Problem 139 - A Complete Resource Book in Chemistry for JEE Main

Question

The oxidation state of chromium, in the final product formed by the reaction between KI and acidified potassium dichromate solution, is (a) \(+2\) (b) \(+3\) (c) \(+4\) (d) \(+6\)

Step-by-Step Solution

Answer

The oxidation state of chromium in the final product is +3.

Step 1: Determine the initial oxidation state of chromium

Potassium dichromate ( K_2Cr_2O_7 ) can be broken down into ( Cr_2O_7^{2-} ) ions; in this ion, the oxidation state of chromium is +6.

Step 2: Write the redox reaction equations

The reaction between KI and acidified potassium dichromate is a redox reaction. Chrome, in the form of ( Cr_2O_7^{2-} ) reduced to Cr^{3+} , while iodine is oxidized from I^- to I_2 .

Step 3: Determine the final oxidation state of chromium

In the reduction half-reaction, the Cr_2O_7^{2-} ion gains electrons to form 2Cr^{3+} , meaning that chromium goes from an initial oxidation state of +6 to a final oxidation state of +3.

Key Concepts

Redox Reactions

The term "redox reactions" refers to reactions involving the transfer of electrons between chemical species. "Redox" is a combination of "reduction" and "oxidation," the two processes that occur simultaneously in such reactions. In reduction, a substance gains electrons, while in oxidation, a substance loses electrons. In every redox reaction, one species gets oxidized, meaning it loses electrons, and another gets reduced, gaining electrons.

In the context of potassium dichromate reacting with KI, chromium in potassium dichromate is reduced, as it gains electrons, while iodine is oxidized, losing electrons to become diatomic iodine \(I_2\).

• Reduction: \(Cr_2O_7^{2-}\) transforms to \(Cr^{3+}\) through electron gain.
• Oxidation: \(I^-\) becomes \(I_2\), releasing electrons.

This dual process is critical to maintaining balance in chemical reactions, ensuring the total number of electrons lost equals the number of electrons gained.

Chromium

Chromium is a transition metal known for its various oxidation states, which can vary from \(-2\) to \(+6\). However, in many chemical reactions, chromium can prominently be found in the oxidation states of \(+3\) and \(+6\). In potassium dichromate, chromium starts in the \(+6\) oxidation state.

During reactions, like the one with potassium iodide (KI), chromium often changes its oxidation state as it participates in electron transfer. As seen in the example reaction, chromium is reduced from its \(+6\) state in dichromate to a \(+3\) state. This change of oxidation state indicates a gain of electrons, which is characteristic of a reduction reaction.

• Common Chromium States: \(+3\) (often in Cr^{3+}) and \(+6\) (often found in Cr_2O_7^{2-})

Such variations in states make chromium an essential element in studying redox processes, where it serves as a good example of oxidation and reduction changes.

Potassium Dichromate

Potassium dichromate \(K_2Cr_2O_7\) is a strongly oxidizing agent commonly used in various chemical reactions. It contains chromium in the \(+6\) oxidation state, which makes it a potent oxidizer as it can accept electrons from other substances.

When dissolved in water, the compound disassociates into its ions, including \(Cr_2O_7^{2-}\). In the presence of an acidic medium, potassium dichromate is highly effective at oxidizing iodide ions \(I^-\) into iodine \(I_2\), while it is itself reduced to chromium in the \(+3\) oxidation state.

• Acidic Medium: Increases potassium dichromate's oxidizing potential.
• Transformation during Reaction: \(K_2Cr_2O_7\) changes to \(2Cr^{3+}\)

This property is used in a variety of applications, from organic chemistry to analytical testing, due to its reliable reaction profile in redox reactions.