Problem 23 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Among the electrolytes \(\mathrm{Na}_{2} \mathrm{SO}_{4}, \mathrm{CaCl}_{2}, \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) and \(\mathrm{NH}_{4} \mathrm{Cl}\), the most effective coagulating agent for \(\mathrm{Sb}_{2} \mathrm{~S}_{3}\) sol is(a) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{CaCl}_{2}\) (c) \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) (d) \(\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

The most effective coagulating agent is (c) \(Al_2(SO_4)_3\).

Step 1: Understanding Coagulation

Coagulation is the process of aggregating particles from a colloidal solution, leading to the formation of a precipitate. The effectiveness of a coagulating agent depends on its charge according to Hardy-Schulze rule.

Step 2: Applying Hardy-Schulze Rule

Hardy-Schulze rule states that the greater the charge on the ions of the coagulating agent, the more effective it will be at causing coagulation. The ions with opposite charge to the colloidal particles are particularly effective.

Step 3: Analyzing the Charge of Electrolytes

The colloidal particles of \(Sb_2 S_3\)sol have a negative charge. According to Hardy-Schulze rule, positively charged ions will neutralize them. Analyzing the given electrolytes:- \(Na_2 SO_4\) provides \(Na^{+}\) ions.- \(CaCl_2\) provides \(Ca^{2+}\) ions.- \(Al_2(SO_4)_3\) provides \(Al^{3+}\) ions.- \(NH_4Cl\) provides \(NH_4^{+}\) ions.

Step 4: Identifying the Most Effective Coagulant

Since \(Al_2(SO_4)_3\) provides \(Al^{3+}\) ions, which has the highest positive charge (+3), it will be the most effective coagulating agent for the negatively charged \(Sb_2 S_3\) sol, according to Hardy-Schulze rule.

Key Concepts

Electrolytes

Electrolytes are substances that, when dissolved in water, produce ions and can conduct electricity. This ability to generate ions is what allows electrolytes to play a crucial role in coagulation. In the context of colloidal solutions, electrolytes help neutralize the charge on colloidal particles, which can cause the particles to aggregate, a process known as coagulation.

Different electrolytes produce ions of varying charges, which affects their ability to coagulate colloidal particles. For example, in the given exercise:

• Sodium sulfate (\(\mathrm{Na}_{2} \mathrm{SO}_{4}\)) produces \(\mathrm{Na}^{+}\) ions.
• Calcium chloride (\(\mathrm{CaCl}_{2}\)) offers \(\mathrm{Ca}^{2+}\) ions.
• Aluminum sulfate (\(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\)) gives \(\mathrm{Al}^{3+}\) ions.
• Ammonium chloride (\(\mathrm{NH}_{4} \mathrm{Cl}\)) provides \(\mathrm{NH}_{4}^{+}\) ions.

Understanding which ions are produced by an electrolyte is key to determining its effectiveness as a coagulating agent.

Hardy-Schulze Rule

The Hardy-Schulze rule is a fundamental principle in colloid chemistry that describes the effectiveness of ions in causing the coagulation of colloidal particles. According to this rule, the coagulating power of an ion is directly related to the magnitude of its charge.

In essence, the greater the positive charge on an ion, the more effective it will be at neutralizing the charge of negatively charged colloidal particles, leading to coagulation. This is because the charged ions work to neutralize the colloidal particles’ surface charges.

• Ions with higher positive charges are more efficient in causing coagulation.
• Opposite charges attract, which is why positively charged ions are needed to coagulate negatively charged colloidal particles.

Thus, when analyzing which electrolyte will be the most effective in coagulating a colloidal solution, it’s crucial to consider the charge on the ions they produce.

Charged Ions

Charged ions play a pivotal role in the process of coagulation. When an electrolyte dissolves in water, it dissociates into ions with either positive or negative charges. These ions interact with colloidal particles in the solution.

Negatively charged colloidal particles require positively charged ions for coagulation. This is because the positive ions will neutralize the negative charges on the colloidal particles' surface through electrostatic attraction, allowing them to aggregate. Here’s a breakdown of the charged ions from the exercise:

• \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) provides \(\mathrm{Na}^{+}\) ions.
• \(\mathrm{CaCl}_{2}\) results in \(\mathrm{Ca}^{2+}\) ions.
• \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) gives \(\mathrm{Al}^{3+}\) ions, the highest positive charge.
• \(\mathrm{NH}_{4} \mathrm{Cl}\) results in \(\mathrm{NH}_{4}^{+}\) ions.

Among these, the \(\mathrm{Al}^{3+}\) ions have the highest charge and therefore, are the most effective for coagulating negatively charged colloids.

Colloidal Particles

Colloidal particles are small, dispersed particles within a colloid that are suspended throughout another substance. These particles are typically charged, which keeps them stable and prevents them from readily aggregating due to repulsion between particles with like charges.

However, to form a precipitate (a visible solid), these colloidal particles need to be neutralized, which is where coagulating agents come in. Coagulation involves reducing the repulsive forces between colloidal particles by neutralizing their charges with ions of opposite charges.

• The effectiveness of coagulation depends largely on the charge of the ions used.
• Negatively charged colloidal particles, such as \(\mathrm{Sb}_{2} \mathrm{~S}_{3}\) sol, are prevalent in many solutions and require positively charged ions for coagulation.
• The higher the positive charge of the ions introduced (from the coagulating agent), the more effective the coagulation process.

Understanding how colloidal particles interact with charged ions helps in the effective use of electrolytes and supports the application of the Hardy-Schulze rule in practical scenarios.