constant Temp. ( Keq can also be Kp , Kc , Ksp , Ka , Kb , etc.)

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

Ka = Eg. Weak Acid: HC2H3O2 (+ H2O) H+ (aq) + C2H3O2- (aq) [H+] = 1.8 x 10-5 Ka = [HC2H3O2] What is there more of……reactant or product? Insoluble salts: PbI2 (s) (+ H2O) Pb+2 (aq) + 2 I- (aq) Ksp = [Pb+2][I-]2 = 8.4 x 10-9 What does this value tell us?

Keq is used to calculate concentrations of species at equilibrium. Given: N2(g) + O2(g) = 2NO(g). At 25°C the Kc = 1.0 x 10-30. [N2] = 0.040 & [O2] = 0.010. What is the concentration of NO?

Orig. Conc (mol/L) Change in Conc. Equilib. Conc. CO2 H2 CO H2O For the system: CO2 + H2 = CO + H2O Kc = 0.64 @ 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. CO2 H2 CO H2O

Heterogeneous equilibrium Reactions in which one or more of the substances involved is a pure liquid or solid. CO2(g) + H2(g) CO(g) + H2O (l)