Problem 7 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The gas evolved on heating \(\mathrm{CaF}_{2}\) and \(\mathrm{SiO}_{2}\) with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), on hydrolysis gives a white gelatinous precipitate. The precipitate is: [Main Online April 9, 2014] (a) hydrofluosilicic acid (b) silica gel (c) silicic acid (d) calciumfluorosilicate

Step-by-Step Solution

Answer

The white gelatinous precipitate is silicic acid (option c).

Step 1: Determine the Reaction Products

When calcium fluoride (\(\mathrm{CaF}_2\)) is heated with silica (\(\mathrm{SiO}_2\)) and concentrated sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)), hydrofluoric acid (\(\mathrm{HF}\)) and silicon tetrafluoride (\(\mathrm{SiF}_4\)) are formed. The balanced chemical equation for this reaction can be written as: \[ \mathrm{CaF}_{2} + \mathrm{SiO}_{2} + 2\,\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{CaSO}_{4} + \mathrm{2}\,\mathrm{HF} + \mathrm{SiF}_{4} + 2\,\mathrm{H}_2\mathrm{O} \]

Step 2: Hydrolysis of Silicon Tetrafluoride

The silicon tetrafluoride (\(\mathrm{SiF}_4\)) gas reacts with water during hydrolysis to form silicic acid (\(\mathrm{H}_2\mathrm{SiO}_3\)) as a white gelatinous precipitate and hydrofluoric acid (\(\mathrm{HF}\)). The hydrolysis reaction is as follows: \[ \mathrm{SiF}_4 + 4\,\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{SiO}_3 + 4\,\mathrm{HF} \]

Step 3: Identify the Precipitate

From the hydrolysis reaction, it is clear that the product formed as a precipitate is silicic acid (\(\mathrm{H}_2\mathrm{SiO}_3\)). It is described as a white gelatinous substance in the context of this reaction.

Key Concepts

CaF2 reaction

The chemical reaction involving calcium fluoride, known as \(\mathrm{CaF}_2\), is fundamental to understanding how some industrial processes work. When \(\mathrm{CaF}_2\) is heated along with silica (\(\mathrm{SiO}_2\)) and concentrated sulfuric acid (\(\mathrm{H}_2\mathrm{SO}_4\)), it results in the formation of silicon tetrafluoride (\(\mathrm{SiF}_4\)) and hydrofluoric acid (\(\mathrm{HF}\)).

This reaction is significant for a couple of reasons. First, it derives crucial compounds, which are used as building blocks in other chemical reactions. Another importance is producing \(\mathrm{SiF}_4\), which is later hydrolyzed to produce silicic acid, valuable in various applications.

• Calcium fluoride reacts with silica and sulfuric acid.
• The reaction yields \(\mathrm{SiF}_4\) and \(\mathrm{HF}\).
• Calcium sulfate (\(\mathrm{CaSO}_4\)) and water are also products.

The overall reaction can be noted as:\[ \mathrm{CaF}_{2} + \mathrm{SiO}_{2} + 2\,\mathrm{H}_{2}\mathrm{SO}_{4} \rightarrow \mathrm{CaSO}_{4} + 2\,\mathrm{HF} + \mathrm{SiF}_{4} + 2\,\mathrm{H}_2\mathrm{O} \]This simplified chemical representation provides insight into what materials one can expect from such a controlled reaction.

SiO2 reaction

Silicon dioxide (\(\mathrm{SiO}_2\)), commonly known as silica, undergoes a significant reaction when mixed with calcium fluoride and sulfuric acid. The goal of this interaction is to produce silicon tetrafluoride, contributing further to silicic acid formation.

Silica is a major component in glass and other critical industrial materials. Its reactions tend to be quite robust and efficient, allowing for practical applications in chemical industries.Important aspects of the \(\mathrm{SiO}_2\) reaction:

• \(\mathrm{SiO}_2\) plays a crucial role in the formation of \(\mathrm{SiF}_4\).
• It serves as a source of silicon in the reaction.
• Interactively reacts with \(\mathrm{CaF}_2\) and \(\mathrm{H}_2\mathrm{SO}_4\) to yield \(\mathrm{HF}\) and \(\mathrm{SiF}_4\).

Silicon dioxide's involvement is vital for hydrolysis, leading to the obtained silicic acid, well-known for its application in desiccants and adsorbents due to its properties.

Hydrolysis of SiF4

The hydrolysis of silicon tetrafluoride (\(\mathrm{SiF}_4\)) marks an important step in the chemical process. Hydrolysis involves breaking down \(\mathrm{SiF}_4\) with water, resulting in the formation of silicic acid (\(\mathrm{H}_2\mathrm{SiO}_3\)) and hydrofluoric acid (\(\mathrm{HF}\)). This step confirms the creation of the white gelatinous precipitate, which is silicic acid.

This process is represented in the chemical reaction: \[ \mathrm{SiF}_4 + 4\,\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{SiO}_3 + 4\,\mathrm{HF} \]Highlights of \(\mathrm{SiF}_4\) hydrolysis:

• The reaction with water results in a clear precipitation of \(\mathrm{H}_2\mathrm{SiO}_3\).
• Hydrolysis is essential for producing these byproducts, particularly \(\mathrm{H}_2\mathrm{SiO}_3\), a common wet gel.
• Both products have significant industrial relevance, with \(\mathrm{H}_2\mathrm{SiO}_3\) being used in everything from toothpaste to filtration systems.

By understanding the hydrolysis of silicon tetrafluoride, students can better grasp the chemistry behind silica-based compounds and their applications in everyday materials and processes.