question_answer 1)

Discuss the principle and method of softening of hard water by synthetic ion exchange resins.

Answer:

Ion exchange resins are synthetic substances. These are of two types: (i) cation exchange resins and (ii) anion exchange resins. The cation exchange resins are granular insoluble organic acid resins having giant molecules with \[-S{{O}_{3}}H\]or \[\text{COOH}\] groups having general formula \[R-S{{O}_{3}}H\] or \[R-COOH\] where R is a giant hydrocarbon network while the anion exchange resins contain giant organic molecules with basic groups derived from amines. These resins remove all soluble minerals from water. The hard water is first passed through a bed of cation exchange which removes the cations like \[N{{a}^{+}},M{{g}^{2+}},C{{a}^{2+}}\] and others by exchanging with \[{{H}^{+}}\] ions. \[2\underset{\operatorname{Re}\sin }{\mathop{RCOOH}}\,+C{{a}^{2+}}\to {{(RCOO)}_{2}}Ca+2{{H}^{+}}\] \[2RS{{O}_{3}}H+M{{g}^{2+}}\to (RS{{O}_{3}})Mg+2{{H}^{+}}\] The water coming from cation exchanger is acidic on account of free \[{{H}^{+}}\] ions. This water is then passed through anion exchange which removes anions like\[C{{l}^{-}},SO_{4}^{2-},NO_{3}^{-}\], etc., by exchanging with \[O{{H}^{-}}\] ions. \[R-N{{H}_{3}}OH+C{{l}^{-}}\to R-N{{H}_{3}}Cl+O{{H}^{-}}\] The \[O{{H}^{-}}\] ions combine with \[{{H}^{+}}\] ions. \[{{H}^{+}}+O{{H}^{-}}\to {{H}_{2}}O\] This process gives distilled water. This water is also called demineralised water. The resins can be regenerated by treated with cone. \[HCl\] and \[\text{NaOH}\]solution respectively and regenerated exchangers can be used over again and again.