Problem 9 - Chemistry in 30 days

Question

Green \(\mathrm{Cr}^{3+}\) changes to yellow \(\mathrm{CrO}_{4}^{2}\) on boiling with (a) alkaline \(\mathrm{H}_{2} \mathrm{O}_{2}\) (b) acidified \(\mathrm{H}_{2} \mathrm{O}_{2}\) (c) both (a) and (b) (d) None of these

Step-by-Step Solution

Answer

The answer is (a) alkaline \(\mathrm{H}_{2}\mathrm{O}_2\).

Step 1: Understanding the Reaction Conditions

The problem asks which condition allows green \(\mathrm{Cr}^{3+}\) to change to yellow \(\mathrm{CrO}_{4}^{2-}\). This involves the oxidation of chromium from +3 oxidation state to +6.

Step 2: Analyzing the Chemical Reagents

\(\mathrm{H}_2\mathrm{O}_2\) can act as both an oxidizing and reducing agent depending on the pH. In alkaline conditions, it usually acts as an oxidizer. In acidic conditions, \(\mathrm{H}_2\mathrm{O}_2\) also acts as an oxidizing agent but the chemistry might differ significantly in oxidation products.

Step 3: Checking Alkaline Conditions (Option a)

In alkaline \(\mathrm{H}_2 \mathrm{O}_2\), chromium can undergo oxidation from \(\mathrm{Cr}^{3+}\) to \(\mathrm{CrO}_{4}^{2-}\) due to the basic environment, facilitating the oxidation process effectively.

Step 4: Checking Acidified Conditions (Option b)

In acidified \(\mathrm{H}_2 \mathrm{O}_2\), chromium typically does not form \(\mathrm{CrO}_{4}^{2-}\), as acidic conditions do not favor this oxidation to the chromate ion. Another possible product could form in acidic conditions.

Step 5: Choosing the Correct Option

Given this analysis, only alkaline \(\mathrm{H}_2 \mathrm{O}_2\) allows the transformation of green \(\mathrm{Cr}^{3+}\) to yellow \(\mathrm{CrO}_{4}^{2-}\). Therefore, option (a) is the correct choice.

Key Concepts

Chromium Oxidation States

Chromium is a fascinating element primarily because of its ability to exist in multiple oxidation states. In chemistry, oxidation states indicate the degree of oxidation of an atom in a chemical compound. For chromium, two of the most notable oxidation states are +3 and +6.
In its +3 oxidation state, represented as \(\mathrm{Cr}^{3+}\), chromium exhibits a green color. This form of chromium is more stable and less toxic compared to its higher oxidation state. The +6 oxidation state, known as chromate or \(\mathrm{CrO}_4^{2-}\), is typically yellow and is formed under specific chemical conditions, like during oxidation reactions. This oxidation state is more reactive and can facilitate many chemical transformations, including acting as an oxidizing agent.
Understanding these states helps predict how chromium will behave in various chemical reactions, especially in oxidation reactions where the chromium can switch from being stable to highly reactive.

Chemical Reagents in Alkaline Solution

Chemical reagents behave differently depending on whether they are in an acidic or alkaline environment. In alkaline solutions, reagents often undergo unique reactions that would not occur in neutral or acidic environments.

• In the presence of an alkaline solution, reagents like hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)) can facilitate oxidation reactions effectively.
• Alkalinity affects the nature and extent of reactions by providing an environment where certain ions and molecules become more stable or reactive.
• For example, hydroxide ions in an alkaline solution can stabilize high oxidation states of metals, such as the +6 state of chromium in \(\mathrm{CrO}_4^{2-}\), facilitating the conversion of \(\mathrm{Cr}^{3+}\) to \(\mathrm{CrO}_4^{2-}\).

This aspect allows reagents to act as stronger oxidizing agents, encouraging the oxidation processes needed to form certain compounds like chromates from lower oxidation states of metals.

Role of Hydrogen Peroxide in Oxidation

Hydrogen peroxide (\(\mathrm{H}_2\mathrm{O}_2\)) is a versatile compound known for its role as both an oxidizing and reducing agent. Its behavior changes depending on the pH level of the solution it is in.

• In alkaline environments, \(\mathrm{H}_2\mathrm{O}_2\) acts predominantly as an oxidizing agent, encouraging the increase of oxidation numbers in other substances.
• When mixed with a compound like \(\mathrm{Cr}^{3+}\), \(\mathrm{H}_2\mathrm{O}_2\) promotes the oxidation to \(\mathrm{CrO}_4^{2-}\) under alkaline conditions.
• This is because, in basic solutions, \(\mathrm{H}_2\mathrm{O}_2\) decomposes slowly, providing a steady source of oxygen to facilitate oxidation.

Understanding this behavior is crucial for predicting outcomes in reactions involving hydrogen peroxide, particularly in synthetic and analytical chemistry, where controlling oxidation states is important.