Fig. 1 shows part of a water-treatment system installed beside a remote field laboratory. The well water contains dissolved calcium chloride. It passes thro...
Question 1 Report
Fig. 1 shows part of a water-treatment system installed beside a remote field laboratory. The well water contains dissolved calcium chloride. It passes through a vessel packed with an acidic ion-exchange resin before it enters a storage tank. Each resin group is represented by RH. As calcium ions are retained by the resin, hydrogen ions enter the water. The treated water is therefore checked before it is used for washing glassware or preparing sodium hydroxide solution.
(a) State the ion released into the water by the resin. [1]
(b) Explain why the water leaving the vessel is acidic. [2]
(c) Complete the equation for the ion-exchange reaction.
2RH + CaCl2 → ............ + ............ [3]
(d) Give the name of a sodium compound that could be added carefully to make the water less acidic. [2]
(e) Explain how passing concentrated hydrochloric acid through the used resin allows it to be used again. [3]
Answer Details
(a) The resin releases hydrogen ions, \(\mathrm{H^+}\), into the water. [1]
(b) The resin releases hydrogen ions into the water. A solution containing hydrogen ions is acidic, so its pH is lowered. [2]
(c) Each \(\mathrm{Ca^{2+}}\) ion replaces two hydrogen ions on the resin:
\[\mathrm{2RH + CaCl_2 \rightarrow R_2Ca + 2HCl}\]
This gives \(\mathrm{R_2Ca}\), \(2\mathrm{HCl}\), and balances all atoms. [3]
(d) Sodium hydroxide can be added carefully to make the water less acidic. Sodium carbonate or sodium hydrogencarbonate are also acceptable sodium compounds because they neutralise acid. [2]
(e) Concentrated hydrochloric acid provides hydrogen ions. These hydrogen ions replace the calcium ions held by the resin. The calcium ions leave as calcium chloride solution and are washed out, leaving the resin groups in the \(\mathrm{RH}\) form again. [3]
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