Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

What would happen when a solution of potassium chromate is treated with an excess of dilute nitric acid? (a) \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) and \(\mathrm{H}_{2} \mathrm{O}\) are formed (b) \(\mathrm{Cr}^{3+}\) and \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) are formed (c) \(\mathrm{CrO}_{4}^{2-}\) is oxidized to \(+7\) state to \(\mathrm{Cr}\) (d) \(\mathrm{CrO}_{4}^{2-}\) is reduced to \(+3\) state of \(\mathrm{Cr}\)

Step-by-Step Solution

Answer

When potassium chromate is treated with excess dilute nitric acid, \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) and \(\mathrm{H}_{2} \mathrm{O}\) are formed, so option (a) is correct.

Step 1: Identify Main Chemical Reaction

Potassium chromate, which contains the chromate ion \((\mathrm{CrO}_4^{2-})\), is treated with an excess of dilute nitric acid (\(\mathrm{HNO}_3)\). This introduces a significant amount of \(\mathrm{H}^+\) ions into the solution, which will convert chromate ions \(\mathrm{CrO}_4^{2-}\) into dichromate ions \(\mathrm{Cr}_2\mathrm{O}_7^{2-}\).

Step 2: Understand the Chemical Equation

The equilibrium between chromate and dichromate in solution can be represented as: \(2 \mathrm{CrO}_{4}^{2-} + 2 \mathrm{H}^{+} \rightleftharpoons \mathrm{Cr}_2\mathrm{O}_7^{2-} + \mathrm{H}_2\mathrm{O}\). This shows that in acidic conditions, chromate ions convert into dichromate ions and water.

Step 3: Analyze the Conditions

Since the nitric acid is in excess, it ensures that the equilibrium shifts completely towards the formation of dichromate ions \(\mathrm{Cr}_2\mathrm{O}_7^{2-}\). This means \(\mathrm{Cr}_2\mathrm{O}_7^{2-}\) and water \(\mathrm{H}_2\mathrm{O}\) are formed completely in this reaction.

Step 4: Determine the Correct Option

From the balanced chemical equation and condition analysis, we see \(\mathrm{Cr}_2\mathrm{O}_7^{2-}\) and \(\mathrm{H}_2\mathrm{O}\) are the final products. Thus, option (a) is correct.

Key Concepts

Chemical Equilibrium

Chemical equilibrium is a fundamental concept in chemistry that describes the state where the concentrations of reactants and products no longer change with time. This occurs when the forward and reverse reactions occur at the same rate, creating a dynamic balance. Consider the reaction that occurs when potassium chromate (\(\mathrm{K}_2\mathrm{CrO}_4^2-\)) is mixed with dilute nitric acid (\(\mathrm{HNO}_3\)). The key equilibrium can be represented as follows:

• \(2 \mathrm{CrO}_{4}^{2-} + 2 \mathrm{H}^+ \rightleftharpoons \mathrm{Cr}_2\mathrm{O}_7^{2-} + \mathrm{H}_2\mathrm{O}\)

This equilibrium illustrates how chromate ions and hydrogen ions combine to form dichromate ions and water. The presence of excess \(\mathrm{H}^+\) ions from the nitric acid pushes the equilibrium towards the formation of more dichromate ions.
Understanding this principle allows us to predict the outcomes of reactions within certain conditions.

Dichromate Formation

Dichromate formation is an interesting aspect of chemistry that arises from the reaction between chromate ions and acidic media. Initially, potassium chromate contains chromate ions, represented as \(\mathrm{CrO}_4^{2-}\). When treated with an excess of nitric acid, the chromate ions undergo a condensation reaction to form dichromate ions \(\mathrm{Cr}_2\mathrm{O}_7^{2-}\).

• The process: \(2 \mathrm{CrO}_{4}^{2-} + 2 \mathrm{H}^+ \rightarrow \mathrm{Cr}_2\mathrm{O}_7^{2-} + \mathrm{H}_2\mathrm{O}\)

This transition from chromate to dichromate is not just a simple exchange; it involves the reorganization of atoms and a change in color. Potassium chromate solution, which is yellow, turns orange when dichromate forms. This visual cue is a convenient indicator of the ongoing chemical reaction.
This transformation is significant in various applications, including laboratory analyses and industrial processes.

Acid-Base Reaction

Acid-base reactions are chemical processes where an acid donates a proton (\(\mathrm{H}^+\)) to a base. In the case of potassium chromate with nitric acid, \(\mathrm{HNO}_3\) acts as the acid. It supplies the hydrogen ions necessary for the conversion of chromate ions into dichromate ions.

• The general reaction: \( \mathrm{Acid} + \mathrm{Base} \rightarrow \mathrm{Conjugate \, Acid} + \mathrm{Conjugate \, Base} \)
• For our specific reaction: \( \mathrm{CrO}_{4}^{2-} \) acts as the base.

This reaction highlights how acids and bases interact in an aqueous solution to achieve equilibrium.
By altering the concentration of \(\mathrm{H}^+\) ions, the reaction's direction can be controlled, making it a practical demonstration of how acid-base balance affects chemical processes. Understanding this allows chemists to manipulate reactions for desired outcomes.