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 \([2005]\) (a) \(+2\) (b) \(+3\) (c) \(+4\) (d) \(+6\)

Step-by-Step Solution

Answer

The oxidation state of chromium in the product is +3. (Answer: b)

Step 1: Write down the initial reaction components

The question involves the reaction between potassium iodide (KI) and acidified potassium dichromate (K2Cr2O7) solution. This is a redox reaction, where KI acts as a reducing agent and K2Cr2O7 acts as an oxidizing agent.

Step 2: Identify oxidation and reduction components

In KI, the iodine is in the -1 oxidation state and can be oxidized to I2. In K2Cr2O7, chromium is in the +6 oxidation state and will be reduced in the reaction.

Step 3: Determine the reduction in oxidation state

Chromium in the dichromate ion is reduced. The half-reaction for the dichromate ion in acidic solution is: \[ \text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{e}^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O} \]This shows that the chromium is reduced from +6 to +3.

Step 4: Balance the full redox equation

Write the balanced redox equation to confirm the changes:The overall balanced equation is: \[ \text{Cr}_2\text{O}_7^{2-} + 6\text{I}^- + 14\text{H}^+ \rightarrow 2\text{Cr}^{3+} + 3\text{I}_2 + 7\text{H}_2\text{O} \] This confirms the reduction of chromium to +3 oxidation state.

Step 5: Final Answer Selection

Based on the reduction process observed in Step 3 and Step 4, the final product has chromium in the +3 oxidation state, the final answer is option (b).

Key Concepts

Redox reactions

Redox reactions, short for reduction-oxidation reactions, are chemical processes in which the oxidation state of atoms change due to the transfer of electrons. In a typical redox reaction, one substance loses electrons (oxidation) while the other gains electrons (reduction). For example, in the reaction between potassium iodide (KI) and potassium dichromate (K2Cr2O7), iodine from KI goes through oxidation, losing electrons to form molecular iodine (I2). Simultaneously, chromium in K2Cr2O7 gets reduced, gaining those electrons to lower its oxidation state. A redox reaction is central when considering processes like energy generation in batteries or biological systems like cellular respiration.

• Oxidation: Involves the loss of electrons; leading to an increase in oxidation state.
• Reduction: Involves the gain of electrons; resulting in a reduction of oxidation state.
• Both processes must occur simultaneously for a redox reaction to happen.

Potassium dichromate

Potassium dichromate, with the chemical formula K2Cr2O7, is an orange-red crystalline solid, and commonly used as an oxidizing agent in chemical reactions. It's particularly powerful due to its ability to accept electrons and change the oxidation state of other substances. In reactions, potassium dichromate can play a crucial role in converting a variety of compounds by accepting electrons and transforming itself. When in acidic solutions, like the reaction with KI, it acts as an efficient oxidizing substance, facilitating changes in oxidation states, specifically decreasing its own chromium atoms from +6 to +3. This property makes potassium dichromate a valuable tool in laboratory settings and industrial applications where controlled oxidation is desired.

• Typically used in solutions with acidic conditions to maximize its oxidizing potential.
• The color change from orange to a greenish hue indicates a change in chromium's oxidation state.
• Safety Note: Handle with care as it is a strong oxidizer and can be hazardous to health.

Oxidizing agent

An oxidizing agent, also known as an oxidant, is a substance that has the ability to accept electrons from another substance, thus getting reduced in the process. In the context of redox reactions, the oxidizing agent is crucial as it facilitates the oxidation of the counterpart reactant. Potassium dichromate (K2Cr2O7) in acidic medium serves as an excellent example of an oxidizing agent. During the reaction with KI, potassium dichromate accepts electrons, ultimately reducing the oxidation state of chromium. This accepting of electrons allows the iodide ions from KI to change their state, exemplifying the intricate dance of electrons that occurs during these reactions.

• The oxidizing agent will always be reduced, gaining electrons during the reaction.
• High electronegativity and the potential for stable reduced forms make for strong oxidizing agents.
• Knowing the oxidizing agent helps predict the outcome and products of the reaction.

Half-reaction balance

The process of balancing half-reactions is a vital step in understanding redox reactions fully. A redox reaction can be split into two separate half-reactions: one for oxidation and one for reduction. Each half-reaction independently shows the electron transfer either gained or lost. For instance, in the reaction between KI and acidified K2Cr2O7, balancing the half-reactions involves ensuring that the number of electrons lost by iodide matches those gained by the chromium. This balance is achieved in an acidic solution by ensuring charge and mass stay consistent throughout the equation.
When performing half-reaction balancing:

• Identify the two separate processes: oxidation and reduction.
• Ensure that the number of electrons lost equals the number gained across the half-reactions.
• Use additives like H+, OH- or water to balance oxygen and hydrogen atoms as needed.
• Combine the balanced half-reactions to derive the full balanced equation.

Understanding this ensures that each atom and charge is accounted for, highlighting the full transformation process inherent to redox reactions.