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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
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33%33%33% 1. Strong acid
2. Weak acid
3. Neither
Identify the molecule as a strong acid, a weak acid, or neither
CH3CH2CH2OH
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33%33%33% 1. Strong acid
2. Weak acid
3. Neither
Identify the molecule as a strong acid, a weak acid, or neither
HCN
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33%33%33% 1. Strong acid
2. Weak acid
3. Neither
Identify the molecule as a strong acid, a weak acid, or neither
HClO4
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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?
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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?
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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…