Problem 50 - A Complete Resource Book in Chemistry for JEE Main

Question

Which compound acts as an oxidizing as well as a reducing agent? (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{MnO}_{2}\) (c) \(\mathrm{Al}_{2} \mathrm{O}_{3}\) (d) \(\mathrm{CrO}_{3}\)

Step-by-Step Solution

Answer

Both \(\mathrm{SO_{2}}\) and \(\mathrm{MnO_{2}}\) can act as both oxidizing and reducing agents.

Step 1: Understand the Concept

Compounds that act as both oxidizing and reducing agents have elements that can donate and accept electrons, hence can undergo oxidation or reduction themselves. They should have an element that exists in an intermediate oxidation state.

Step 2: Analyze the Options

Look at each compound to see if the element present can be both oxidized and reduced. This will require checking the oxidation state of the main element and understanding its possible changes in oxidation state.

Step 3: Evaluate \\(\mathrm{SO}_{2}\mathrm{SO_{2}}\\)

In \(\mathrm{SO_{2}}\), sulfur is in an oxidation state of +4. It can be reduced to -2 (\(\mathrm{H_{2}S}\)) or oxidized to +6 (\(\mathrm{SO_{3}}\)), hence it can act as both an oxidizing and reducing agent.

Step 4: Evaluate \\(\mathrm{MnO}_{2}\\)

In \(\mathrm{MnO_{2}}\), manganese is in a +4 state. It can be reduced to +2, such as in \(\mathrm{Mn}^{2+}\), or oxidized to +7, as in \(\mathrm{MnO_{4}^{-}}\), allowing it to act as an oxidizing or reducing agent.

Step 5: Evaluate \\(\mathrm{Al}_{2}\mathrm{O}_{3}\mathrm{Al_{2}O_{3}}\\)

In \(\mathrm{Al_{2}O_{3}}\), aluminum is in its highest oxidation state of +3, and cannot be reduced or oxidized further, so it cannot act as both an oxidizing and reducing agent.

Step 6: Evaluate \\(\mathrm{CrO}_{3}\mathrm{CrO_{3}}\\)

In \(\mathrm{CrO_{3}}\), chromium is in its highest oxidation state of +6. It cannot be oxidized further, but can be reduced to lower states like +3. However, it primarily acts as an oxidizing agent.

Step 7: Determine the Correct Compound

After analyzing the oxidation states and potential electron transfers for each compound, select the one that can undergo both oxidation and reduction.

Key Concepts

Oxidizing agents

An oxidizing agent is essential in redox reactions as it facilitates the oxidation of other substances by accepting electrons. This causes the oxidizing agent itself to be reduced. Usually, oxidizing agents are substances containing a high oxidation state that can decrease, such as

• Oxygen (O₂), which can accept electrons to form O^2-
• Chlorine (Cl₂), which can be converted to Cl^-
• Other examples include molecules like hydrogen peroxide (H₂O₂) and nitric acid (HNO₃)

The ability of an oxidizing agent to accept electrons is its primary function in a chemical reaction. To determine if a compound can serve as an oxidizing agent, one can look at the oxidation state of the elements involved and verify whether they have electrons to gain.

Reducing agents

Reducing agents are the unsung heroes of redox reactions. They give away electrons, causing themselves to be oxidized while reducing the oxidation state of another substance. Common reducing agents involve elements at a lower or intermediate oxidation state, enabling them to give away electrons.

• Hydrogen gas (H₂) is a well-known reducing agent that provides electrons to other substances.
• Carbon monoxide (CO) acts similarly in metallurgic processes by converting metal oxides to pure metal.
• Some metals, like sodium (Na) and magnesium (Mg), are excellent reducing agents due to their propensity to release electrons to stabilize their electron structure.

By identifying a substance capable of donating electrons, one can identify potential reducing agents in a chemical equation. To conclude, reducing agents are vital in driving redox reactions forward as they enable the transfer of electrons by willingly sacrificing their own electrons.

Oxidation states

Oxidation states (or oxidation numbers) are numerical values assigned to an element in a molecule to indicate its partial gain or loss of electrons. This concept is crucial in distinguishing how electrons are allocated in compounds and to balance redox equations.

• Rules govern these assignments, such as oxygen typically having an oxidation state of -2 or hydrogen having +1, unless they form peroxides or hydrides, respectively.
• An element may have different oxidation states depending on the compound it is in, as in the case for sulfur in
  • SO₂ where it shows a +4 state
  • H₂S where it has an oxidation state of -2.

Understanding oxidation states helps determine how electrons are transferred in a reaction and thus identifies which elements or compounds act as oxidizing or reducing agents. The careful analysis of oxidation states in compounds is key to predicting their behavior in chemical reactions.