Kinetics and Equilibria By definition, kinetic processes are not equilibrium processes. In fact, we may think of kinetic processes as the mechanism that nature uses to reach the equlibrium state. has 2 rate constants, we can write, assuming these are ELEMENTARY reaction steps: (Equilibrium condition) Where [A]e etc. are the equilibrium concentrations of [A] etc. Binary Collision rate in reverse direction. Binary Collision rate in forward direction.

Using the Arrhenius form for the rate constants kf and kr But later we will learn (or you already know from high school): ln[Keq]= -G0/RT G0= H0 - T S0 Where    H0 is the enthalpy change for the reaction and S0 is the entropy change for the reaction. G0 is called the Free Energy k f  A f eEAf/RTk r A r eEAr/RT

Equating these two forms for the equilibrium constant allows us to connect thermodynamics and kinetics! “Identify” Af / Ar with {e (S/R)} (T “independent” assuming ∆Sº indep of T). A + B Act State E Af E Ar C + D +∆Ho = EAf - EAr (∆H o = Enthalpy change for A+B  C+D)  Thermodynamic form of Keq Kinetic form of Keq

Acid-Base Equilibria Several ways to define acid, base: Evidence exists for presence of H3O+ in solution. Small size of H+ allows it to be incorporated into the structure of the solvent. 1) Oversimplified: H+ ~ cm in diameter because is a free proton (unique in + charged species) HCl  H+ + Cl- CH3COOH  CH3COO- + H+ NaOH  Na+ (aq) + OH- (aq)

H3O+ is called the hydronium ion-is particularly stable. Less evidence for species like H9O4+ (4H2O+H+) Some non-OH- species can neutralize acids: HCl(aq)+NH3  NH4+ +Cl- 2) More accurate view: HCl (aq) + H2O  H3O+ (aq) + Cl- (aq) acid base acid base Note: H2O is a base here.

Conjugate pairs: CO32-, HCO3- and H2O, OH- Strengths of Acids and Bases: Equate to tendency to transfer a proton to H2O Need a quantitative measure of acidity or H+ donating power. CO32- + H2O  HCO3- + OH- HCl + H2O  H3O+ + Cl- HSO4- + H2O  H3O+ + SO42-

3) Lewis Concept (most general): Acid is any substance that can accept electrons and a base is any substance that can donate electrons. If acid is strong, e.g., HCl, then conjugate base is weak (Cl-) Will prove this latter in a quantitative fashion. Note that large K is associated with strong acid since it means numerator is large compared to denominator. Large KHA is a good proton donor to H2O. HA + H2O  H3O+ + A-

Acid-Base Equilibria Considerations Ka is called the acid ionization constant Acid ionization constant NH3 + H2O  NH4+ + OH - (K’ is a true equilibrium constant)

pH= -log 10 [H 3 O + ]

Bonus * Bonus * Bonus * Bonus * Bonus * Bonus

Suppose you have [H3O+] from added HCl = 0.1 M = 10-1 M Then [10 -1][OH-] = In pure H2O, [OH-] = [H3O+] = 10-7, but Weak Acids and Bases Add acetic acid to H2O: What are [CH3COOH], [H3O+], [CH3COO-]? K=Ka