Problem 19 - Chemistry in 30 days

Question

When a solution of potassium ferricyanide is added to an aqueous solution of ferrous lulphate, a deep blue colour, known as Turnbull's blue, is produced. The formula of the compound responsible for this deep blue colour is (a) \(\mathrm{KFe}^{\mathrm{mI}}\left[\mathrm{Fe}^{\mathrm{Il}}(\mathrm{CN})_{6}\right]\) (b) \(\mathrm{Fe}_{4}^{\mathrm{m}}\left[\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_{6} \mathrm{l}_{3}\right.\) (c) \(\mathrm{KFe}^{\mathrm{II}}\left[\mathrm{Fe}^{\mathrm{IH}}(\mathrm{CN})_{6}\right]\) (d) \(\mathrm{Fe}_{3}{\underline{\phantom{xx}}}^{\mathrm{II}}\left(\mathrm{Fe}^{\mathrm{II}}(\mathrm{CN})_{1} \mathrm{l}_{3}\right.\)

Step-by-Step Solution

Answer

The formula of Turnbull's blue is (b) \(\mathrm{Fe}_{4}[\mathrm{Fe}^{ ext{II}}(\mathrm{CN})_6]_3\).

Step 1: Understand the Reaction

When potassium ferricyanide (\(\text{K}_3[\text{Fe}(\text{CN})_6]\)) is added to ferrous sulfate (\( \text{FeSO}_4 \)), a double displacement reaction occurs to form Turnbull's blue. This happens as the ferrous ions \((\text{Fe}^{2+})\) react with ferricyanide ions\([\text{Fe}^{3+}(\text{CN})_6]^{3-}\), leading to the formation of Turnbull's blue.

Step 2: Identify Product of the Reaction

The product of this reaction is Turnbull's blue, a complex compound formed due to the formation of a precipitate. Given the double displacement reaction, Turnbull's blue results in the compound \([\text{Fe}_3^{2+}[\text{Fe}^{3+}(\text{CN})_6]_3]\). This involves the formation of a lattice structure that gives the deep blue color.

Step 3: Match with Options

Now, compare the formed compound \([\text{Fe}_3^{2+}[\text{Fe}^{3+}(\text{CN})_6]_3]\) with the given options. It can be seen to match with option (b), \(\mathrm{Fe}_{4}^{ ext{III}}[\mathrm{Fe}^{ ext{II}}(\mathrm{CN})_6]_3\), which represents Turnbull's blue closely in composition.

Key Concepts

Potassium Ferricyanide

Potassium ferricyanide is a red-colored compound with the chemical formula \( \text{K}_3[\text{Fe}(\text{CN})_6] \). It is composed of potassium ions and the ferricyanide complex anion. This compound is part of a group known as transition metal complexes, where the iron ion is in the 3+ oxidation state.

• The potassium ions are positively charged single atoms, each carrying a +1 charge.
• The ferricyanide part consists of a central iron ion bonded to six cyanide ions, forming a stable octahedral structure.

Potassium ferricyanide is widely used in the chemical industry for its ability to form colored compounds. It plays a critical role in the double displacement reaction that leads to the creation of Turnbull's blue.

Ferrous Sulfate

Ferrous sulfate is an important iron compound with the chemical formula \( \text{FeSO}_4 \). It consists of iron in the +2 oxidation state, known as the ferrous ion, and the sulfate ion.

• Ferrous ions are key in various chemical reactions due to their reactivity and ability to easily donate electrons.
• The sulfate ion is a polyatomic ion composed of sulfur and oxygen atoms with a -2 overall charge.

In the reaction with potassium ferricyanide, ferrous sulfate provides \( \text{Fe}^{2+} \) ions, which are essential for forming Turnbull's blue. This reaction highlights the versatility of ferrous sulfate as a reactant in color change and precipitation reactions.

Turnbull's Blue

Turnbull's blue is a distinctive deep blue compound produced through the interaction between potassium ferricyanide and ferrous sulfate. Chemically, it is represented as \( [\text{Fe}_3^{2+}[\text{Fe}^{3+}(\text{CN})_6]_3] \).This involves complexing of ferrous and ferricyanide ions.

• The color originates from the formation of a three-dimensional lattice structure where ferrous ions are coordinated with ferricyanide ions.
• Originally thought to differ from Prussian blue, experiments and studies reveal they are practically identical but produce varied colors based on the oxidation states involved.

Due to its vibrant color and historical significance, Turnbull's blue finds application in the fields of paints, inks, and similarly the detection of iron ions in solutions.

Chemical Precipitate

In chemistry, a precipitate is a solid that forms out of a solution during a chemical reaction. In the context of the reaction between potassium ferricyanide and ferrous sulfate, a precipitate known as Turnbull's blue is formed.

• A precipitate typically forms when the product of the reaction is insoluble in the solvent used.
• This is seen in the double displacement reaction, where ions from both reactants swap partners and create an insoluble compound.

The formation of a precipitate can be influenced by various factors like temperature, concentration of the reactants, and the nature of the ions involved. Recognizing when a precipitate will form is key to predicting and controlling outcomes in many chemical processes.