lecture 20 – chapter 17, sections 4-5 more weak acids and...
TRANSCRIPT
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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
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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.
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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.
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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)
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Carboxylic Acids
• RCO2H
• All carboxylic acids are weak acids.
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Other Weak Acids
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Polyprotic Acids
Have multiple acidic protons (more later)
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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
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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
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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
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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
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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.
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Representative Organic Bases
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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–
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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 +
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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)
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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
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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
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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
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Ka and Kb are related (iii)
14ppp then
If
==+
=
wba
wba
KKK
KKK
(for a conjugate acid-base pair)
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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
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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
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Table 17-5 p.746
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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
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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
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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
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How acidic is the second proton in oxalic acid compared to formic acid?
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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
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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
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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…