Problem 106 - A Complete Resource Book in Chemistry for JEE Main

Question

In acidic medium, dichromate ion oxidize ferrous ion to ferric ion. If the gram molecular weight of potassium dichromate is \(294 \mathrm{~g}\), its equivalent weight is (a) 19 (b) 49 (c) 99 (d) 294

Step-by-Step Solution

Answer

The equivalent weight of potassium dichromate is 49 g/equiv.

Step 1: Identify the Reaction

In acidic medium, the reaction between dichromate ion \(( ext{Cr}_2 ext{O}_7^{2-})\) and ferrous ion \(( ext{Fe}^{2+})\) involves the oxidation of ferrous ion to ferric ion \(( ext{Fe}^{3+})\). Meanwhile, the dichromate ion is reduced to chromium ion \(( ext{Cr}^{3+})\).

Step 2: Determine the Change in Oxidation States

In the reaction, each Cr in \( ext{Cr}_2 ext{O}_7^{2-}\) is reduced from an oxidation state of +6 to +3, leading to a total change of \(3 imes 2 = 6\) per dichromate ion as it consists of two Cr atoms. This means there is a transfer of 6 electrons per molecule of potassium dichromate.

Step 3: Calculate Equivalent Weight of Potassium Dichromate

To find the equivalent weight of a compound in a redox reaction, divide its molar mass by the number of electrons lost or gained in the reaction. Thus, the equivalent weight of potassium dichromate in this reaction is \(\frac{294\ ext{g/mol}}{6} = 49\ ext{g/equiv}\).

Key Concepts

Dichromate Ion

The dichromate ion, \(\text{Cr}_2\text{O}_7^{2-}\), is a powerful oxidizing agent in acidic solutions.
It is composed of two chromium atoms and seven oxygen atoms.The chromium atoms in the dichromate ion have an oxidation state of +6. This means they are highly oxidized and can accept electrons during a chemical reaction.
In many redox reactions, dichromate ions are reduced from their +6 oxidation state to +3.
In these reactions, they play a crucial role by accepting electrons and hence oxidizing other substances.In the specific reaction where dichromate ion oxidizes ferrous ion to ferric ion, it accepts electrons and is reduced, making it an essential part of oxidation-reduction chemistry discussed in this context.
Students can explore further examples of this ion's properties and behavior in different conditions for a clearer understanding of its applications.

Ferrous Ion

The ferrous ion, \(\text{Fe}^{2+}\), is a common form of iron in aqueous solutions.
It is often characterized by its +2 oxidation state, which indicates it has lost two electrons.
In redox reactions, such as the one with dichromate ions, ferrous ions often act as reducing agents.When ferrous ions are oxidized, they lose an electron and become ferric ions, \(\text{Fe}^{3+}\).
This change in oxidation state represents a typical oxidation process where the ferrous ion undergoes electron loss.
In the given reaction, ferrous ions give up one electron each, contributing to the reduction of dichromate ions.

Equivalent Weight

Equivalent weight is a concept used extensively in redox chemistry to relate the reactive capacity of a compound to its mass.
It refers to the mass of a substance that will react with or supply one mole of electrons in a redox reaction.
To calculate equivalent weight, divide the molar mass by the number of electrons involved in the redox process.In the current exercise, the equivalent weight of potassium dichromate is evaluated.
With a gram molecular weight of 294 g/mol and a total of 6 electrons transferred in its reaction with ferrous ions, the equivalent weight is calculated as \(\frac{294\ \text{g/mol}}{6} = 49\ \text{g/equiv}\).
Understanding this helps in balancing redox reactions and determining the amount of substance needed for a reaction.

Oxidation States

Oxidation states, also known as oxidation numbers, indicate the degree of oxidation of an atom in a chemical compound.
They are used to keep track of electron transfer in oxidation-reduction reactions.
Oxidation states can be positive, negative, or zero, depending on the electron arrangement in atoms or ions. In the oxidation of ferrous ions to ferric ions, the oxidation state of iron changes from +2 to +3.
This reflects electron loss, which is the essence of oxidation.
On the other hand, for dichromate ions, each chromium’s oxidation state reduces from +6 to +3, indicating it has gained electrons. Understanding oxidation states is crucial as it helps in identifying how electrons are shared among different atoms in a compound or during a reaction.
This process lays the foundation for studying more complex redox reactions and balancing chemical equations.