Recap – Last Lecture
Acids and Bases
•  Intermolecular forces occur between molecules.
•  Arrhenius definition of acids and bases (1884):
An acid releases H+ ions when dissolved in water
A base releases HO– ions when dissolved in water
Force
Attraction
Energy/kJ mol-1
Example
Dispersion
forces
Fluctuations in e- cloud
0.05 - 40
All molecules
Dipole-dipole
forces
Partial charges
5 - 25
H-Cl
Hydrogenbonding
X :::::::: H-X
10 - 40
H2O
1
Conjugate pair
NH3
pH = – log [H+ (aq)]
In neutral solution
[H+] = [OH–] = 1 x10–7 and pH = 7 at 25 °C
Acid
NH4+
2
•  [H+] is used as a measure of acidity and for
convenience a log scale is often employed:
conjugate pair
HCl +
Note: H+(aq) and H3O+(aq) are used interchangeably.
pH scale
•  A conjugate acid – base pair differ by H+.
•  For example NH4+ is the conjugate acid of NH3
and NH3 is the conjugate base of NH4+.
Base
•  Brφnsted – Lowry definition of acids and bases (1923):
An acid is a proton donor
A base is a proton acceptor
+
Acid
Cl-
In acid solution
[H+] >1 x10–7 and [OH–] <1 x10–7 and pH < 7
Base
conjugate pair
3
Example
In basic solution
[H+] < 1 x10–7 and [OH–] > 1 x10–7 and pH > 7
4
Autoionisation of Water
•  Calculate the pH of a 0.002 M solution of sulfuric acid.
H2O(l) ⇌ H+(aq) + OH–(aq)
H2SO4 → 2H+ + SO42(one mole of H2SO4 dissociates to give two moles of protons)
Kw = [H+(aq)][OH–(aq)] = 1 x 10-14 at 25 °C
pKw = -log Kw = -log (1 x 10-14) = 14.0
∴  [H+] = 0.004 M
∴  pH = – log (0.004) = 2.4
(NB number of significant figures is given by the mantissa of
log term.)
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pH and pOH may be interconverted using pKw
consequently it is convention to report all
solutions – both acidic and basic ones in terms
of pH.
pKw = pH + pOH = 14
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1
Example
Strong & Weak Acids & Bases
•  Calculate the pH of a 0.002 M solution of sodium
hydroxide.
•  The terms strong and weak have a specific meaning in
an acid – base context.
NaOH → Na+ + OH(one mole of NaOH dissociates to give one mole of
hydroxide ions)
•  Strong indicates complete dissociation into ions,
e.g. HNO3
→ H+(aq) + NO3–(aq)
Ca(OH)2 → Ca2+(aq) + 2 OH–(aq)
and the concentration of the ions, and hence pH, is
obtained directly from the amount of starting material.
∴  [OH-] = 0.002 M
∴  pOH = – log (0.002) = 2.7
∴  pH = 14 – 2.7 = 11.3
Common strong acids are:
Common strong bases are:
HCl, HBr, HI, HNO3, H2SO4, HClO4
NaOH, KOH, Ca(OH)2, Ba(OH)2
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Strong & Weak Acids & Bases
Strong & Weak Acids & Bases
•  Weak indicates an equilibrium exists between the ions
and the undissociated compound in solution with, very
often, the undissociated compound dominating.
•  The pH of this solution may only be calculated if the
equilibrium constant, K, as well as the concentration of
the starting material is known.
e.g.
CH3COOH(aq)
⇌ CH3COO–(aq) + H+(aq)
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq)
Common weak acids include:
other carboxylic acids
Common weak bases include:
organic amines
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•  There is a direct relationship between the strength
of an acid and its conjugate base.
•  So a strong acid (e.g. HCl) is completely
dissociated in water and its conjugate base (Cl–) is
a very, very weak conjugate base and shows no
tendency to accept a proton.
HCl → H+ + Cl–
and
Cl– + H2O ––X→ HCl + OH–
(no reaction)
HF, HNO2, H3PO4, CH3COOH and
F–, NO2– , CH3COO–, NH3 and most
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Strong & Weak Acids & Bases
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Quantification
•  A weak acid (e.g. CH3COOH) is in equilibrium with
its ions in water and its conjugate base (CH3COO–,
a weak base) is also in equilibrium in water.
•  The position of equilibrium of a weak acid is given
by the value of Ka.
e.g. CH3COOH(aq) ⇌ CH3COO–(aq) + H+(aq)
Ka = [CH3COO–][H+] / [CH3COOH]
CH3COOH ⇌ CH3COO– + H+
and
CH3COO– + H2O ⇌ CH3COOH + OH–
For convenience the values are usually recorded as
pKa values where pKa = -log Ka.
The stronger the acid the smaller the pKa value (and
the larger Ka value)
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12
2
pH and pKa
Quantification
The position of equilibrium of a weak base is given by the
value of Kb.
e.g.
•  In the same way it is convenient to report both
acid and base concentration on the same scale
(pH), data books often report weak base strength
in terms of the pKa (of the conjugate acid).
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq)
Kb = [NH4+][OH–] / [NH3]
OH
O
C
CH
NH 2
H 2C
Once again, the values are usually recorded as pKb values.
The stronger the base the smaller the pKb value (and the
larger Kb value)
CH2
H 2C
CH2
H 2N
lysine
(Recap: pH + pOH = pKw = 14
but also pKa + pKb = pKw = 14)
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Applications
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Learning Outcomes:
• 
•  Many pharmaceuticals are composed of molecules with acidic or
basic groups. When in their charged form, this may aid
administration as they are likely to be water soluble.
•  In particular drugs containing an amine (nitrogen) group are often
sold as the hydrochloride salt. This not only increases water
solubility by also hinders the oxidation of the amine group and
hence extends shelf life of the product.
By the end of this lecture, you should be able to:
−  list common acids and bases.
−  define acids and bases according to the Arrhenius and
Bronsted-Lowry models.
−  use the definitions of pH and Kw to quantify the acidity and
basicity of aqueous solutions.
−  explain the difference between a strong and weak acid and
the difference between a strong and weak base in terms of
the percentage dissociation in solution.
HO
−  be able to complete the worksheet (if you haven’t already
done so…)
ClN
HO
N
H2
NH
Cl
OH
salbutamol
OH
O
O
HSO4-
Sold as a hydrosulfate and used for
asthma relief (eg ventolin)
cetirizine
Sold as a chloride and used for
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hayfever and allergy relief (eg zyrtec)
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Questions to complete for next lecture:
1.  Consider 1 M water solutions of NaOH, H2SO4, H3PO4
(pKa1 = 2.15), HF (pKa = 3.17) and CH3COOH (pKa = 4.76).
Arrange these solutions in order of increasing pH.
2.  Calculate the pH of the following solutions:
a. 
b. 
c. 
d. 
0.015 M HCl
0.0015 M HCl
0.03 M KOH
The solution formed from mixing 250 mL of 0.03 M HNO3 with 150
mL of 0.02 M Ca(OH)2.
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