Problem 115 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following sequence of reactions. Identify (C) here? \(\mathrm{FeCO}_{3} \stackrel{\text { Heat in air }}{\longrightarrow}(\mathrm{A}) \stackrel{\text { Boil with } \mathrm{HCl}}{\longrightarrow}(\mathrm{B}) \stackrel{\mathrm{K}_{\mathrm{Fe}}(\mathrm{CN})_{6}}{\longrightarrow}(\mathrm{C})\) (a) \(\mathrm{K}_{3} \mathrm{Fe}(\mathrm{CN})_{6}\) (b) \(\mathrm{Fe}(\mathrm{CNS})_{3}\) (c) \(\mathrm{Fe}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]_{3}\) (d) Both (b) and (c)

Step-by-Step Solution

Answer

The compound (C) is \(\mathrm{Fe}_{4}[\mathrm{Fe}(\mathrm{CN})_{6}]_{3}\), option (c).

Step 1: Identify Compound A

The first reaction involves heating iron(II) carbonate (\(\mathrm{FeCO}_{3}\)) in air. Heating in air typically results in the decomposition of metal carbonates into metal oxides. Therefore, \(\mathrm{FeCO}_{3}\) when heated in air forms iron(II) oxide (\(\mathrm{FeO}\)) and carbon dioxide. Thus, compound (A) is \(\mathrm{FeO}\).

Step 2: Identify Compound B

Next, compound (A), which is \(\mathrm{FeO}\), is boiled with hydrochloric acid (HCl). Metal oxides react with acids to form salt and water. Thus, \(\mathrm{FeO}\) reacts with \(\mathrm{HCl}\) to form iron(II) chloride \(\mathrm{FeCl}_2\) and water. Therefore, compound (B) is \(\mathrm{FeCl}_2\).

Step 3: Identify Compound C

The last step involves treating compound (B), \(\mathrm{FeCl}_2\), with potassium ferricyanide \(\mathrm{K}_3\mathrm{Fe}(\mathrm{CN})_6\). Iron(II) ions react with ferricyanide anions to form a complex salt known as Prussian blue, which is \(\mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3\). Thus, compound (C) is \(\mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3\).

Key Concepts

Metal Carbonate Decomposition

Metal carbonate decomposition is a key chemical reaction that involves the breakdown of metal carbonates into metal oxides and carbon dioxide when heated. This process often requires heat because the energy input helps to break the bonds in the carbonate compounds.
When iron(II) carbonate, \( \mathrm{FeCO}_{3} \), is heated in air, it undergoes decomposition to form iron(II) oxide, \( \mathrm{FeO} \), and releases carbon dioxide gas, \( \mathrm{CO}_2 \). This can be represented as:

• \( \mathrm{FeCO}_{3} \rightarrow \mathrm{FeO} + \mathrm{CO}_{2} \)

The release of carbon dioxide is what makes the reaction interesting and significant in various chemical processes. Understanding this reaction is crucial in industrial applications such as extracting metals from their ores.
Additionally, recognizing metal carbonate decomposition reactions is essential for predicting how different compounds behave under certain conditions, like heating.

Complex Ion Formation

Complex ion formation is a fascinating area in chemistry where metal ions bond with other molecules or ions to form complex compounds. These complexes consist of a central metal ion surrounded by organic or inorganic ligands. This process is crucial in the study of coordination chemistry.
In our example, the compound \( \mathrm{FeCl}_2 \), which is the intermediate product, reacts with potassium ferricyanide \( \mathrm{K}_3\mathrm{Fe}(\mathrm{CN})_6 \). This reaction leads to the formation of a complex ion known as Prussian blue, \( \mathrm{Fe}_4[\mathrm{Fe}(\mathrm{CN})_6]_3 \). Here:

• Iron(II) ions in \( \mathrm{FeCl}_2 \) interact with ferricyanide anions to form an intricate network of atoms.
• This forms the iconic blue complex, commonly utilized in pigments and analytical chemistry due to its distinct color.

Complex ion formation is vital for many industrial and environmental applications and provides significant insights into the chemistry of transition metals.

Salt Formation Reaction

Salt formation is a chemical process where acids react with bases, resulting in salt and often water. A classic example is the reaction between metal oxides or hydroxides and acids.
In the given sequence, the iron(II) oxide \( \mathrm{FeO} \) from the decomposition reaction then reacts with hydrochloric acid \( \mathrm{HCl} \) to form iron(II) chloride \( \mathrm{FeCl}_2 \) and water. This can be illustrated as follows:

• \( \mathrm{FeO} + 2 \mathrm{HCl} \rightarrow \mathrm{FeCl}_2 + \mathrm{H}_2\mathrm{O} \)

This type of reaction is fundamental to many fields, such as metallurgy, where it is part of processes used to refine metals.
The ability to predict the products of salt formation reactions is key to controlling chemical processes, developing new compounds, and understanding the behavior of reactants under different conditions. Salt formation reactions like this are basic yet crucial for students to master in chemistry.