Animated Solution for Chemistry - Metallurgy: Among the reactions (A) - (D), the reaction(s) that does/do not occur in the blast furnace during the extraction of iron is/are (A) $\text{CaO}+{\text{SiO}}_2⟶{\text{CaSiO}}_3$ (B) $3{\text{Fe}}_2{\text{O}}_3+\text{CO}⟶2{\text{Fe}}_3{\text{O}}_4+{\text{CO}}_2$ (C) $\text{FeO}+{\text{SiO}}_2⟶{\text{FeSiO}}_3$ (D) $\text{FeO}⟶\text{Fe}+\frac{1}{2}{\text{O}}_2$

A Chemical Marvel

The extraction of iron from its ore is a fascinating journey that takes place inside a colossal structure known as the blast furnace. Imagine a towering inferno where raw materials—iron ore, coke, and limestone—are continuously fed into the top, while a blast of hot air is forced in from the bottom.

Inside this roaring reactor, a complex symphony of chemical reactions occurs at different temperature zones. Our task is to act as chemical detectives and evaluate four specific reactions to determine which ones truly belong in this process and which ones are imposters.

The Role of Flux

Let's examine the first reaction:

In the blast furnace, the iron ore often contains earthy impurities, primarily acidic silica (${\text{SiO}}_2$). If left unchecked, these impurities would interfere with the extraction. To combat this, we add limestone (${\text{CaCO}}_3$), which decomposes in the heat to form calcium oxide ($\text{CaO}$).

This calcium oxide acts as a basic flux. It eagerly reacts with the acidic silica to form calcium silicate (${\text{CaSiO}}_3$), commonly known as slag. Because slag is lighter than molten iron, it floats on top and can be easily drained away. Therefore, this reaction is a vital part of the iron extraction process.

The Step-Wise Reduction

Next, we look at reaction B:

As the hot carbon monoxide ($\text{CO}$) gas rises from the lower combustion zones, it encounters the descending iron ore, hematite (${\text{Fe}}_2{\text{O}}_3$). In the upper, relatively cooler regions of the furnace (around $500^{\circ }\text{C}$ to $800^{\circ }\text{C}$), the carbon monoxide begins the reduction process.

It doesn't strip away all the oxygen at once. Instead, it reduces hematite to magnetite (${\text{Fe}}_3{\text{O}}_4$) in a step-wise manner. This initial reduction is a hallmark of blast furnace chemistry, meaning this reaction definitely occurs.

The Copper Mix-Up

Now, let's scrutinize reaction C:

At first glance, this looks like a slag formation reaction. However, there is a critical catch! In the extraction of iron, we want to keep the iron, not turn it into slag. Here, iron oxide ($\text{FeO}$) is reacting with silica to form iron silicate (${\text{FeSiO}}_3$).

This specific reaction actually belongs to copper metallurgy. During the extraction of copper from copper pyrites, iron oxide is the unwanted impurity. In that scenario, silica (${\text{SiO}}_2$) is added as a flux to remove the iron as iron silicate slag. Thus, this reaction is an imposter in the iron blast furnace.

Thermodynamic Reality

Finally, we evaluate reaction D:

This equation depicts the direct thermal decomposition of iron oxide into pure iron and oxygen gas. While it looks simple on paper, thermodynamics tells a different story.

Breaking the strong bonds between iron and oxygen without any chemical assistance requires an astronomically high temperature, far beyond the operating limits of a standard blast furnace. To make this reduction thermodynamically feasible, we absolutely need a reducing agent like carbon ($\text{C}$) or carbon monoxide ($\text{CO}$) to couple with the oxygen. Therefore, this direct decomposition does not occur.

By carefully analyzing the chemical principles and thermodynamic constraints, we have successfully unmasked the imposters.

Reactions (A) and (B) are essential steps in the extraction of iron. However, reaction (C) is a process from copper metallurgy, and reaction (D) is thermodynamically impossible under these conditions.

Therefore, the reactions that do not occur in the blast furnace are (C) and (D), making option (a) the correct answer.