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

(b) 49

Step 1: Understand the Reaction

The reaction in acidic medium is \( ext{Cr}_2 ext{O}_7^{2-} + 14 ext{H}^+ + 6 ext{Fe}^{2+} \rightarrow 2 ext{Cr}^{3+} + 6 ext{Fe}^{3+} + 7 ext{H}_2 ext{O} \). The dichromate ion \( ext{Cr}_2 ext{O}_7^{2-} \) is reduced to two \( ext{Cr}^{3+} \) ions. Therefore, one dichromate ion involves a change of 6 electrons.

Step 2: Use the Equivalent Weight Formula

Equivalent weight of a compound in a redox reaction can be found using the formula: Equivalent Weight = \( \frac{\text{Molar Mass of the Compound}}{n} \), where \( n \) is the change in moles of electrons per molecule.

Step 3: Calculate Equivalent Weight

The molar mass of potassium dichromate \( ext{K}_2 ext{Cr}_2 ext{O}_7 \) is given as \( 294 \) g/mol, and it involves 6 electrons, so the equivalent weight is \( \frac{294}{6} = 49 \) g/equiv.

Key Concepts

Understanding the Dichromate Ion

The dichromate ion is a well-known ion in chemistry, characterized by its vibrant orange color. It has an empirical formula of \( \text{Cr}_2\text{O}_7^{2-} \).
This ion comprises two chromium atoms bonded with seven oxygen atoms, forming a double tetrahedron structure. Here are some important highlights about dichromate ions:

• Dichromate ions often act as strong oxidizing agents, which means they can easily accept electrons from other substances in a reaction.
• This makes them very useful in industrial applications, analytical chemistry, and redox titrations.
• Under acidic conditions, dichromate ions are used to oxidize other compounds, which is evident from their reaction with ferrous ions to form ferric ions.

Oxidation-Reduction Reactions in Chemistry

Oxidation-reduction reactions, or redox reactions, are fundamental chemical processes where the oxidation state of one or more atoms changes. These reactions encompass a transfer of electrons between substances.
Here's what you need to know about oxidation-reduction reactions:

• **Oxidation** is the loss of electrons. The substance that loses electrons is known as the reducing agent and becomes oxidized.
• **Reduction** is the gain of electrons. The substance that gains electrons acts as the oxidizing agent and becomes reduced.
• In the reaction involving the dichromate ion, the chromium is reduced, simultaneously oxidizing the iron from ferrous (\( \text{Fe}^{2+} \)) to ferric (\( \text{Fe}^{3+} \)) state.

Understanding both processes is key to mastering redox reactions.

Diving into Potassium Dichromate

Potassium dichromate, with the chemical formula \( \text{K}_2\text{Cr}_2\text{O}_7 \), is an inorganic compound often used in laboratories. Here are some vital points about potassium dichromate:

• It is a crystalline ionic solid, known similarly for its vibrant orange color.
• It is a potent oxidizing agent and is commonly used in chemical analysis and synthesis, especially in organic chemistry for oxidizing alcohols.
• Because of its dangerous nature, handling this compound requires diligent safety precautions such as gloves and goggles.
• In redox reactions, potassium dichromate releases \( \text{Cr}_2\text{O}_7^{2-} \), which plays a crucial role in transferring electrons.

Exploring Redox Reactions in Detail

Redox reactions, as mentioned previously, involve electron transfer between species. They are not just limited to laboratory settings but also occur in everyday life, like rusting or photochemical processes. Here's why redox reactions are significant:

• They are vital for biochemical processes, such as cellular respiration and photosynthesis.
• Redox reactions are the principles behind batteries, where chemical energy is converted to electrical energy.
• In the laboratory, such as with the reaction of dichromate ions, redox reactions help determine the equivalent weight of compounds, crucial for stoichiometric calculations.
• Analyzing redox reactions, like those involving potassium dichromate with ferrous ions, provides insight into the transfer of electrons and the calculation of equivalent masses based on electron change.

Understanding these processes allows us to harness redox reactions for practical and scientific purposes.