Acids and Bases
23-3-2011
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Definition of Acids
Arrhenius acid: A substance that releases H+ in
water ( e.g. HCl)
H+ + H2O
g
H3O+
Hydronium Ion
Acid: A substance which increases [H+]
when dissolved in water.
Base: A substance which increases [OH-]
when dissolved in water.
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Arrhenius Acids
- Strong acids completely ionize in water to
release H+ (H3O+) and an anion;
Strong Acids:
HClO4 H2SO4
HI
HBr
HCl
HNO3
HI + H2O
g
H 3O + + I or
HI g H+ + I- (100%)
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Arrhenius Acids
- Weak acids only partially ionize in water;
Weak Acids:
HC2H3O2 HF
HC2H3O2
H3PO4
HCN
H2S
H+ + C2H3O2-
H O
H C C O H
H
4
HNO2
NH4+
Brønsted-Lowry Acids and Bases
The Arrhenius definition does not apply
when water is not the solvent; so, a broader
definition (Brønsted-Lowry) was generated.
Acid: A proton donor
Base: A proton acceptor
Example:
HCl(aq) g H+(aq) + ClH+(aq) + :NH3(aq) g NH4+(aq)
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Conjugate Acids and Bases:
• From the Latin word conjugare, meaning “to
join together.”
• Reactions between acids and bases always yield
their conjugate bases and acids.
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Examples:
HF + HCO3-
H2CO3 + F-
HF = Acid
F- = Conjugate Base
HCO3- = Base
H2CO3 = Conjugate Acid
CH3NH2 + HI g CH3NH3+ + IWhich is conjugate acid & which is conjugate
base?
I- = conjugate base
CH3NH3+ = conjugate acid
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Strong Acids
You will recall that the six strong acids are
HCl, HBr, HI, HNO3, H2SO4, and HClO4.
These are, by definition, strong electrolytes
and exist totally as ions in aqueous
solution.
For the monoprotic strong acids,
[H3O+] = [acid].
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Strong Bases
Strong bases are the soluble hydroxides,
which are the alkali metal and heavier
alkaline earth metal hydroxides (Ca2+, Sr2+,
and Ba2+).
Again, these substances dissociate completely
in aqueous solution.
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Autoionization of Water
Autoionization is the self-ionization of a neutral
molecule into positive and negative ions.
H2O(l) + H2O(l)
H3O+(aq) + OH−(aq)
What is the equilibrium-constant expression for this
equilibrium?
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The expression is….
2 H2O
H3O+ + OHK = [H3O+][OH-]/[H2O]2
At 25°C, Kw = 1.0  10−14 and [H+] = [OH-] = 10-7
Kw is also called ion-product of water as well
as autoprotolysis of water
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pH Scale
In most cases, the hydrogen ion
concentration is very small which
makes it difficult to practically express
a meaningful concept for such a small
value. Currently, the pH scale is used to
better have an appreciation of the value
of the hydrogen ion concentration
where:
pH = - log [H+]
pH + pOH = 14
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In the case of pure water at 25oC , the pH is 7 and
the solution is said to be neutral.
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What is the concentration of OH- ions in
a HCl solution whose hydrogen ion
concentration is 0.013 M?
Kw = [H+][OH-] = 1.0 x 10-14
[H+] = 0.013 M
-14
K
1
x
10
w
= 7.7 x 10-13 M
[OH-] = + =
[H ]
0.013
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The OH- ion concentration of a blood sample is
2.5 x 10-7 M. What is the pH of the blood?
pH + pOH = 14.00
pOH = -log [OH-] = -log (2.5 x 10-7)
= 6.60
pH = 14.00 – pOH = 14.00 – 6.60 = 7.40
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What is the pH of a 2 x 10-3 M HNO3 solution?
HNO3 is a strong acid – 100% dissociation.
HNO3 (aq) + H2O (l)
Start 0.002 M
End
0.0 M
H3O+ (aq) + NO3- (aq)
0.0 M
0.0 M
0.002 M
0.002 M
pH = -log [H+] = -log [H3O+] = -log(0.002) = 2.7
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More detailed manipulation:
Find the pH of a 0.1 M HCl solution.
Solution
HCl is a strong acid that completely
dissociates in water, therefore we have
HCl g H+ + ClH2O D H+ + OH[H+]Solution = [H+]from HCl +[H+]from water
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However, [H+]from water = 10-7 in absence of
a common ion, therefore it will be much
less in presence of HCl and can thus be
neglected as compared to 0.1
(0.1>>[H+]from water)
Always neglect [H+]from water when
[H+]from ACID is larger than 10-6M
[H+]solution = [H+]HCl = 0.1
pH = -log 0.1 = 1
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Calculate the pH of the solution that is 0.04 M
Ca(OH)2 in water.
[OH-] = [OH-]Ca(OH)2 + [OH-]water
Always neglect [OH-]water when [OH-]base is
larger than 10-6M
[OH-]Ca(OH)2 = 0.08 M
[H+] = 10-14/0.08 = 1.2x10-12 M, Therefore:
[OH-] = [OH-]Ca(OH)2 = 0.08 M
pOH = 1.097
pH = 14 – 1.097 = 12.9
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What is the pH of a 1.8 x 10-2 M Ba(OH)2 solution?
Ba(OH)2 is a strong base – 100% dissociation.
Ba(OH)2 (s)
Start 0.018 M
End
0.0 M
Ba2+ (aq) + 2OH- (aq)
0.0 M
0.0 M
0.018 M
0.036 M
pH = 14.00 – pOH = 14.00 + log(0.036) = 12.6
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The pH of rain water in northeastern
United States on a particular day was
4.82. Find [H+].
pH = -log [H+]
[H+] = 10-pH
[H+] = 10-4.8
[H+] = 1.5*10-5 M
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Acid - Base Strengths
- Two factors influence acid strength:
1) Bond Polarity- The more polar the bond, the
stronger the acid.
- HCl (∆EN = 0.9) > > H2S (∆EN = 0.4)
2) Bond Strength
- The weaker the bond, the
stronger the acid.
- HI >> HF. HI bond is weaker than that of HF
bond.
- Bond strength is the larger of the two factors.
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Strength of Oxyacids
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