The coagulating power of electrolytes having ions Na+, Al3+ and Ba2+ for arsenic sulphide sol increa

Q: How do I memorize which sols are positive and which are negative?

Generally, metal sulphides (like $As_2S_3$, $CdS$) and metal sols (like $Au$, $Ag$) are negative. On the other hand, metal hydroxides (like $Fe(OH)_3$, $Al(OH)_3$) and basic dyes are positive.

Q: Why does a higher charge mean higher coagulating power?

A higher charged ion can neutralize more colloidal particles per ion. This makes the coagulation process much more efficient, requiring a significantly lower concentration of the electrolyte to achieve precipitation.

Q: Does the size of the ion matter in coagulation?

While the size of the hydrated ion plays a minor role, the valency or charge of the ion is the overwhelmingly dominant factor according to the Hardy-Schulze rule.

Q: What is the difference between coagulating power and coagulation value?

They are inversely related. Coagulation value is the minimum concentration (usually in millimoles per liter) required to cause coagulation. A higher coagulating power means a lower coagulation value is needed.

Q: Would $NaCl$ or $Na_2SO_4$ be better for coagulating this sol?

Since $As_2S_3$ is a negative sol, the active ion is the cation ($Na^+$). Both electrolytes provide $Na^+$, so their primary coagulating power would be very similar, though the anion might have a very slight, negligible secondary effect.

Question

The coagulating power of electrolytes having ions $	ext{Na}^+$, $	ext{Al}^{3+}$ and $	ext{Ba}^{2+}$ for arsenic sulphide sol increases in the order

Options:

(A) $	ext{Al}^{3+} < 	ext{Ba}^{2+} < 	ext{Na}^+$

(B) $	ext{Na}^+ < 	ext{Ba}^{2+} < 	ext{Al}^{3+}$

(C) $	ext{Ba}^{2+} < 	ext{Na}^+ < 	ext{Al}^{3+}$

(D) $	ext{Al}^{3+} < 	ext{Na}^+ < 	ext{Ba}^{2+}$

Sigma Percentile · Accepted Answer

### The Nature of the Sol

To master the concept of coagulation, we must first understand the battlefield. Colloidal solutions, or sols, are incredibly stable because the dispersed particles carry an identical electrical charge. This mutual electrostatic repulsion prevents them from coming together and settling down.

In our specific problem, we are dealing with an **arsenic sulphide sol** ($As_2S_3$). A crucial piece of factual knowledge you must carry into the exam is that metal sulphides generally form **negatively charged sols**. Imagine a central colloidal particle of $As_2S_3$ surrounded by a protective, invisible shield of negative charges.

### The Weapon of Choice: Coagulation

To destroy this stability—a process called coagulation or precipitation—we need to neutralize that protective negative shield. We do this by introducing an electrolyte, which splits into positive and negative ions. For a negatively charged sol, the active agents are the positive ions (cations), also known as **flocculating ions**.

### The Master Equation: Hardy-Schulze Rule

This brings us to the elegant **Hardy-Schulze Rule**. It dictates a very simple but powerful principle: the coagulating power of an ion is directly proportional to its valency (or the magnitude of its charge).

$$	ext{Coagulating Power} \propto 	ext{Valency of Flocculating Ion}$$

Simply put, an ion with a higher charge is a much more potent weapon for neutralizing the sol than an ion with a lower charge. It can neutralize more colloidal particles per ion, making the process highly efficient.

### Final Calculation and Conclusion

Let's inspect the arsenal provided by the electrolytes in the question:
1. Sodium ion: $Na^+$ (Charge = $+1$)
2. Barium ion: $Ba^{2+}$ (Charge = $+2$)
3. Aluminum ion: $Al^{3+}$ (Charge = $+3$)

Applying the Hardy-Schulze rule, we simply arrange these ions in increasing order of their positive charge:

$$+1 < +2 < +3$$

Therefore, the increasing order of their coagulating power is:

$$Na^+ < Ba^{2+} < Al^{3+}$$

This perfectly matches option (b). Always remember to first identify the charge of the sol, and then look for the oppositely charged ion with the highest valency!

Animated Solution for Chemistry - Surface Chemistry: The coagulating power of electrolytes having ions $Na^{+}$ , $Al^{3 +}$ and $Ba^{2 +}$ for arsenic sulphide sol increases in the order

Select Answer:

Visualized Solution

$N a t u r e o f A r se ni c S u l p hi d e S o l$

$H a r d y - S c h u l z e R u l e$

$I d e n t i f y in g t h e F l occ u l a t in g I o n s$

$C o m p a r in g C o a g u l a t in g P o w er$

$F ina l C o n c l u s i o n$

$W ha t i f t h e so l w a s p os i t i v e ?$

The Sigma Insight: Colloids, Micelles and and Emulsions

The Nature of the Sol

The Weapon of Choice

The Master Equation

Final Calculation and Conclusion

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Animated Solution for Chemistry - Surface Chemistry: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The coagulating power of electrolytes having ions Na+, Al3+ and Ba2+ for arsenic sulphide sol increases in the order