Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

When zeolite, which is hydrated sodium aluminium silicate, is treated with hard water the sodium ions are exchanged with 1\. \(\mathrm{H}^{+}\)ions 2\. \(\mathrm{Ca}^{2+}\) ions 3\. \(\mathrm{SO}_{4}^{-2}\) ions 4\. \(\mathrm{Mg}^{2+}\) ions (a) 2,4 (b) \(1,2,3\) (c) \(2,4,5\) (d) all are correct

Step-by-Step Solution

Answer

The sodium ions are exchanged with \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions. The correct answer is (a) 2,4.

Step 1: Understanding Zeolite Ion Exchange

Zeolite is a hydrated sodium aluminum silicate which is known for its ion exchange capabilities. In water softening processes, zeolite can exchange its sodium ions for positive ions in solution.

Step 2: Identifying Ions in Hard Water

Hard water contains high concentrations of calcium \(\mathrm{Ca}^{2+}\) and magnesium \(\mathrm{Mg}^{2+}\) ions. These cations are typically exchanged with zeolite in water softening.

Step 3: Determining Ion Exchange Products

When zeolite exchanges sodium ions, it typically does so with other cations, such as \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\), but not with anions like \(\mathrm{SO}_{4}^{-2}\). \(\mathrm{H}^{+}\) ions are also unlikely to be a major exchange ion in this context.

Step 4: Identifying the Correct Answer

The ions that are exchanged with sodium in a zeolite during water softening are \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions. Therefore, the correct answer includes both of these ions.

Key Concepts

Water Softening Process

Water softening is a crucial process to remove hardness from water which is primarily caused by calcium and magnesium ions. In this context, hardness refers to the presence of these minerals which can cause scaling in pipes or affect the soap's effectiveness.

To soften hard water, various methods can be employed, but ion exchange using zeolites is one of the most common and effective methods. This process helps in replacing the calcium and magnesium ions with sodium ions, which results in water that is softer and more compatible with soap and other detergents.

During the water softening process:

• Hard water containing significant amounts of calcium and magnesium ions is circulated through a tank containing zeolite beads.
• The sodium ions present in the zeolite are exchanged with the calcium and magnesium ions in the water.
• Once the zeolite is exhausted of sodium ions, it can be regenerated using a strong salt solution, typically sodium chloride (common salt).

This cycle effectively reduces the hardness of the water, preventing issues associated with hard water usage.

Calcium and Magnesium Ions

Calcium (Ca^{2+}) and magnesium (Mg^{2+}) ions are the primary culprits behind water hardness. These ions are typically introduced into water systems as it percolates through mineral-rich soil and rocks.

The presence of these ions presents a few challenges:

• React with soap to form a sticky scum, reducing cleaning efficiency.
• Cause scale buildup in appliances and boilers, reducing their lifespan and efficiency.
• Interfere with various industrial processes requiring soft water.

To manage these ions, water softening strategies focus on removing calcium and magnesium through different treatment methods, the most effective being ion exchange with zeolite.

Ion Exchange Chemistry

Ion exchange chemistry is a fascinating branch of chemistry that involves the reversible interchange of ions between a solid and a liquid in which there is no substantial change to the structure of the solid.

In the context of water softening with zeolite:

• Zeolite acts as a medium that facilitates the exchange of ions. It consists of a framework of aluminum, silicon, and oxygen with voids that house sodium ions.
• These sodium ions are easily exchangeable with other cations in the surrounding solution, such as calcium and magnesium ions from hard water.
• The exchanged ions are then trapped in the zeolite structure allowing sodium ions to be released into the water, transforming it into soft water.
• This exchange does not affect the structural integrity of the zeolite, allowing it to be regenerated and reused repeatedly.

The result is a chemical process that efficiently removes hardness from water, demonstrating how ionic interactions can be harnessed for practical applications.