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Question

Which of the following colloidal solution is prepared when very dilute solution of ferric chloride reacts with potassium ferrocyanide? (a) Prussian blue (b) Arsenic sulphide (c) Ferric hydroxide (d) Extrinsic colloid

Step-by-Step Solution

Answer

Prussian blue is formed when ferric chloride reacts with potassium ferrocyanide.

Step 1: Identify the Reaction Components

First, recognize the chemical components involved in the reaction. We have a very dilute solution of ferric chloride (\(FeCl_3\)) reacting with potassium ferrocyanide (\(K_4[Fe(CN)_6]\)).

Step 2: Understand the Reaction Possibility

Understanding typical products of the reaction between ferric chloride and potassium ferrocyanide. Ferric ions \(Fe^{3+}\) from ferric chloride can react with the ferrocyanide ions \([Fe(CN)_6]^{4-}\) from potassium ferrocyanide to form a compound.

Step 3: Determine the Precipitate Formed

When \(Fe^{3+}\) ions react with \([Fe(CN)_6]^{4-}\), it results in the formation of a blue precipitate known as "Prussian Blue," which is chemically represented as \(Fe_4[Fe(CN)_6]_3\).

Step 4: Conclude with the Correct Option

Based on the reaction between the ions, the correct colloidal solution formed is Prussian Blue. Thus, the answer is option (a) Prussian blue.

Key Concepts

Ferric Chloride

Ferric chloride, or iron(III) chloride, is a chemical compound with the formula \(FeCl_3\). It is deliquescent, meaning it can absorb moisture from the air and dissolve in it. In its anhydrous form, it appears as dark crystals, but it is commonly used in a solution form.

**Applications of Ferric Chloride**

• It's widely used as a flocculant in water and wastewater treatment processes.
• In a laboratory setting, it often acts as a Lewis acid catalyst in various chemical reactions.

When dissolved in water, it forms \[Fe^{3+}\]ions and \[Cl^{-}\] ions, which play a crucial role in its chemical reactivity. A distinctive feature of ferric chloride is its yellowish-brown color in solution, which is used as a qualitative test for the presence of phenols in organic compounds, as it forms intensely colored complexes.

Potassium Ferrocyanide

Potassium ferrocyanide, known chemically as \(K_4[Fe(CN)_6]\), is a compound made of potassium ions and ferrocyanide ions. It appears as a light yellow crystalline salt and is primarily used in industries.

**Uses of Potassium Ferrocyanide**

• It is commonly used as an anti-caking agent in table salt, known as additive E536.
• In laboratories, it serves as a reagent to detect metals.

Although it contains the cyanide group (CN), it is not inherently toxic due to the ferrocyanide's stable chemical nature. In chemical reactions, the \([Fe(CN)_6]^{4-}\) groups are of particular interest as they react with various metal ions to form complex compounds.

Prussian Blue

Prussian blue is a deep blue pigment, known chemically as \(Fe_4[Fe(CN)_6]_3\). It has historically been used in dyes, paints, and even as a coloring agent in printing and textiles. Its vibrant color is due to the unique electron transfer between iron ions within its structure.

**Applications**

• Beyond its aesthetic uses, Prussian blue is vital in medicine. It is used as a treatment for heavy metal poisoning, particularly thallium and radioactive cesium.
• Its ability to form a stable colloidal solution makes it applicable in various scientific studies and industrial processes.

Prussian blue exemplifies how a seemingly simple pigment can have diverse applications extending far beyond just an art material.

Chemical Reactions

Chemical reactions are transformations where reactants change into products. Observations, such as color changes, formation of precipitates, or gas evolution, often indicate a reaction. The reaction of ferric chloride with potassium ferrocyanide, for instance, exemplifies a classic precipitation reaction.

**Key Characteristics of Chemical Reactions**

• Reactants interact to form one or more new products.
• The initial and final materials often possess different chemical properties.
• They may either absorb or release energy, detected through temperature changes.

Understanding reactions is essential in predicting product formation and reaction conditions. In a laboratory context, reactions are often represented by balanced chemical equations. For this specific exercise, the formation of Prussian blue from ferric chloride and potassium ferrocyanide shows a clear transition from reactants to a product with a striking color change.