question_answer

The sol formed on adding silver nitrate solution to excess of potassium iodide solution is negatively charged while the same prepared in the reverse manner has positive charge. Explain.

Answer:

                The charge on the colloidal particles depends upon the preferential adsorption of the ions from the electrolyte that are common with their own lattice ions. When \[AgN{{O}_{3}}\] is added to an aqueous solution of KI, the yellow precipitate of \[AgI\] adsorbs \[{{I}^{-}}\] ions from the electrolyte KI and forms negatively charged colloidal particles. \[AgI+KI\xrightarrow{\,}\,\underset{\text{(fixed}\,\,\text{part)}}{\mathop{[AgI]{{I}^{-}}}}\,\,\,\,:\,\,\,\,\,\underset{\text{(diffused}\,\,\text{part)}}{\mathop{{{K}^{+}}}}\,\] When formed on adding KI solution to an aqueous solution of \[AgN{{O}_{3}}\] positively charged \[A{{g}^{+}}\] ions are adsorbed from the electrolyte and the colloidal particles acquire negative charge. \[AgI+AgN{{O}_{3}}\xrightarrow{{}}\,\underset{\text{(fixed}\,\,\text{part)}}{\mathop{[AgI]A{{g}^{+}}}}\,\,\,\,\,\,:\underset{\text{(diffused}\,\text{part)}}{\mathop{NO_{3}^{-}}}\,\]