Question

\(CrO_3\) is a/an

• A. acidic oxide
• B. basic oxide
• C. neutral oxide
• D. amphoteric oxide
• E. reducing agent

Hint:

Metal oxides in high oxidation states are acidic, while those in low oxidation states are basic.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The nature of metal oxides (acidic, basic, or amphoteric) generally depends on the oxidation state of the metal. As the oxidation state of a metal increases, the acidic character of its oxide increases.
Step 2: Detailed Explanation:
First, let's determine the oxidation state of Chromium (Cr) in CrO\(_3\). Let the oxidation state of Cr be 'x'. Oxygen usually has an oxidation state of -2. The overall charge on the molecule is zero.
\[ x + 3(-2) = 0 \] \[ x - 6 = 0 \] \[ x = +6 \] The oxidation state of Cr in CrO\(_3\) is +6. This is a very high oxidation state for chromium.
The general trend for oxides is as follows:

Low oxidation states: Metal oxides in low oxidation states (e.g., +1, +2) are typically basic. Example: CrO (oxidation state +2) is basic.

Intermediate oxidation states: Metal oxides in intermediate oxidation states (e.g., +3, +4) are often amphoteric, meaning they can react with both acids and bases. Example: Cr\(_2\)O\(_3\) (oxidation state +3) is amphoteric.

High oxidation states: Metal oxides in high oxidation states (e.g., +5, +6, +7) are acidic. Example: CrO\(_3\) (oxidation state +6) is acidic.

Since Cr is in a high oxidation state of +6, CrO\(_3\) is an acidic oxide. It dissolves in water to form chromic acid (H\(_2\)CrO\(_4\)), a strong acid and oxidizing agent.
\[ \text{CrO}_3(s) + \text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{CrO}_4(aq) \] Step 3: Final Answer:
Because chromium is in its highest oxidation state (+6), chromium trioxide (CrO\(_3\)) is an acidic oxide. This corresponds to option (A).