Lecture 20 Chapter 17, Sections 4-5 More weak acids and bases. Identifying acids and bases Conjugate acids and bases Salts of weak acids and bases

Transcription

1 Lecture 20 Chapter 17, Sections 4-5 More weak acids and bases Identifying acids and bases Conjugate acids and bases Salts of weak acids and bases

2 Acids and Bases Strong vs. Weak K a vs. K b ph = -log([h]) ph + poh = 14 Conjugate acids and bases Should be able to calculate ph (or poh) of some solution given that you have some concentration of some weak acid.

3 Common Strong Acids & Strong Bases Strong Acids (Strong Electrolytes) Strong Bases (Strong Electrolytes) HCl Hydrochloric acid LiOH Lithium hydroxide HBr Hydrobromic acid NaOH Sodium hydroxide HI Hydroiodic acid KOH Potassium hydroxide HNO 3 Nitric acid Ca(OH) 2 Calcium hydroxide HClO 4 Perchloric acid H 2 SO 4 Sulfuric acid Strong acids will donate a proton to water to form hydronium ion. The hydronium ion concentration will be equal to the acid concentration. Strong bases will dissociate in solution to form hydroxide ion, the concentration of which can be calculated from the base s molarity.

4 Recognizing Acids and Bases Oxoacids an acid that contains an inner atom bonded to some oxygen atoms and acidic OH groups. H x EO y (Labels are reversed)

5 Carboxylic Acids RCO 2 H All carboxylic acids are weak acids.

6 Other Weak Acids

7 Polyprotic Acids Have multiple acidic protons (more later)

8 Identify the molecule as a strong acid, a weak acid, or neither Cl 3 CCO 2 H 33% 1. Strong acid 33% 2. Weak acid 33% 3. Neither

9 Identify the molecule as a strong acid, a weak acid, or neither CH 3 CH 2 CH 2 OH 33% 1. Strong acid 33% 2. Weak acid 33% 3. Neither

10 Identify the molecule as a strong acid, a weak acid, or neither HCN 33% 1. Strong acid 33% 2. Weak acid 33% 3. Neither

11 Identify the molecule as a strong acid, a weak acid, or neither HClO 4 33% 1. Strong acid 33% 2. Weak acid 33% 3. Neither

12 Weak Bases Water is a weak base Another common weak base is NH 3 Many other weak bases are derivatives of ammonia called amines. (Hey, we ve seen them before!) Some of the N H bonds have been replaced with C H bonds.

13 Representative Organic Bases

14 Conjugate Acid Base Pairs An acid donates an H + when in water. The resulting species could accept an H + from water to yield hydroxide. That resulting species is the conjugate base of the original acid HA + H 2 O A + H 3 O + acid conj. base A + H 2 O HA + OH

15 HF is a weak acid. What is its conjugate base? 20% 1. OH 20% 2. H 3 O + 20% 3. H 2 O 20% 4. F 20% 5. H 2 F

16 Is NaF an acid or a base? Neither at first glance But notice that in aqueous solution two ions are formed Na + and F Since F is a base (the conjugate base of HF) NaF must act as a base. NaF (s) + H 2 O Na + (aq) + F (aq) F (aq) + H 2 O HF (aq) + OH (aq)

17 What about Na + NaOH is a base, so Na + must be a conjugate acid Na + (aq) + 2H 2 O NaOH (aq) + H 3 O + (aq) Why doesn t this matter? Because NaOH is a strong base, Na + is essentially not acidic at all F acts as a base because HF is a weak acid

18 K a and K b are related Note the two equilibria related to hydrofluoric acid HF (aq) + H 2 O F (aq) + H 3 O + (aq) [ F ][ H O ] + 3 K a = [ HF] F (aq) + H 2 O HF (aq) + OH (aq) K b = [ ][ ] HF OH [ ] F

19 K a and K b are related (ii) If we add these two equations HF (aq) + H 2 O F (aq) + H 3 O + (aq) F (aq) + H 2 O HF (aq) + OH (aq) 2 H 2 O OH (aq) + H 3 O + (aq) If we add equations, we multiply equilibrium constants a b [ ][ ][ ][ ] + F H3O HF OH HF [ F ] [ ] [ ][ ] + H3O OH Kw K K = = =

20 K a and K b are related (iii) If K a K b = K w then pk a + pk b = pk w = 14 (for a conjugate acid-base pair)

21 Important points about K a and K b Acid Strength K a Conjugate Base Strength K b Strong >1 Very Weak < Weak to 1 Weak to 1 Very Weak < Strong >1 Of course, we could also have a similar table with Base Strength and Conjugate Acid Strength

22 Example: Bleach The bleach we used for the kinetics demos was standard 5 % NaOCl solution (aqueous) approx M What are the concentrations of ALL species in this solution? Identify all species Recognize conjugate base and find K b Set up equilibrium table Solve for species concentrations

23 Table 17-5 p.746

24 What makes an acid stronger or weaker? Charge Strength of X-H bond Charge is pretty simple Something positive is a good acid, lousy base Something negative is a lousy acid, good base Something neutral is somewhere between

25 Acid strength? More polar bond stronger acid HF stronger acid than CH 4 Weaker X-H bond stronger acid HCl stronger acid than HF HClO 4 stronger acid than HOCl

26 How acidic is the bold proton in oxalic acid compared to formic acid? Oxalic Formic 33% 1. Oxalic is more acidic than formic 33% 2. Oxalic is less acidic than formic 33% 3. They are essentially the same

27 How acidic is the second proton in oxalic acid compared to formic acid? Formic Oxalic 33% 1. 2 nd Oxalic proton is more acidic than formic acid 33% 33% 2. 2 nd Oxalic proton is less acidic than formic acid 3. They are essentially the same

28 Polyprotic Acids form Multiple Equilibria H 2 SO 4 has two protons that can come off Three species are all in equilibrium along with H 3 O + and OH H 2 SO 4 HSO 4 SO 4 2 Two pairs of conjugate acids and bases See Example Solve equilibrium problem for first proton Plug answers in as starting point for second equilibrium problem Continue if there are still more acidic protons

29 Today Finish CAPA #12 (due tomorrow) Friday Seminar (student-invited speaker) We ll finish up Chapt 17 It s not too early to start thinking about exam 2