Chem. 1B – 9/15 Lecture

Announcements I Lab Report for Lab #1 due Monday/Tuesday Second Mastering Set due Tuesday (should cover material to do most problems today) Exam #1 Two weeks from today (9/29) Covers material covered through next Thurs. (including buffers)

Announcements I Today’s Lecture [H+] and pH in strong acid and weak acid solutions % ionization of weak and strong acids Base solutions + solving equilibrium problems Acid-base properties of ions and salts

Chem 1B – Aqueous Chemistry Equilibrium Problems Involving Acids Strong Acids (large Ka values) Example: Determine the pH of a 0.014 M HCl solution We could set up an ICE table, but with strong acids, we assume 100% dissociation HCl(aq) → H+(aq) + Cl- (aq) So [H+] = [HCl(aq)]o = 0.014 M pH = -log[H+] = 1.85 Note: this approach works as long as [H+] from HA is greater than [H+] from H2O

Chem 1B – Aqueous Chemistry Equilibrium Problems Involving Acids Weak Acids (Ka values < 1) Example: Determine the pH of HCHO2 (formic acid, Ka = 1.8 x 10-4) solution initially made to be 0.20 M.

Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium % Ionization of Weak Acids (or % dissociation) Given by [A-]*100/[HA]all forms Show example from the 0.20 M formic acid problem covered last time

Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium Generalities about % Ionization The greater Ka, the greater % ionized (example for 0.05 M) The lower the concentration, the greater the % ionized (example for formic acid) Acid HNO2 HCHO2 HC2H3O2 HClO HCN Ka 4.60E-04 1.80E-04 1.80E-05 2.90E-09 4.90E-10 %ionized 9.14 5.82 1.88 0.02 0.01 [HA]o 0.5 M 0.05 M 5E-03 5E-04 5E-05 %ionized 1.88 5.82 17.26 44.64 81.53

Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium Mixtures of Acids Example: what is the pH of a raindrop if its sources of acid are from H2CO3 (from CO2 gas) and NH4HSO4? This is one of the few general chemistry texts covering this However, in general we cannot solve these problems using the approach covered in this class as the approach can fail when more than one equilibrium occurs In selected cases (where concentrations are similar but Ka values are very different), it is possible to solve these problems

Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium Mixtures of Acids – back to example: What is the pH of a raindrop if [H2CO3]o = 1.4 x 10-5 and (from CO2 gas) and [NH4HSO4]o = 1.0 x 10-4 M? In this case there are 4 sources of H+: HSO4- (Ka2 = 0.012), H2CO3 (Ka1 = 4.3 x 10-7), NH4+ (Ka = 5.7 x 10-10), and HCO3- (Ka1 = 5.6 x 10-11) (actually 5 if we include water) Because the Ka value for HSO4- is much greater than the others, we can assume that determines the pH Show ICE approach for HSO4- Once we know [H+], we can determine the concentration of other species (e.g. [HCO3-])

Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium Other Types of Problems An unknown acid is prepared so its concentration is 0.010 M. Its pH is measured and found to be 3.90. What is the Ka of this acid? What is its % ionization?

Chem 1B – Aqueous Chemistry Base Reactions and Equilibrium Strong Bases Most Strong Bases are Metal Hydroxides (MOH) Since metal hydroxides are ionic compounds, they totally dissociate Some are not very soluble (e.g. Zn(OH)2), but considered strong bases because what does dissolve 100% dissociates Example: Calculate the pH if we have an aqueous Ba(OH)2 solution prepared to be 0.0030 M.

Chem 1B – Aqueous Chemistry Base Reactions and Equilibrium Weak Bases Generic Reaction: B(aq) + H2O(l) ↔ BH+(aq) + OH-(aq) (B = base) Kb = [BH+(aq)][OH-(aq)]/[B(aq)] (why no H2O?) Stronger weak base has larger Kb value Examples: NH3 (ammonia) CH3NH2 (methylamine) C5H5N (pyridine) Most of these examples smell bad (e.g. tuna fish) Molecule is volatile but cation form is not Le Châtelier’s Principle Question: Explain why adding acid (mustard) to tuna fish reduces its smell

Chem 1B – Aqueous Chemistry Base Reactions and Equilibrium Weak Bases – Example Problem Example: Determine pH of a NH2OH (hydroxylamine) solution (Kb = 9.1 x 10-9) initially at 0.010 M

Chem 1B – Aqueous Chemistry Acid-Base Properties of Ions In looking at acids and bases, we have restricted ourselves to uncharged compounds However, the ions produced by the acid and base reactions are acids and bases Example: Acetate (C2H3O2-) is the conjugate base of acetic acid. Is it an acid or a base? What is its Kb if Ka(HC2H3O2) = 1.8 x 10-5? (Hint: combine the acid reaction with the water protolysis reaction to determine Kb) How is conjugate base strength related to acid strength?

Chem 1B – Aqueous Chemistry Acid-Base Properties of Ions In general, cations will be acidic (conjugate acids of bases or water/hydroxide complexing metals) or neutral Group I and II metals complex poorly with hydroxide and can be considered neutral Anions will be basic (conjugate bases of weak acids), or neutral (conjugate bases of strong acids) “Hydrogen anions” can be acidic (HSO4-) or basic (HCO3-) For salts, must examine both cation and anion

Chem 1B – Aqueous Chemistry Acid-Base Properties of Ions Example Question: Determine if the ionic compounds are acidic or basic in the following examples: NaCl NH4Cl NaC2H3O2 Fe(NO3)3 NH4CN

Chem 1B – Aqueous Chemistry Polyprotic Acids Generic Example: H2A – has two protons that can be lost through acid reactions (diprotic) Some Examples: H2SO4 (sulfuric – first H+ loss is strong acid) H2SO3 (sulfurous) H2CO3 (carbonic) H3PO4 (phosphoric – triprotic) Reaction of generic diprotic example H2A(aq) ↔ H+(aq) + HA-(aq) K = Ka1 HA- (aq) ↔ H+(aq) + A2-(aq) K = Ka2

Chem 1B – Aqueous Chemistry Some Practice Which solution will have a greater fraction of ionization? 0.10 M HClO vs. 0.10 M HF Ka(HClO) = 2.9 x 10-8 Ka(HF) = 3.5 x 10-4 An unknown base is dissolved in water so that its initial molarity is 0.050 M. The pH is measured and found to be 10.13. What is its Kb value? The Kb for NH3 is 1.76 x 10-5. What is the pH of a solution initially made to 0.10 M NH4Cl?

Chem 1B – Aqueous Chemistry Polyprotic Acids – in Problems Solving polyprotic acid problems can be challenging (the concentrations of the products from the first reaction affect the equilibrium in the second reaction) To simplify the problem, we assume the two reactions occur independently (valid if Ka1 >> Ka2) Example Problem: calculate [H2CO3], [HCO3-], pH, and [CO32-] for a 1.0 x 10-3 M solution of H2CO3