Chem. 1B – 9/17 Lecture Updated Announcements I page to list exam date as 10/1

Announcements I Lab (Next Mon./Tues.): –Experiment 1 Lab Reports Due –Quiz 3 (on Ch Experiment 7) Exam 1 –On Oct. 1 (2 weeks from today) –Will ask Sect. 7 students to take exam in lab (if I can get a proctor) to give everyone more space –I plan to spend some time on Sept. 29 th reviewing material for the exam

Announcements II Today’s Lecture – Chapter 15 Topics –Equilibrium Problems involving Acids (more on weak acids + % ionization and acid mixtures) –Base Solutions + Solving Equilibrium Problems –Acid-Base Properties of Ions and Salts –Polyprotic Acids

Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium Weak Acids in Water (covered last time) Question about solving problems and assumption of [HA] o >> x –Can solve all using quadratic equation –Can make assumption and check later –If [HA] o > 1000·K a (barely met last time), assumption is usually valid % 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 K a, the greater % ionized (example for 0.05 M) –The lower the concentration, the greater the % ionized (example for formic acid) AcidHNO 2 HCHO 2 HC 2 H 3 O 2 HClOHCN KaKa 4.60E E E E E-10 %ionized [HA] o 0.5 M0.05 M5E-035E-045E-05 %ionized

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 H 2 CO 3 (from CO 2 gas) and NH 4 HSO 4 ? –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 K a 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 [H 2 CO 3 ] o = 1.4 x and (from CO 2 gas) and [NH 4 HSO 4 ] o = 1.0 x M? –In this case there are 4 sources of H + : HSO 4 - (K a2 = 0.012), H 2 CO 3 (K a1 = 4.3 x ), NH 4 + (K a = 5.7 x ), and HCO 3 - (K a1 = 5.6 x ) (actually 5 if we include water) –Because the K a value for HSO 4 - is much greater than the others, we can assume that determines the pH –Show ICE approach for HSO 4 - –Once we know [H + ], we can determine the concentration of other species (e.g. [HCO 3 - ])

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 M.

Chem 1B – Aqueous Chemistry Base Reactions and Equilibrium Weak Bases –Generic Reaction: B(aq) + H 2 O(l) ↔ BH + (aq) + OH - (aq) (B = base) –K b = [BH + (aq)][OH - (aq)]/[B(aq)] (why no H 2 O?) –Stronger weak base has larger K b value –Examples: NH 3 (ammonia) CH 3 NH 2 (methylamine) C 5 H 5 N (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 NH 2 OH (hydroxylamine) solution (K b = 9.1 x ) initially at 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 (C 2 H 3 O 2 - ) is the conjugate base of acetic acid. –Is it an acid or a base? –What is its K b if K a (HC 2 H 3 O 2 ) = 1.8 x ? (Hint: combine the acid reaction with the water protolysis reaction to determine K b ) 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 (HSO 4 - ) or basic (HCO 3 - ) 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: 1.NaCl 2.NH 4 Cl 3.NaC 2 H 3 O 2 4.Fe(NO 3 ) 3 5.NH 4 CN

Chem 1B – Aqueous Chemistry Polyprotic Acids Generic Example: H 2 A – has two protons that can be lost through acid reactions (diprotic) Some Examples: –H 2 SO 4 (sulfuric – first H + loss is strong acid) –H 2 SO 3 (sulfurous) –H 2 CO 3 (carbonic) –H 3 PO 4 (phosphoric – triprotic) Reaction of generic diprotic example 1)H 2 A(aq) ↔ H + (aq) + HA - (aq) K = K a1 2)HA - (aq) ↔ H + (aq) + A 2- (aq) K = K a2

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 K a1 >> K a2 ) Example Problem: calculate [H 2 CO 3 ], [HCO 3 - ], pH, and [CO 3 2- ] for a 1.0 x M solution of H 2 CO 3