Chem 121
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J. Acids and Bases
Definitions of acids and bases
Acid and base strength
pH, pOH, pKa, pKw
Factors that determine the strength of acids and bases
Calculations
Definitions: Arrhenius
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The simplest definition of an acid and a base is that of
Arrhenius:
Definitions: Brønsted–Lowry
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Why do we need a more general definition?
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Species that can behave as either an acid or a base are
What Happens When an Acid Dissolves in Water?
Conjugate Acids and Bases
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From the Latin word conjugare, meaning “to join together.”
Acid and Base Strength
Acid and Base Strength
In any acid-base reaction, the equilibrium will favour the reaction
that moves the proton to the stronger base.
HCl(aq) + H2O(l)
⇄ H3O+(aq) + Cl−(aq)
CH3COOH(aq) + H2O(l)
⇄
H3O+(aq) + CH3COO−(aq)
Autoionisation of Water
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Water is amphoteric.
H2O(l) + H2O(l)
⇄
H3O+(aq) + OH−(aq)
pH
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pH is defined as the negative logarithm (to base 10) of the [H+]
ion concentration.
pH of acids and bases
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For water:
Kw = [H+] [OH−] = 1.0  10−14
Other “p” Scales
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The “p” in pH is an instruction to take the negative log to
base 10 of the quantity (in this case, hydrogen ions).
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Other important examples are:
More on pH
Because
we know that, after taking logs,
(−log [H+]) + (−log [OH−]) = (−log Kw ) = 14.00
Or, in other words,
How Do We Measure pH?
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For approximate measurements, we can use:
For more accurate measurements,
we use a pH meter, which measures
the voltage of an electrode in the
solution.
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Strong Acids and Bases
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The seven strong acids are:
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These are strong electrolytes so dissociate fully in aqueous solution.
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For monoprotic strong acids:
Dissociation Constants
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For a generalized acid dissociation:
HA(aq) + H2O(l)
⇄
A−(aq) + H3O+(aq)
the acid dissociation constant is
Calculations 1: Calculating Ka from pH
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E.g. The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is
2.38. Calculate Ka for formic acid at this temperature.
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To calculate Ka, we need the equilibrium concentrations of all three
species.
Calculations 1: 2nd part
Calculations 2: Calculating Percent Ionization
Percent Ionization =
In this example:
Calculations 3: Calculating pH from Ka
E.g. Calculate the pH of a 0.30 M solution of acetic acid, CH3COOH, at
25°C. Ka for acetic acid at 25°C is 1.8  10−5.
Assume that the concentration of the acid is almost unchanged
when it dissociates (why is this likely to be a good assumption?)
Polyprotic Acids
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Polyprotic acids have more than one acidic proton.
H2SO3(aq) ⇄ H+(aq) + HSO3−(aq) Ka1 = 1.7 X 10-2
HSO3−(aq) ⇄ H+(aq) + SO32-(aq)
Ka2 = 6.4 X 10-8
If successive Ka values differ by a factor of 103 or more, the pH is
determined by the first dissociation constant, Ka1.
Weak Bases
Bases react with water to produce
hydroxide ion.
[NH4+] [OH−]
Kb =
[NH3]
Kb is the basedissociation
constant. It can
be used to find
[OH−] and, thus,
pH.
Calculations 4: pH of Basic Solutions
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What is the pH of a 0.15 M solution of NH3? Kb is 1.8 x 10-5
Ka and Kb
Ka and Kb for an acid and its conjugate base are related
Reactions of Cations with Water
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Attraction between nonbonding
electrons on oxygen and a cation
causes a shift of the electron
density in water.
Factors that Affect Acid Strength: binary acids
Binary acids are stronger:
Factors that Affect Acid Strength: Oxyacids
In oxyacids, the OH group and any further oxygen atoms are bound to a
central atom, Y, e.g. H2SO4 {O2S(OH)2}, ClOH, HNO3 {O2NOH}.
Factors that Affect Acid Strength: Carboxylic Acids
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Resonance in the conjugate bases of carboxylic acids stabilizes the
base and makes the conjugate acid more acidic.
Lewis Acids and Bases