Question

A colloidal solution is subjected to an electrical field. The particles move towards anode. The coagulation of same sol is studied using \(\mathrm{NaCl}\), \(\mathrm{BaCl}_{2}\) and \(\mathrm{AICl}_{3}\) solutions. Their coagulating power should be : (1) \(\mathrm{NaCl}>\mathrm{BaCl}_{2}>\mathrm{AlCl}_{3}\) (2) \(\mathrm{BaCl}_{2}>\mathrm{AICl}_{3}>\mathrm{NaCl}\) (3) \(\mathrm{AlCl}_{3}>\mathrm{BaCl}_{2}>\mathrm{NaCl}\) (4) \(\mathrm{BaCl}_{2}>\mathrm{NaCl}>\mathrm{AlCl}_{3}\)

Short answer

(3) \(\text{AlCl}_3 > \text{BaCl}_2 > \text{NaCl}\)

Step-by-step solution

- Identify the nature of particles

The particles in the colloidal solution are moving towards the anode in an electrical field, which means they are negatively charged.

- Select the appropriate rule

According to Hardy-Schulze rule, the coagulating power of an ion increases with the valence (charge) of the ion. Higher the valence of the counter-ion, stronger is its coagulating power.

- Determine valences of ions

Identify the valences of the positive ions (cations) in \(\text{NaCl}\), \(\text{BaCl}_2\), and \(\text{AlCl}_3\): \(\text{Na}^+\) has a valence of +1, \(\text{Ba}^{2+}\) has a valence of +2, and \(\text{Al}^{3+}\) has a valence of +3.

- Rank the coagulating powers

Based on the Hardy-Schulze rule and the valences, rank the coagulating powers: \(\text{AlCl}_3 \) with the highest valence \(+\text{3}\), followed by \(\text{BaCl}_2\) with valence \(+\text{2}\), and \(\text{NaCl}\) with valence \(+\text{1}\). Therefore, \( \text{AlCl}_3 > \text{BaCl}_2 > \text{NaCl} \).

Key concepts

Hardy-Schulze rule

The Hardy-Schulze rule is fundamental when understanding colloidal solutions and their coagulation behavior. This rule states that the greater the valence (charge) of the ions causing coagulation, the more effective they will be in precipitating the colloid.
For example, ions with higher charges will neutralize the charges on the colloidal particles more effectively, leading to coagulation. This is because they have stronger electrostatic attraction forces.
To put this into context, consider a colloidal solution with negatively charged particles, as identified in our exercise. When adding different solutions such as \(\text{NaCl}\), \(\text{BaCl}_2\), and \(\text{AlCl}_3\), the cations (\(\text{Na}^+\), \(Ba^{2+}\), and \(Al^{3+}\)) will neutralize the colloid particles to varying degrees. Following the Hardy-Schulze rule, \(Al^{3+}\) with a valence of +3 will have the highest coagulating power, followed by \(Ba^{2+}\) and then \(Na^+\).

Coagulation power

Coagulation power refers to the ability of an electrolyte to destabilize and aggregate particles in a colloidal solution. The coagulating power is directly influenced by the charge of the ions in the electrolyte.
According to our exercise, we are looking at \(\text{NaCl}\), \(\text{BaCl}_2\), and \(\text{AlCl}_3\). Each contains cations of different valences: \(Na^+ (valence +1)\), \(Ba^{2+} (valence +2)\), and \(Al^{3+} (valence +3)\).
The higher the valence, the more powerful the ion in causing coagulation. This is because such ions can neutralize the charge on the colloidal particles more effectively. Thus, \(Al^{3+}\) has the highest coagulating power, followed by \(Ba^{2+}\), and then \(Na^+\). In essence, the higher the charge of the ion, the stronger its coagulating power.

Electrical field in colloids

When a colloidal solution is subjected to an electrical field, the charged particles will move towards the electrode of opposite charge. In our exercise, the particles are moving towards the anode. This indicates that the colloidal particles are negatively charged.
Movement of particles in an electrical field helps in studying their charge properties and behavior. It’s a useful method for identifying the type of charge on colloidal particles.
As the charged particles migrate, they can interact with oppositely charged ions, which plays a role in the coagulation process. Understanding this behavior is crucial for applications that require controlled coagulation, such as in water purification.

Valence of ions

The valence of an ion, which is its charge, is a key concept in understanding its interaction with colloids. In the context of coagulation, ions with higher valences have a stronger capability to neutralize the charges on colloidal particles.
For example, in the given exercise, \(Na^+\) has a valence of +1, \(Ba^{2+}\) has a valence of +2, and \(Al^{3+}\) has a valence of +3. The effectiveness of these ions in causing coagulation increases with the valence.
Higher valence means stronger electrostatic force to attract and neutralize the charges on the colloidal particles, leading to faster and more effective coagulation. This principle is in direct accordance with the Hardy-Schulze rule.