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

Acids and Bases

• Strong vs. Weak

• Ka vs. Kb• 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.

Sulfuric acidH2SO4

Perchloric acidHClO4

Calcium hydroxideCa(OH)2Nitric acidHNO3

Potassium hydroxideKOHHydroiodic acidHI

Sodium hydroxideNaOHHydrobromic acidHBr

Lithium hydroxideLiOHHydrochloric acidHCl

Strong Bases (Strong Electrolytes)

Strong Acids (Strong Electrolytes)

Common Strong Acids & Strong Bases

• 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.

Recognizing Acids and Bases

• Oxoacids– an acid that contains an inner atom bonded to some oxygen atoms and acidic OH groups.

• HxEOy

(Labels are reversed)

Carboxylic Acids

• RCO2H

• All carboxylic acids are weak acids.

Other Weak Acids

Polyprotic Acids

Have multiple acidic protons (more later)

Identify the molecule as a strong acid, a weak acid, or neither

Cl3CCO2H

54321

33%33%33% 1. Strong acid

2. Weak acid

3. Neither

Identify the molecule as a strong acid, a weak acid, or neither

CH3CH2CH2OH

54321

33%33%33% 1. Strong acid

2. Weak acid

3. Neither

Identify the molecule as a strong acid, a weak acid, or neither

HCN

54321

33%33%33% 1. Strong acid

2. Weak acid

3. Neither

Identify the molecule as a strong acid, a weak acid, or neither

HClO4

54321

33%33%33% 1. Strong acid

2. Weak acid

3. Neither

Weak Bases

• Water is a weak base

• Another common weak base is NH3• 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.

Representative Organic Bases

Conjugate Acid – Base Pairs

• An aciddonates an H+ when in water. The resulting species could accept an H+ from water to yield hydroxide.

– That resulting species is the conjugate baseof the original acid

HA + H2O � A– + H3O+

acid conj. base

A– + H2O � HA + OH–

HF is a weak acid. What is its conjugate base?

54321

20%20%20%20%20% 1. OH –

2. H3O +

3. H2O

4. F –

5. H2F +

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) + H2O � Na+(aq) + F –(aq)F –(aq) + H2O � HF(aq) + OH–(aq)

What about Na+

• NaOH is a base, so Na+ must be a conjugate acid

Na+(aq) + 2H2O � NaOH(aq) + H3O+(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

Ka and Kb are related

• Note the two equilibria related to hydrofluoric acid

HF(aq) + H2O � F –(aq) + H3O+(aq)

F –(aq) + H2O � HF(aq) + OH–(aq)

[ ][ ][ ]HF

OHFKa

+−

= 3

[ ][ ][ ]−

−

=F

OHHFKb

Ka and Kb are related (ii)

• If we add these two equations…

HF(aq) + H2O � F –(aq) + H3O+(aq)

F –(aq) + H2O � HF(aq) + OH–(aq)

2 H2O � OH –(aq) + H3O+(aq)

• If we add equations, we multiply equilibrium constants

[ ][ ][ ]

[ ][ ][ ] [ ][ ] wba KOHOHF

OHHF

HF

OHFKK === −+−

−+−

33

Ka and Kb are related (iii)

14ppp then

If

==+

=

wba

wba

KKK

KKK

(for a conjugate acid-base pair)

Important points about Ka and Kb

>1Strong< 10-16Very Weak

< 10-16Very Weak>1Strong

10-16 to 1

Kb

Weak

Conjugate Base Strength

10-16 to 1

Ka

Weak

Acid Strength

Of course, we could also have a similar table with Base Strength and Conjugate Acid Strength

Example: Bleach

• The bleach we used for the kinetics demos was standard

5 % NaOCl solution (aqueous) approx. 0.67 M

• What are the concentrations of ALL species in this solution?

– Identify all species

– Recognize conjugate base and find Kb– Set up equilibrium table

– Solve for species concentrations

Table 17-5 p.746

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

Acid strength?

• More polar bond � stronger acid

– HF stronger acid than CH4• Weaker X-H bond � stronger acid

– HCl stronger acid than HF

– HClO4 stronger acid than HOCl

How acidic is the bold proton in oxalic acid compared to formic acid?

54321

33%33%33% 1. Oxalic is more acidic than formic

2. Oxalic is less acidic than formic

3. They are essentially the same

FormicOxalic

How acidic is the second proton in oxalic acid compared to formic acid?

54321

33%33%33% 1. 2nd Oxalic proton is more acidic than formic acid

2. 2nd Oxalic proton is less acidic than formic acid

3. They are essentially the same

Formic Oxalic

Polyprotic Acids form Multiple Equilibria

• H2SO4 has two protons that can come off

• Three species are all in equilibrium along with H3O+ and OH–

– H2SO4– HSO4–

– SO42–

• Two pairs of conjugate acids and bases

• See Example 17-14

– Solve equilibrium problem for first proton

– Plug answers in as starting point for second equilibrium problem

– Continue if there are still more acidic protons

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…