Question

Which of the following is least effective in causing flocculating of ferric hydroxide sol? (a) \(\mathrm{K}_{3} \mathrm{Fe}(\mathrm{CN})_{6}\) (b) \(\mathrm{K}_{2} \mathrm{CrO}_{4}\) (c) \(\mathrm{KBr}\) (d) \(\mathrm{K}_{2} \mathrm{SO}_{4}\)

Short answer

The compound \(KBr\) is the least effective in causing flocculating of ferric hydroxide sol.

Step-by-step solution

Understanding Flocculation

Flocculation is the process where particles, such as those present in a sol, come out of suspension to sediment under the influence of flocculating agents. The effectiveness of a flocculating agent depends on the charge and the size of ions it contains.

Assessing the Compounds

From the given options (a) \(K_{3}Fe(CN)_{6}\), (b) \(K_{2}CrO_{4}\), (c) \(KBr\), (d) \(K_{2}SO_{4}\), let's assess them individually: (a) \(K_{3}Fe(CN)_{6}\): This is a complex compound that releases \(Fe^{3+}\) ions which can causes decent flocculation of ferric hydroxide sol. (b) \(K_{2}CrO_{4}\): This compound forms \(CrO^{2-}_{4}\) ions which are large and negatively charged anions. They are efficient flocculating agents. (c) \(KBr\): This compound dissociates into single positive and negative ions (\(K^{+}\) and \(Br^{-}\)), which are not very effective in flocculation. (d) \(K_{2}SO_{4}\): This compound forms \(SO^{2-}_{4}\) ions, which, due to being large and charged negatively, are efficient in causing flocculation.

Concluding the Least Effective Flocculating Agent

Having studied each compound individually, it's evident that \(KBr\) is likely to be least effective as a flocculating agent because it forms single positive and negative ions, which are less effective in causing flocculation of ferric hydroxide sol.

Key concepts

Ferric Hydroxide Sol

Ferric hydroxide sol is a colloidal solution consisting of very small particles of iron(III) hydroxide, Fe(OH)₃, dispersed in water. When we talk about colloids, we're discussing mixtures where one substance is evenly dispersed within another and where these tiny particles do not settle out quickly. The particles in a ferric hydroxide sol carry a certain charge which helps to keep them apart because they repel each other, thus maintaining the 'stability' of the sol.

Understanding colloidal stability is essential in fields like water treatment and in industries where suspensions are common. To get these fine particles to clump together and settle down—so they can be removed from the mixture—we use a process called flocculation. This involves adding substances that will cause these particles to come out of suspension, essentially decreasing the colloidal stability.

Flocculating Agents

Flocculating agents, also known as flocculants, play a pivotal role in water purification, wastewater treatment, and in processes where we need to separate fine solid particles from a liquid. These agents encourage a process where colloidal particles come together to form larger particles in a phenomenon known as flocculation. But how do they do this?

Flocculants are chemicals that have charges opposite to that of the colloidal particles. So in the issue of ferric hydroxide sol, where particles carry a positive charge, an effective flocculant would have a negative charge. Ions that are larger and have higher charges are considered strong flocculating agents because they can effectively neutralize the charges of colloidal particles leading to aggregation. Keeping this in mind, simple ions like K⁺ and Br⁻ are often not powerful enough to act as flocculants, while more complex anions, such as CrO₄²⁻ or SO₄²⁻, can be very effective.

Colloidal Stability

The term 'colloidal stability' refers to the propensity of particles in a sol to remain uniformly dispersed and resist sedimentation. It largely depends on the balance between repulsive and attractive forces among particles. Factors determining this stability include the nature of the solute particles, their charge, the presence of electrolytes, pH of the solution, and temperature.

Revising our example of ferric hydroxide sol, the colloidal particles naturally repel each other due to their like charges, and thus, remain dispersed. When flocculating agents are introduced, they interact with these charges, reducing repulsive interactions and thus destabilizing the colloid. If an agent isn't effective—like in the case of KBr—it can't adequately neutralize the charge on the colloidal particles, and the system remains stable. Other factors, such as the ionic strength of the flocculant and the specific interactions it may have with the colloidal particles, are also critical in determining the efficacy of the flocculation process.