Question

Show how the successive use of cation- and anionexchange resins could be used to remove sodium chloride from water.

Short answer

To remove sodium chloride from water using cation- and anion-exchange resins, pass the solution through a cation-exchange resin first. The resin attracts and holds sodium ions (Na+), releasing an equivalent amount of hydrogen ions (H+) in exchange. Reaction: \(Na^+_{(aq)} + Resin^-H^+ \rightarrow Resin^-Na^+ + H^+_{(aq)}\) Then, pass the solution through an anion-exchange resin, which attracts and holds chloride ions (Cl-) and releases an equivalent amount of hydroxide ions (OH-) in exchange. Reaction: \(Cl^-_{(aq)} + Resin^+OH^- \rightarrow Resin^+Cl^- + OH^-_{(aq)}\) Finally, the hydrogen ions (H+) and hydroxide ions (OH-) present in the solution combine to form water (H2O), effectively removing sodium chloride and leaving purified water. Reaction: \(H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}\)

Step-by-step solution

Understand Ion Exchange Resins

Ion exchange is a process where ions are exchanged between a solution and a solid (usually a resin). There are two types of ion exchange resins: cation-exchange resins which exchange positively charged ions (cations) and anion-exchange resins which exchange negatively charged ions (anions). In this case, we will be using both cation- and anion-exchange resins to remove sodium (Na+) and chloride (Cl-) ions, respectively, from a sodium chloride dissolved in water.

Cation-Exchange Resin

Cation exchange resins have negatively charged functional groups that can attract and hold positively charged ions. In our case, sodium ions (Na+) present in the sodium chloride solution will be attracted to the cation-exchange resin. The resin has a fixed negative charge and will release an equivalent amount of hydrogen ions (H+) in exchange for the sodium ions. This will result in the removal of sodium ions from the water and their replacement by hydrogen ions. Reaction: \(Na^+_{(aq)} + Resin^-H^+ \rightarrow Resin^-Na^+ + H^+_{(aq)}\)

Anion-Exchange Resin

Anion exchange resins have positively charged functional groups that can attract and hold negatively charged ions. In our case, chloride ions (Cl-) present in the sodium chloride solution will be attracted to the anion-exchange resin. The resin has a fixed positive charge and will release an equivalent amount of hydroxide ions (OH-) in exchange for the chloride ions. This will result in the removal of chloride ions from the water and their replacement by hydroxide ions. Reaction: \(Cl^-_{(aq)} + Resin^+OH^- \rightarrow Resin^+Cl^- + OH^-_{(aq)}\)

Formation of Water

After passing the solution through both the cation- and anion-exchange resins, the hydrogen ions (H+) and hydroxide ions (OH-) present in the solution will combine to form water (H2O), a neutral molecule. Reaction: \(H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}\)

Removal of Sodium Chloride from Water

By using successive cation- and anion-exchange resins, the sodium chloride dissolved in the water is effectively removed. The sodium ions are replaced by hydrogen ions, and the chloride ions are replaced by hydroxide ions. As a result, the hydrogen and hydroxide ions combine to form neutral water molecules, leaving us with purified water free from sodium chloride.

Key concepts

Cation-Exchange Resin

Cation-exchange resins are a type of resin utilized in ion exchange processes. They have negatively charged sites that attract positively charged ions, also known as cations. These resins are especially useful in water treatment for removing unwanted cations like sodium (Na+) from solutions. How do they work? When a sodium chloride solution passes through a cation-exchange resin, the resin's negative sites attract and hold onto the positively charged sodium ions. This is achieved by exchanging these sodium ions with hydrogen ions (H+) mutually attached to the resin.

As the sodium ions bind to the resin, the hydrogen ions are released into the solution, effectively removing sodium ions from the water. This conversion can be represented by the reaction: \[Na^+_{(aq)} + \text{Resin}^-\text{H}^+ \rightarrow \text{Resin}^-\text{Na}^+ + H^+_{(aq)}\]The outcome? Sodium ions are replaced, and hydrogen ions take their place in the liquid, setting the stage for the next step in purification.

Anion-Exchange Resin

Anion-exchange resins are the counterparts to cation-exchange resins and are equally important in the ion exchange process. These resins possess positively charged sites that effectively capture and hold negatively charged ions, known as anions. In the context of removing sodium chloride from water, anion-exchange resins do the heavy lifting of capturing chloride ions (Cl^-).

When the water containing dissolved chloride ions flows through an anion-exchange resin, the resin exchanges its hydroxide ions (OH^-) for chloride ions. This action purifies the water by removing chlorides and replacing them with hydroxides, as expressed in the following chemical equation:\[Cl^-_{(aq)} + \text{Resin}^+\text{OH}^- \rightarrow \text{Resin}^+\text{Cl}^- + OH^-_{(aq)}\]Thus, the chloride ions are sequestered by the resin, leaving the liquid with hydroxide ions in their stead. This crucial step contributes to the overall purification and prepares the water for transformation into its pure form.

Sodium Chloride Removal

The removal of sodium chloride from water via ion exchange is a seamless process using both cation- and anion-exchange resins. The systematic replacement of sodium and chloride ions with hydrogen and hydroxide ions respectively, effectively extracts sodium chloride from the solution. Initially, cation-exchange resins remove sodium ions (Na+) from the water. Subsequently, anion-exchange resins eliminate chloride ions (Cl^-).

This two-step ion exchange process results in sodium chloride removal, rendering the water free from these ions. Moreover, the exchange results in ions that are capable of forming water:\[NaCl_{(aq)} \rightarrow Na^+_{cation-exchange} \rightarrow H^+_{(aq)}\] \[Cl^-_{anion-exchange} \rightarrow OH^-_{(aq)}\] \[H^+_{(aq)} + OH^-_{(aq)} \rightarrow H_2O_{(l)}\]Through the careful combination of both ion-exchange resins, the entire removal process is efficient and results in thoroughly purified water.

Water Purification

Water purification using ion exchange involves transforming impurities into harmless products or removing them altogether. The integration of cation- and anion-exchange resins in water purification leverages their ability to substitute unwanted ions with less harmful ones.

In this process, the resins contribute hydrogen ions (H+) and hydroxide ions (OH^-) in place of sodium and chloride ions. When these ions unite, they form water, a neutral and clean compound. Importantly, this method of purification does not involve adding additional chemicals, leading to clean water without residue.

A hallmark of this process is its ability to target and remove specific undesired ions. This ensures customized adeptness in addressing varied water quality issues, making ion exchange an effective and adaptable means of achieving high-purity water. Once the sodium chloride is removed, the resultant water is significantly purified and safe for consumption or usage in various applications.