The Chemistry of Acids and Bases
Acids
Acids
Bases
Some Properties of Acids• Produce H+
• Taste sour • Corrode metals• Electrolytes• React with bases to form a salt and water• pH is less than 7• Turns blue litmus paper to red “Blue to Red A-CID”• React with certain metals to produce hydrogen gas.• React with carbonates and bicarbonates to produce
carbon dioxide gas
Some Common AcidsMonoprotic acids: one acidic (ionizable)hydrogenHNO3 - nitric acid HCl - hydrochloric acid
Diprotic acids: two acidic (ionizable)hydrogensH2SO4 - sulfuric acid
Triprotic acids: three acidic (ionizable)hydrogensH3(PO4)phosphoric acidH3(C6H5O7) citric acid
Some Properties of Bases
• Produce OH- ions in water• Taste bitter, chalky• Are electrolytes• Feel soapy, slippery• React with acids to form salts and water• pH greater than 7• Turns red litmus paper to blue “Basic Blue”
Some Common BasesNaOH sodium hydroxide lye
KOH potassium hydroxide liquid soap
Ba(OH)2 barium hydroxide stabilizer for plastics
Mg(OH)2 magnesium hydroxide Milk of magnesia
Al(OH)3 aluminum hydroxide Maalox (antacid)
Acid/Base definitionsDefinition #1: Arrhenius (traditional)
Acids – produce H+ ions (H3O+)HX H+
(aq) + X-(aq)
Bases – produce OH- ionsXOH -----> X+
(aq) + OH-(aq)
(problem: some bases don’t have hydroxide ions!)
Arrhenius acid is a substance that produces H+ (H3O+) in water
Arrhenius base is a substance that produces OH- in water
But: some bases don’t have hydroxide ions! Now what?????
Acid/Base DefinitionsDefinition #2: Brønsted – Lowry
Acids – proton donorBases – proton acceptor
A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor
acid
conjugate base
base
conjugate acid
H+ + Base = Conjugate acid of Base+
Acid = H+ + Conjugate base of Acid-
Conjugate Pairs
Water can be an acid or a baseAmphoteric
Learning Check!
Label the acid, base, conjugate acid, and conjugate base in each reaction:
HCl + OH- Cl- + H2O
H2O + H2SO4 HSO4- +
H3O+
Acids & Base DefinitionsDefinition #3 – Lewis Acids and Bases
Lewis base – a substance that donates an electron pair
Lewis acid – a substance that accepts an electron pair
Lewis Acids & BasesFormation of hydronium ion is also an excellent
example.
H H
H
BASE
••••••
O—HO—HH+
ACID
Electron pair of the new O-H bond originates on the Lewis base
Lewis Acid Base Reaction
Water
Water molecules collide with one another: self-ionization reaction 2H2O H3O+ + OH- The reaction does not form very much H3O+ or OH-.
1 Liter water = ~55 moles of water molecules, but only forms 1.0 x 10-7 moles of H3O+ and OH-
[H3O+] and [OH-] in pure water = 1.0 x 10-7 M. It is the 7 in the exponent or power of this number
that gives neutral water a pH of 7.
Ionization constant of water
Why isn’t water in the equilibrium constant Kw ?
Neutral solution: [H3O+] = [OH-] = [10-7]pH = - log [H+] pOH = - log [OH-]
Neutral solution: pH = pOH = 7Acidic Solutions: [H+] > [OH-]Basic Solutions (Alkaline): [OH-] > [H+] pH + pOH must always add up to 14
pH Scale: indicates strength of acid or base
Example: If [H+] = 1 X 10-10
pH = - log 1 X 10-10
pH = - (- 10)pH = 10 basicpOH = 4
Example: If [H+] = 1.8 X 10-5
pH = - log 1.8 X 10-5
pH = - (- 4.74)pH = 4.74 acidicpOH = 9.26
Try These!Find the pH of these:1) A 0.15 M solution of Hydrochloric acid2) A 3.00 X 10-7 M solution of Nitric acid3) A 0.15 M solution of sodium
hydroxide
pH testing• There are several ways to test pH
– Blue litmus paper (red = acid)– Red litmus paper (blue = basic)– pH paper (multi-colored)– pH meter (7 is neutral, <7 acid, >7 base)– Universal indicator (multi-colored)– Indicators like phenolphthalein– Natural indicators like red cabbage,
radishes
pH indicators• Indicators are dyes that can
be added that will change color in the presence of an acid or base.
• Some indicators only work in a specific range of pH
TitrationBuret with standardized solution
Unknown solution
End point
Past End point
End point – Visual Equivalence Point: StoichiometricThe point at which neutralization is
achieved (acid = base)
Titration Curves
Strong Acids vs Weak Acids• An acid that
nearly completely dissociates
• All molecules of the acid break up to form the ions soluble in water
• An acid that only slightly dissociates in a water solution
• Only a small percent of acid molecules donate their hydrogen, and most remain the same.
Weak acids have much lower percent ionization.CH3COOH(l) + H2O(l) H3O+
(aq) + CH3COO-(aq)
0.10 mol << 0.10 mol << 0.10 molmany molecules few ionsOnly about 5% ionization at 25C
A strong acid essentially ionizes 100%. HCl(g) + H2O(l) H3O+
(aq) + Cl-(aq)
0.10 mol 0.10 mol 0.10 molAfter ionization: a few molecules; but many ions
Strong Acids and BasesStrong Acids Strong Bases
HClO4 perchloric acid LiOH lithium hydroxide
HCl hydrochloric acid NaOH sodium hydroxide
HNO3 nitric acid KOH potassium hydroxide
H2SO4 sulfuric acid RbOH rubidium hydroxide
HBr hydrobromic acid CsOH cesium hydroxide
HI hydriodic acidCa(OH)2 calcium hydroxideHClO3Chloric acid* Sr(OH)2strontium hydroxide Ba(OH)2 barium
hydroxideNot included in every list
H+ A - H+ A - HA
A - H+ A - H+ A –
H+ A - H+ A - H+
A - HA H+ A -
H+ A - H+ A - H+
HA HA HA HA
HA HA HA
H+ A - HA HA
HA HA H + A –
HA H + A – HA HA
H+ A- H+ A- H+ A- H+ A- HAA- H+ A- H+ A- H+ A- H+ A -
H+ A- HA H+ A- H+ A- H+ A-
A- H+ A- H+ A- H+ A- H+ A- H+ H+ A - H + A - H + A - HA H + A -
A- H+ A- H+ A- H+ A- H+ A–
H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A-
HA A- H+ A- H+ A- H+ A- H+
HA HA H+ A- HA HA HA HA HA HA HA HA H+ A- H+ A- HA HA HA HA HA
HA HA H+ A- HA HA HA HA HA HA H+
A- HA HA H+ A- HA HA HA HA HA HA HA HA H+
A- HA HA H+ A- HA HA HA HA HA HA HA H+ A- HA HA HA
DILUTECONCENTRATED
STR
ON
GW
EAK
Acids: Concentration vs. Strength
Acid Dissociation Constant
Bases
• All information and equations reference bases also.
Strengths Of Conjugate Acid-Base Pairs• The stronger an acid, the weaker is its
conjugate base.
• The stronger a base, the weaker is its conjugate acid.
• An acid-base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid-base pair.
For biological systems:
• Ionization of a strong acid is TOO BIG!• Ionization of water itself is way TOO LITTLE!
• Ionization of a weak acid is JUST RIGHT!
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Weak acids, their conjugate bases, and buffers…
• Weak acids have only a modest tendency to shed their protons (definition of an acid).
• When they do, the corresponding negatively charged anion becomes a willing proton acceptor, and is called the conjugate base.
• The properties of a buffer rely on a balance between a weak acid and its conjugate base.
• And a titration curve looks like this…
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Buffers• A buffer is a solution of a weak acid and its
conjugate base that resists changes in pH in both directions—either up or down.
• A buffer works best in the middle of its range, where the amount of undissociated acid is about equal to the amount of the conjugate base.
• One can soak up excess protons (acid), the other can soak up excess hydroxide (base).
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pKa = 4.76
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pH 7 _
_
_
_
_
_
_
0 equiv. of NaOH 1.0added
Buffering range: only small pH changes
result from addition of base or acid
Titration of acetic acid with sodium hydroxide
50% dissociation
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