Problem 11 - Basic Principles of Organic Chemistry

Question

Consider a "hard" water comprised of dilute \(\mathrm{MgCl}_{2}\). Ion exchange with resin \(-\mathrm{SO}_{3}\) Na replaces \(\mathrm{Mg}^{2+}\) with \(\mathrm{Na}^{+}\), and with resin \(-\mathrm{SO}_{3} \mathrm{H}, \mathrm{Na}^{+}\) is replaced by \(\mathrm{H}^{+}\), thereby producing a dilute \(\mathrm{HCl}\) solution. What kind of an ion-exchange resin would you need to remove the \(\mathrm{Cl}^{-}\) from the HCl solution and produced "deionized" water? (Consider exchanging \(\mathrm{Cl}^{-}\) for \(-\mathrm{OH}\).)

Step-by-Step Solution

Answer

Use an anion exchange resin capable of exchanging Cl⁻ for OH⁻ ions.

Step 1: Understanding Ion Exchange

The process described involves replacing ions in the solution using resins. Initially, MgCl extsubscript{2} loses Mg extsuperscript{2+} ions which are replaced by Na extsuperscript{+} using a resin that contains -SO extsubscript{3}Na. The Na extsuperscript{+} ions are then replaced by H extsuperscript{+} ions using a resin that contains -SO extsubscript{3}H.

Step 2: Resulting Solution

After the first two ion-exchange processes, the resulting solution is a dilute HCl solution. This is because Mg extsuperscript{2+} was replaced by Na extsuperscript{+}, and subsequently, Na extsuperscript{+} was replaced by H extsuperscript{+}.

Step 3: Objective: Remove Cl⁻ Ions

To produce deionized water, we need to remove the Cl⁻ ions from the resulting HCl solution. The simplest way to achieve this is by using a resin that can exchange Cl⁻ for -OH.

Step 4: Choosing the Right Resin

The appropriate ion-exchange resin to remove Cl⁻ and exchange it for OH⁻ must be an anion exchange resin. This type of resin has functional groups such as -NR extsubscript{3} extsuperscript{+}, where they can exchange chloride ions (Cl⁻) with hydroxide ions (OH⁻). This will remove Cl⁻ from the solution, releasing OH⁻.

Key Concepts

Water Deionization

Water deionization is a purification process used to remove unwanted ions from water, resulting in highly pure water. In the scenario involving the removal of chloride ions from an HCl solution, water deionization plays a key role.

The process involves multiple steps where unwanted ions are exchanged for more desirable ions using ion exchange resins. Typically, deionization removes both cations and anions to produce water that is free from these ions.

• The first ion exchange replaces unwanted cations with hydrogen ions, \(\mathrm{H}^{+}\), making the water acidic.
• Next, unwanted anions such as \(\mathrm{Cl}^{-}\) are exchanged with hydroxide ions, \(\mathrm{OH}^{-}\), to neutralize the acidity.
• The resulting water is typically very pure and often referred to as "deionized water."

For this specific exercise, the goal is to effectively remove these ions to ensure the water's purity, making it suitable for various sensitive applications, such as in laboratories or for industrial use.

Anion Exchange Resins

Anion exchange resins are a critical component in the ion exchange process, specifically designed to exchange anions present in water with desirable counter-ions. These resins are usually made from a matrix of plastic beads that contain positive functional groups capable of attracting and holding negative ions, like chloride ions (\( \mathrm{Cl}^{-} \)).

In our exercise scenario, the anion exchange resin is used to remove \(\mathrm{Cl}^{-}\) ions from a solution and replace them with hydroxide ions, \(\mathrm{OH}^{-}\). This exchange is crucial because it helps neutralize the resulting solution, removing the chloride ions completely.

• Anion resins often have quarternary ammonium groups, like -\( \mathrm{NR}_{3}^{+} \), which effectively trap anions.
• The trapped anions are then replaced by hydroxide ions provided by the resin, thus achieving the purification goal.
• This process can also help remove other undesirable anions from water, making it versatile for various applications.

Choosing the right anion exchange resin ensures the successful completion of the water purification process.

Ion Exchange Process

The ion exchange process is a chemical reaction that involves the swapping of ions between an electrolyte solution and an ion exchange resin. This technique is widely used for water purification, including deionization, softening, and other applications where ion removal is desired.

In our specific example, the process starts with the removal of \(\mathrm{Mg}^{2+}\) ions, which are replaced by sodium ions, \(\mathrm{Na}^{+}\). Then, these sodium ions are exchanged for hydrogen ions, resulting in a hydrochloric acid (HCl) solution.

• The process is completed by an anion exchange, which replaces chloride ions with hydroxide ions, neutralizing the solution.
• Through careful selection of appropriate resins, various ions can be targeted and removed selectively.
• This process is highly efficient and can be tailored to meet very specific water quality requirements.

Understanding the ion exchange process is crucial for the effective removal of specific ions, thereby achieving the desired purity in water treatments.