Question

When \(\mathrm{SO}_{2}\) gas is passed through an acidified solution of \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (a) the solution becomes blue (b) the solution becomes colourless (c) \(\mathrm{SO}_{2}\) is reduced (d) green \(\mathrm{Cr}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) is formed

Short answer

The solution becomes green, indicating \(\text{Cr}_2\left(\text{SO}_4\right)_3\) formation.

Step-by-step solution

Identify the Reaction Components

The problem involves the reaction of sulfur dioxide \(\text{SO}_2\) with an acidified potassium dichromate \(\text{K}_2\text{Cr}_2\text{O}_7\) solution. This is a redox reaction.

Understand the Role of SO2

In this reaction, \(\text{SO}_2\) acts as a reducing agent. It gets oxidized to \(\text{SO}_4^{2-}\), while the dichromate ion \(\text{Cr}_2\text{O}_7^{2-}\) gets reduced.

Reduction of Dichromate Ions

\(\text{Cr}_2\text{O}_7^{2-}\), in an acidic medium, gets reduced to \(\text{Cr}^{3+}\) ions, which give a green color. The reaction can primarily be written as: \(\text{Cr}_2\text{O}_7^{2-} + 6\text{e}^- + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}\).

Formation of Product

The \(\text{Cr}^{3+}\) ions combine with the sulfate ions from the \(\text{H}_2\text{SO}_4\) in the solution to form green \(\text{Cr}_2\left(\text{SO}_4\right)_3\).

Conclude the Reaction Outcome

The reaction results in a color change due to the formation of \(\text{Cr}_2\left(\text{SO}_4\right)_3\), turning the solution green. Therefore, the correct answer is that green \(\text{Cr}_2\left(\text{SO}_4\right)_3\) is formed.

Key concepts

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds.
In the given exercise, we deal with a type of chemical reaction called a redox reaction.
Redox reactions involve the transfer of electrons between two species.
In this process, one substance is oxidized and loses electrons, while the other is reduced and gains electrons.

• **Oxidation** refers to the loss of electrons by a molecule, atom, or ion.
• **Reduction** refers to the gain of electrons by a molecule, atom, or ion.

The chemical reaction between sulfur dioxide \((\text{SO}_2)\) and potassium dichromate \((\text{K}_2\text{Cr}_2\text{O}_7)\) showcases these principles.
Understanding how these reactions work is fundamental to grasping the changes that happen at a molecular level, including color changes in reaction solutions.

Dichromate Ions

Dichromate ions, \((\text{Cr}_2\text{O}_7^{2-})\), are an important part of redox chemistry in an acidic medium.
They are known for their distinct orange color in solution, which changes upon undergoing a redox reaction.
In the acidified \((\text{K}_2\text{Cr}_2\text{O}_7)\) solution, dichromate ions get reduced to chromium ions, \(\text{Cr}^{3+}\). This electron transfer results in a notable color change from orange to green in the solution, which indicates the completion of the reaction.

• The reduction of dichromate involves gaining electrons:

\[\text{Cr}_2\text{O}_7^{2-} + 6\text{e}^- + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}\]This reaction is a clear example of how dichromate ions function as oxidizers, driving the transformation observed in our chemical reaction.

Acidic Medium

An acidic medium is crucial for the dichromate reaction in redox processes.
In the exercise, the acidity provided by an acid like sulfuric acid \((\text{H}_2\text{SO}_4)\) stabilizes the reaction environment.
The acidic condition is essential for converting dichromate ions into chromium ions, and promotes the smooth transition during the electron transfer process.
Using an acidic medium also helps in ensuring that all reactive electrons and protons are available for effective reaction of the dichromate and sulfur dioxide.
Without the proton-rich environment provided by acids, this specific type of transformation wouldn't proceed as observed.

• The acidic environment ensures the reduction of dichromate to green \(\text{Cr}^{3+}\) ions.
• It provides the protons required for balancing the redox equation.

Through understanding why an acidic medium is required, students can get insights into how specific conditions influence chemical reactions.

Sulfur Dioxide

Sulfur dioxide, \((\text{SO}_2)\), plays the role of a reducing agent in this chemical equation.
As a reducing agent, it donates electrons to the dichromate ions, facilitating their reduction.
During the reaction, sulfur dioxide itself gets oxidized into sulfate ions \(\text{SO}_4^{2-}\). This transformation exemplifies a common role for \(\text{SO}_2\) in chemical reactions.

• Sulfur dioxide is commonly involved in redox processes due to its electron donating ability.
• In the exercise, this role is key in driving the reaction towards the formation of \(\text{Cr}^{3+}\) ions.

By understanding sulfur dioxide's function, students can appreciate its importance both in industrial applications and environmental chemistry.
It is a classic example of how substances can influence and facilitate overall chemical changes.