Surface Chemistry
1. Theory: Surface Chemistry
{Important Points to Remember}
- If one of the products acts as a catalyst, it is called autocatalysis, e.g., in the reaction of oxalic acid with acidified \(\mathrm{KMnO}_{4}\) solution, \(\mathrm{Mn}^{2+}\) ions produced act as autocatalyst. The reaction speeds up with time.
- The greater the valence of the flocculating ion added, the greater is its power to cause precipitation. This is known as Hardy-Schulze rule. In the coagulation of a negative sol, the flocculating power is in the order: \(\mathrm{Al}^{3+}>\mathrm{Ba}^{2+}>\mathrm{Na}^{+}\)Similarly, in the coagulation of a positive sol, the flocculating
power is in the order: \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}>\mathrm{PO}_{4}^{3-}>\mathrm{SO}_{4}^{2-}>\mathrm{Cl}^{-}\)
- Higher is the valency of active ion, the greater is its coagulating power.
- Induced catalyst : A substance which induces the similar reaction in another which is otherwise not possible act as induced catalyst, e.g., sodium sulphite solution oxidises in air to sodium sulphate but sodium arsenite is not oxidised by air. In presence of sodium sulphite it is also oxidised.
- Peptization : The dispersal of freshly precipitated substance into colloidal solution by the addition of some electrolyte having one ion common is known as peptization. The electrolyte used is called peptizing agent e.g. \(\mathrm{AgCl}\) is peptized by \(\mathrm{HCl}\) giving negative sol of \([\mathrm{AgCl}] \mathrm{Cl}^{-}\)
- Flocculation Value : The minimum concentration in millimoles per litre required to cause the precipitation of a sol in 2 hours. The smaller the flocculation value, the higher is the precipitating power. Gold Number : The number of milligrams of a hydrophilic colloid that will prevent the precipitation of \(10 \mathrm{~mL}\) of a gold sol on the \(1 \mathrm{~mL}\) of \(10 \%\) sodium chloride solution is known as gold number. The smaller the value of gold number of lyophilic sol, the greater is the protective action.
- Associated Colloids : The molecules of certain substances are smaller than particles but aggregate spontaneously in a given solvent particles of colloidal size. The aggregates are known as micelles.
рдо According to freundlich adsorption isotherm:
\(\mathrm{x} / \mathrm{m}=\mathrm{k} . \mathrm{p} 1 / \mathrm{n}\); where \((n>1)\)
Taking log of the above equation, the following equation will be observed
\(\log \mathrm{x} / \mathrm{m}=\log \mathrm{k}+1 / \mathrm{n} \log \mathrm{p}\)
\(\mathrm{x} / \mathrm{m}\) is plotted on \(\mathrm{y}\) axis and \(\log \mathrm{p}\) is on \(\mathrm{x}\) axis. If straight line is observed then only freundlich isotherm is verified. Slope gives \(1 / \mathrm{n}\) and intercept gives \(\log \mathrm{k}\). The value of \(1 / \mathrm{n}\) varies from 0 to 1 .
If \(1 / \mathrm{n}\) is 0 , adsorption is independent of pressure. If \(1 / \mathrm{n}\) is 1 , adsorption changes with pressure.