H5
Acids – Changing Concepts, Use in Analysis and Making Esters From
The definitions of acids and bases have developed as our understanding of these
reactions has increased. Volumetric analysis is an important method of quantitative
analysis that frequently involves acid-base reactions. Esterification and neutralisation
are compared.
Historical
development



Lowry-Brönsted
concepts


Lavoisier (1780) proposed that acids were substances that
contained oxygen
Davy (1815) proposed that acids were substances that
contained replaceable hydrogen
Arrhenius (1884) proposed that acids were substances
that ionised in solution to produce hydrogen ions.

An acid-base reaction is a proton transfer reaction
An acid is a substance, which in solution, tends to give up
protons (hydrogen ions)
An acid is a proton donor
A base is a substance, which in solution, tends to accept
protons (hydrogen ions)
A base is a proton acceptor
Conjugates bases



An acid gives up a proton to form its conjugate base.
The conjugate base of a strong acid is a weak base.
The conjugate base of a weak acid is a strong base.
Conjugate acids



A base accepts a proton to form its conjugate acid
The conjugate acid of a strong base is a weak acid
The conjugate acid of a weak base is a strong acid
The number of
hydrogen in acids

Monoprotic acids form one proton (hydrogen ion) per
molecule. (HCl, CH3COOH, HNO3)
Diprotic acids form two protons per molecule (H2SO4,
H2S, H2CO3)
Triprotic acids form three protons per molecule. (H3PO4)




Salts as acids and
bases





The solutions of many salts have a pH different from 7
because many anions and cations can act as acid or bases.
In a solution of a salt formed from a weak acid and a
strong base, the anion is a weak base and so the solution
has a pH greater than 7.
In a solution of a salt formed from a strong acid and a
weak base, the cation is a weak acid and the solution has
a pH less than 7.
The pH of a solution of a salt formed from a strong acid
and a strong base is 7.
The pH of a solution of a salt formed from a weak acid
and a weak base is 7.
Amphiprotic
substances
A substance that can act as a proton donor and a proton
acceptor. (HCO3-, HSO4-, H2O)
Volumetric analysis
Form of chemical analysis in which the concentrations of
substance A is determined by measuring the volume of a
solution of known concentration of another substance B which
is just sufficient to react with all of the sample of A.
Equivalence point
The point at which the amounts of the two reactants are just
sufficient to cause complete consumptions of both reactants.
Titration
The process of determining the equivalence point in a
reaction.
The basic steps in titration include
 Fill the burette with solution A
 Place an aliquot of solution B in a conical flask
 Add one or two drops of a suitable indicator to the
conical flask
 Slowly add solution A to the conical flask until one drop
of solution A causes the indicator to change colour
permanently.
 Record the volume of solution A used
 Calculate the concentration of the unknown solution.
Primary standard in
volumetric analysis
A substance of sufficiently high purity and stability that a
solution of it, of accurately known concentration, can be
prepared by
 Weighing out the desired mass
 Dissolving it in water
 Making up the volume to an accurately known value
Volumetric glassware



Indicators
The indicator selected for use in a titration must undergo a
sharp colour change at the equivalence point.
Titration curves
Plot the changes in pH during the progress of an acid-base
titration.
Characteristic curves are obtained depending on the strengths
of the acids and the bases.
Conductance curves
Plot the changes in conductivity of the solution during an
acid-base titration.
The equivalence point has minimum conduction because of
the size of the ions present.
Pipette
Burette
Volumetric flask
Buffer solution
Solution that contains comparable amounts of a weak acid and
its conjugate base.
A buffer solution is able to maintain an approximately
constant pH even when significant amounts of strong acid or
base are added to it.
Modern analytic
methods








Lewis acids and bases 





Atomic absorption spectroscopy (metal ions at ppm
concentrations)
Flame emission spectroscopy (metals in alloys)
Ultraviolet-visible absorption spectroscopy (organic and
inorganic compounds)
Infrared spectroscopy (organic compounds)
X-ray fluorescence (metals in minerals)
Chromatography combined with mass spectroscopy
(organic and biological sample)
Neutron activity
Electrochemical methods (environmental water samples)
A base is a substance that has a lone pair of electrons that
can be used to form a covalent bond with some other
substance.
An acid is a substance that is able to form a covalent
bond using a lone pair of electrons from another
substance.
A lone pair of electrons is a pair of valence electrons not
involved in bonding with some other atom.
A Lewis base is an electron-pair donor
A Lewis acid is an electron-pair acceptor
A complex ion is an ion formed when a Lewis base
attaches itself to a cation to form a combined ion.
Esters
Compounds formed when alkanoic acids react with alkanols.
They have pleasant fruity odours and are used as perfumes and
flavouring agents
Esterification
reactions



Refluxing
Reactions between alkanols and alkanoic acids to form
ester.
Esterification are moderately slow at room temperature
and do not go to completion
Concentrated sulfuric acid is used as a catalyst and to
shift the position of the equilibrium to the right by
reacting with the water formed.
Process of heating a reaction mixture in a vessel with a
cooling condenser attached in order to prevent loss of any
volatile reactant or product.
Refluxing allows the reaction to occur at higher temperatures.