Discuss the principle and the method of softening of hard water by synthetic, ionexchange resins.

Explanation

Synthetic ion - exchange resins method :
`to` In present days, this method is mostly used for softening the hard water by using synthetic resins.
`to` This method is more useful than zeolite (or) permutit process.
Principle : Formation of de-ionised water (or) de mineralised water by passing the water successively through cation exchange and anion exchange resins.
De-ionised water means, water which is free from all soluble mineral salts.
Process : The de-ionisation of water can be done in two steps by this process.
Step - I : Cation exchange process
Step - II : Anion exchange process
Step - I : Cation exchange process.
`to` In this process synthetic resins used are - `So_(3)H` group containing large organic molecule. `(R-SO_(3)H)`
Here R = organic group (or) resin anion.
`to` At first the synthetic resin converted to RNa by reacting it with NaCl.
`to` This resin i.e, RNa exchanges `Ca^(+2) and Mg^(+2)` ions of hard water and the softening of water takes place.
`2RNa_((s))+M_((aq))^(+2)toR_(2)M_((s))+2Na_((aq))^(+)`
`to` The resin can be regenerated by using aq. NaCl solution.
`to` In this step `H^(+)` ions exchanges `Na^(+),Ca^(+2),Mg^(+2)` and proton formation takes place.
`2RH_((s))+M_((aq))^(+2)hArrMR_(2(s))+2H_((aq))^(+)`
Step - II : Anion exchange process :
`to` In this process resins used are `RNH_(3)OH` i.e., a basic compound.
`to` The `OH^(-)` ions of resin exchanges the anions `Cl^(-),SO_(4)^(-2),HCO_(3)^(-)` etc, and `OH^(Ө)` ion formation takes place.
`RNH_(2(s))+H_(2)O_((l))hArrRNH_(3)^(+)*OH_((s))^(-)`
`RNH_(3)^(o+)OH_((s))^(Ө)+x_((aq))^(-)hArrRNH_(3)^(+)x_((s))^(-)+OH_((aq))^(-)`
`to` The `H^(+)` ions and `OH^(-)` ions obtained in the two steps are get neutralised to form de-ionised water.
`H_((aq))^(+)+OH_((aq))^(-)toH_(2)O_((l))`
`to` The cation exchange resins and anion exchange resins are regenerated by treatment with dil acid and base respectively.