H5Acids – 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 /
  • 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.

Lowry-Brönsted concepts /
  • 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 /
  • Pipette
  • Burette
  • Volumetric flask

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.
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 /
  • 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)

Lewis acids and bases /
  • 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 /
  • 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.

Refluxing / 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.