Problem 17 - Chemistry in 30 days

Question

When hydrogen peroxide is added to acidified potassium dichromate, a blue colour is produced due to the formation of (a) \(\mathrm{CrO}_{3}\) (b) \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{CrO}_{5}\) (d) \(\mathrm{CrO}_{4}^{2-}\)

Step-by-Step Solution

Answer

(c) \( \mathrm{CrO}_{5} \) is responsible for the blue color.

Step 1: Understanding the Reaction

When hydrogen peroxide (7H_2O_2F7) is added to acidified potassium dichromate (F7K_2Cr_2O_7F7), a redox reaction occurs. In this process, F7Cr_2O_7^{2-}F7 ions are reduced.

Step 2: Identify the Reduced Form of Chromium

During the reaction in acidic medium, F7Cr_2O_7^{2-}F7 is reduced to chromium species. Hydrogen peroxide acts as a reducing agent, converting chromium to a lower oxidation state.

Step 3: Determine the Product Responsible for Blue Color

The formation of the blue color is due to the formation of F7CrO_5F7. It is characterized by the linkage of F7-Cr(O-O)_2-F7 structure and appears temporarily in the solution.

Step 4: Compare with Given Options

Among the provided choices, option (c) F7CrO_5F7 matches with the product formed that is responsible for the blue color observed in the reaction.

Key Concepts

Hydrogen Peroxide

Hydrogen peroxide, commonly denoted as \( \text{H}_2\text{O}_2 \), is a simple chemical compound that plays a crucial role in many chemical reactions due to its oxidative and reductive properties.
In the context of redox reactions, it mainly acts as a reducing agent, which means it helps in reducing other substances while getting oxidized itself.

• This molecule is made up of two hydrogen atoms and two oxygen atoms.
• It exhibits a peroxide bond (\(-O-O-\)), which is reactive and is responsible for its chemical behavior.

In aqueous solutions, hydrogen peroxide breaks down into water and oxygen, often releasing energy. However, under acidic conditions as in this reaction, its tendency to donate electrons (reduce another substance) becomes prominent.

Potassium Dichromate

Potassium dichromate (\( \text{K}_2\text{Cr}_2\text{O}_7 \)) is an orange-red colored compound that is widely used in laboratories as an oxidizing agent. It contains dichromate ions \( (\text{Cr}_2\text{O}_7^{2-}) \), which are key to many redox reactions.

• The compound is effective in acidic environments where these ions exhibit strong oxidizing properties.
• The reaction with hydrogen peroxide involves the transfer of electrons, leading to changes in the oxidation states of chromium.

The dichromate ions will accept electrons and be transformed into other chromium species. This transformation is crucial for observing the changes in color that occur.

Chromium Compounds

Chromium compounds, particularly in varying oxidation states, are at the heart of the color changes observed in chemical reactions. In this particular redox reaction, chromium from \( \text{Cr}_2\text{O}_7^{2-} \) is reduced by hydrogen peroxide.

• This reduction process leads to the formation of \( \text{CrO}_5 \), a complex compound featuring chromium in a lower oxidation state compared to dichromate.
• The bond structure in \( \text{CrO}_5 \) includes the distinctive peroxide linkage \((-O-O-)\), which is instrumental in the temporary blue color observed.

Chromium can form several different compounds via electron exchange processes, influencing their respective visual characteristics and chemical functions.

Blue Color Formation

The fleeting blue color that emerges during the reaction between hydrogen peroxide and potassium dichromate is due to the formation of a unique compound named \( \text{CrO}_5 \).

• This intermediate forms through interaction under acidic conditions, where the presence of the peroxide linkage \((-O-O-)\) within \( \text{CrO}_5 \) is key.
• Though this blue hue is temporary and quickly diminishes, it serves as a visual indication of the redox process occurring within the solution.

The compound \( \text{CrO}_5 \) itself is unstable and thus decomposes, causing the blue color to fade. Understanding this phenomenon allows one to grasp how spectator ions and short-lived intermediates play crucial roles in chemical reactions.