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Theoretical-Physical Advanced Level Chemistry - Equilibria - Chemical Equilibrium Revision Notes PART 4.3

4.3 Ion Exchange systems and cationic/anionic ion exchange resin theory

What is an ion exchange resin? How do cationic and anionic ion exchange materials work? What can we use ion exchange resins for? The theory of ion exchange and how it is used in various applications is described and explained.

(c) doc b GCSE/IGCSE Notes on reversible reactions and chemical equilibrium

Part 4 sub-index 4.1 Partition between two phases * 4.2 Solubility product Ksp & common ion effect * 4.3 Ion-exchange systems

Advanced Equilibrium Chemistry Notes Part 1. Equilibrium, Le Chatelier's Principle-rules * Part 2. Kc and Kp equilibrium expressions and calculations * Part 3. Equilibria and industrial processes * Part 4 sub-index (this section): 4.1 Partition between two phases * 4.2 Solubility product Ksp and common ion effect * 4.3 Ion-exchange systems * Part 5. pH, weak-strong acid-base theory and calculations * Part 6. Salt hydrolysis, Acid-base titrations-indicators, pH curves and buffers * Part 7. Redox equilibria, half-cell electrode potentials, electrolysis and electrochemical series * Part 8. Phase equilibria-vapour pressure, boiling point and intermolecular forces * M = old fashioned shorthand for mol dm-3

4.3 Ion Exchange systems and cationic/anionic ion exchange resin theory

cation and anion exchange systems

  • Ion-exchange materials have the capacity to hold ions in a dynamic equilibrium with the same ions present in an aqueous solution.

    • They may be synthetic polymer resins with immobile negative groups e.g. based on the sulphonic acid group R-SO2O-H+(s) acting as a cation exchanger. R represents the molecular backbone of the polymer resin. (immobile, exchangeable)

    • A resin with immobile positive groups like R-N(CH3)3+Cl-(s) can act as an anion exchanger.

    • Cation exchangers occur naturally in the sheet structures of clay minerals in soil which have excess immobile negative groups based on oxygen (e.g. clay-O-) which hold cations like H+ or Ca2+.

  • How strongly are ions held on the resin?

    • For singly charged ions the binding order from strongest to weakest bound is:

      • Cs+ > Rb+ > K+ > NH4+ > Na+ > Li+

    • For doubly charged ions the binding order from strongest to weakest bound is:

      • Ba2+ > Sr2+ > Ca2+ > Cu2+ > Zn2+ > Mg2+

    • For change in cation charge: Not surprisingly, the general binding order from strongest to weakest is M3+ > M2+ > M+ as the increasing charge density of the hydrated ion increases, so will the attraction of the ion to the immobile negative groups on the resin.

    • Effect of cationic radius and extent of hydration for constant charge:

      • If you consider the trends for Group 1 cations (M+) or Group 2 cations (M2+) things don't seem to add up? The group trend is for increasing radius down the group. This produces a decreasing charge density trend which should result in weaker binding to the negatively charged resin. However, the radius of the isolated ion does not count here, but what does matter is the effective radius of the hydrated cation. The smaller the ion, with its greater charge density, the greater its attraction for water molecules and the larger the resulting hydrated ion. Therefore the effective hydrated ionic radius actually decreases down the group, and the effective surface charge density increases to give the binding strength order.

  • TOP and LINKSIon exchange case studies

  • Case study 4.3.1 Ion-exchange processes are extremely important in soil chemistry.

    • Clay minerals are based on sheets of linked silicate units.

    • Here the simple tetrahedral silicate(IV) ion is SiO44- is linked together via -O-Si-O- bonds in two dimensions, the resulting silicate sheets have the general formula (Si2O52-)n where n is very large number.

    • The excess negative charge is balanced by various cations e.g. H+, K+, Mg2+, Ca2+, Al3+ which are adsorbed on or can fit in between the silicate sheets. The 'equation' below shows how potassium ions might be exchanged with magnesium ions

      • 2[clay-O]-K+(s) + Mg2+(aq) (c) doc b [clay-O]-Mg2+-[O-clay](s) + 2K+(aq)

    • One of the many unfortunate consequences of acid rain from fossil fuel burning, is the extra hydrogen ions will displace or wash out poisonous aluminium ions from clay soils which are harmful to plants and animals.

      • [(clay-O)3]3-Al3+(s) + 3H+(aq) (c) doc b 3[clay-O]-H+(s) + Al3+(aq)

    • Lime is added to soil to reduce its acidity. The lime (calcium oxide) forms hydroxide ions which will neutralise hydrogen ions held on the clay, so increasing the pH. The hydrogen ions on the clay are replaced by calcium ions, Ca2+.

      • The overall neutralisation and ion exchange can be summarised as ...

      • 2[clay-O]-H+(s) + Ca2+(aq) + 2OH-(aq) (c) doc b [clay-O]-Ca2+-[O-clay](s) + 2H2O(l)

    • Since clay minerals act as cation exchangers, anions like chloride and nitrate are readily washed out in rainwater, the latter ion from artificial ammonium nitrate fertilisers can cause pollution problems like eutrophication, though the ammonium cation is more likely to be retained being a positive ion.

    • Radioactive cations can be retained for quite some time in soil and only slowly displaced and dispersed to non-harmful levels. Even now (2006) in Northern England (Cumbria) sheep from a few farms cannot be sold for meat because of radioactive caesium-137, strontium-90 and iodine-? deposited on the soil they graze on. The radioactive contamination came from rain containing radio-isotopes a few days after the Russian Chernobyl nuclear reactor explosion in 1986. Caesium is more strongly bound than most other singly charged cations and some M2+ cations too? All the caesium will eventually end up in the Irish Sea and very diluted and harmless to aquatic life, but it takes time. The equation below shows the adsorption of the caesium ions onto an alumino-silicate sheets in clay by displacing less strongly held potassium ions,

      • [clay-O]-K+(s) + Cs+(aq) (c) doc b [clay-O]-Cs+(s) + K+(aq)

      • and the strontium ion Sr2+ is also strongly binding and will also displace other ions to remain in the soil for some time (see Mg2+...K+ exchange, 1st equation in section 4.3 above). However the radioactive iodine is likely to end up as the iodide ion. I-, so, being an anion, is more readily washed out of the soil by rainwater and not retained by the negatively charged alumino-silicate sheets.

  • Case study 4.3.2 Removing hardness from water:
    • Packs of ion exchange resins can hold or release ions in an ion exchange process.
    • Negative polymer resin columns hold hydrogen ions or sodium ions. These can be replaced by calcium and magnesium ions when hard water passes down the column. The more highly charged calcium or magnesium ions are more strongly held on the negatively charged resin. The freed or displaced hydrogen or sodium ions do not form a scum with soap (see GCSE/IGCSE notes on hard and soft water).
    • e.g. 2[resin]-H+(s) + Ca2+(aq) (c) doc b [resin]-Ca2+-[resin](s) + 2H+(aq)
    •  or 2[resin]-Na+(s) + Mg2+(aq) (c) doc b [resin]-Mg2+-[resin](s) + 2Na+(aq) etc.
  • Case study 4.3.3 Water purification:
    • You can also use an ion-exchange resin to replace negative ions by using a positively charged resin initially holding hydroxide ions (OH-) e.g. to remove chloride (Cl-), nitrate (NO3- and potentially harmful) and sulphate ions (SO42-) etc.
      • [resin]+OH-(s) + Cl-(aq) (c) doc b [resin]+Cl-(s) + OH-(aq)
      • [resin]+OH-(s) + NO3-(aq) (c) doc b [resin]+NO3-(s) + OH-(aq)
      • 2[resin]+OH-(s) + SO42-(aq) (c) doc b [resin]+SO42-[resin]+(s) + 2OH-(aq) etc.
    • Now, by using both a positive ion-exchange resin (here) and a negatively charged ion-exchange resin (see 4.3.2), you can completely de-ionise water because the released hydrogen ions and hydroxide ions combine to form very pure water.
      • H+(aq) + OH-(aq) ==> H2O(l) 
      • The ionic equation for neutralisation.
    • However, it will NOT remove non-ionic substances like organic pesticides etc.

TOP and LINKS

Part 4 sub-index 4.1 Partition between two phases * 4.2 Solubility product Ksp & common ion effect * 4.3 Ion-exchange systems

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