Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

What product is formed by mixing the solution of \(\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) with the solution of \(\mathrm{FeCl}_{2} ?\) (a) ferric ferrocyanide (b) ferri ferricyanide (c) ferro ferricyanide (d) none of these

Step-by-Step Solution

Answer

The product formed is (a) ferric ferrocyanide.

Step 1: Understanding the Reactants

We are given two reactants: potassium ferrocyanide ( K4[Fe(CN)6]) and iron (II) chloride ( FeCl2). These two substances are going to react together.

Step 2: Determining the Reaction Type

The reaction between K4[Fe(CN)6] and FeCl2 is a precipitation reaction. This means that a solid, called a precipitate, will form when these two are mixed.

Step 3: Balancing the Chemical Equation

The chemical reaction can be represented by the equation:\[3 \times K_4[Fe(CN)_6] + 4 \times FeCl_2 \rightarrow Fe_4[Fe(CN)_6]_3 \times 12H_2O + 8 \times KCl\]This equation shows that mixing the solutions will produce the solid product ferric ferrocyanide (Fe_4[Fe(CN)_6]_3) as the main product, along with potassium chloride (KCl).

Step 4: Identifying the Product

The main insoluble product formed by the reaction is ferric ferrocyanide (Fe_4[Fe(CN)_6]_3), which is also known as Prussian Blue. This is a complex iron compound formed when Fe2+ ions from FeCl2 react with [Fe(CN)6]4- ions from K4[Fe(CN)6].

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is maintained. This law states that matter cannot be created or destroyed, so the quantity of each element must be the same on both sides of a chemical equation. When dealing with a precipitation reaction, like the one between \(\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]\) and \(\mathrm{FeCl}_2\), balancing the equation is crucial to accurately depicting the transformation of reactants to products.

Begin by writing down the unbalanced equation with the reactants and products:

• Reactants: \(\mathrm{K}_4\left[\mathrm{Fe}(\mathrm{CN})_6\right]\) and \(\mathrm{FeCl}_2\)
• Products: \(\mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3\) and \(\mathrm{KCl}\)

Next, balance each type of atom on both sides of the equation:

• Ensure that the number of potassium \(\mathrm{K}\), iron \(\mathrm{Fe}\), cyanide \(\mathrm{CN}\), and chlorine \(\mathrm{Cl}\) atoms are equal.
• The equation \(3 \times \mathrm{K}_4[\mathrm{Fe}(\mathrm{CN})_6] + 4 \times \mathrm{FeCl}_2 \rightarrow \mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3 + 8 \times \mathrm{KCl}\) is balanced and reflects that 3 molecules of potassium ferrocyanide react with 4 formula units of iron(II) chloride, forming 1 molecule of ferric ferrocyanide and 8 formula units of potassium chloride.

Complex Iron Compounds

Complex iron compounds are fascinating coordination compounds formed between iron and groups of atoms, such as cyanide in our case. These compounds involve metal ions bonded to a set of ligands which can greatly affect their properties and reactions.

In the example of potassium ferrocyanide, written as \(\mathrm{K}_4[\mathrm{Fe}(\mathrm{CN})_6]\), the central iron atom is bonded to six cyanide (\(\mathrm{CN^-}\)) ions. This forms a stable, complex ion \([\mathrm{Fe}(\mathrm{CN})_6]^{4-}\). These types of compounds often display distinctive colors; many complex iron cyanides are known for their deep hues due to electronic transitions within the metal ions.

Understanding these compounds is important for recognizing how different coordination environments can influence chemical reactions and product formation in precipitation reactions. Therefore, by combining these solutions with other compounds like \(\mathrm{FeCl}_2\), the formation of a new complex, such as ferric ferrocyanide, becomes achievable, respecting the typical stoichiometry of such reactions.

Prussian Blue

Prussian Blue is a well-known compound, famous not just for its deep blue color, but also for its historical significance and diverse applications. Chemically identified as ferric ferrocyanide (\(\mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3\)), this compound forms through a reaction between potassium ferrocyanide and iron(II) ions.

Prussian Blue's formation is a classic example of a precipitation reaction, where the soluble reactants combine to form an insoluble solid—the vivid blue precipitate that presents itself as a seemingly magical transformation. Originally discovered in the early 18th century, Prussian Blue has been used as a pigment in paints and dyes, as well as in medical applications, such as treating certain cases of heavy metal poisoning.

The Prussian Blue reaction is more than just a visual spectacle; it showcases the complex chemistry of coordination compounds and their ability to form new structures and shades through chemical interactions, significantly enhancing our understanding of both inorganic chemistry and material science.