Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reactions is used to make a fuel cell? (a) \(\mathrm{Cd}(\mathrm{s})+2 \mathrm{Ni}(\mathrm{OH})_{3}(\mathrm{~s}) \longrightarrow \mathrm{CdO}(\mathrm{s})\) \(+2 \mathrm{Ni}(\mathrm{OH})_{2}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{I})\) (b) \(\mathrm{Pb}(\mathrm{s})+\mathrm{PbO}_{2}(\mathrm{~s}) 2 \mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) \longrightarrow\) \(2 \mathrm{PbSO}_{4}(\mathrm{~s})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{s})\) (c) \(2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(l)\) (d) \(2 \mathrm{Fe}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{~g})+4 \mathrm{H}^{+} \longrightarrow 2 \mathrm{Fe}^{2+}(\mathrm{aq})\) \(+2 \mathrm{H}_{2} \mathrm{O}(1)\)

Step-by-Step Solution

Answer

Reaction (c) is used in a fuel cell.

Step 1: Understanding Fuel Cells

A fuel cell converts chemical energy into electrical energy through a chemical reaction involving between hydrogen and oxygen. The key reaction in most fuel cells involves hydrogen gas reacting with oxygen to produce water, typically represented by the equation: \( 2 \text{H}_2(g) + \text{O}_2(g) \rightarrow 2 \text{H}_2\text{O}(l) \).

Step 2: Identifying Hydrogen-Oxygen Reaction

Examine each given reaction to identify which one involves hydrogen and oxygen combining to form water. This is a hallmark of a fuel cell reaction.

Step 3: Analyzing Option (a)

Reaction (a) does not involve any forms of hydrogen and oxygen gas reacting directly to form water, but rather involves cadmium and nickel hydroxide compounds.

Step 4: Analyzing Option (b)

Reaction (b) features lead, lead dioxide, and sulfuric acid reacting but does not prominently feature a hydrogen gas and oxygen gas combination; it is indicative of a lead-acid battery.

Step 5: Analyzing Option (c)

Reaction (c) is \( 2 \text{H}_2(g) + \text{O}_2(g) \rightarrow 2 \text{H}_2\text{O}(l) \), which exactly describes the reaction of hydrogen with oxygen found in fuel cells.

Step 6: Analyzing Option (d)

Reaction (d) involves iron and oxygen, but the result is an oxidation state change in iron rather than combining hydrogen and oxygen gas to form water.

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances interact to form new substances with different properties. They involve the breaking and forming of chemical bonds, ultimately leading to changes in the chemical structure of the molecules involved. In the context of fuel cells, these reactions are at the heart of how they generate energy.

Balanced chemical equations represent the conservation of mass where the number of atoms for each element is the same on both sides of the equation. This is a crucial concept as it ensures that matter is neither created nor destroyed during the reaction. The formation of new chemical compounds from reactants may involve energy changes, with energy either being absorbed or released.

In a fuel cell, the chemical reaction is specially designed to efficiently convert stored chemical energy into electrical energy, minimizing energy loss as heat. Fuel cell reactions are carefully orchestrated to harness electricity without the combustion process, making them cleaner alternatives to traditional methods of power generation.

Hydrogen-Oxygen Reaction

The hydrogen-oxygen reaction is fundamental in most fuel cells. This reaction involves the combination of hydrogen gas (9;_2(g)) with oxygen gas (4;_2(g)) to form water (9;_2O(l)). It is one of the simplest and cleanest chemical reactions used for producing energy.

During this reaction, electrons are transferred from hydrogen to oxygen, which leads to the formation of water. This electron exchange is the key to generating electricity in the fuel cell. Previously stored chemical energy in hydrogen is thereby converted into electrical energy when these electrons move through an external circuit.

Advantages of this reaction include:

• High efficiency and eco-friendliness, with water being the only by-product.
• The abundance of hydrogen and oxygen as reactants, making it a renewable resource.
• No harmful emissions like carbon dioxide, making it a clean energy source.

Electrochemistry

Electrochemistry is a vital field in understanding fuel cells, as it deals with the interplay between electrical and chemical changes. In a fuel cell, electrochemical reactions are what allow the conversion of chemical energy directly to electrical energy efficiently and cleanly.

This process involves:

• Oxidation reactions, where hydrogen gas loses electrons (H_2 52; 2H^+ + 2e^-).
• Reduction reactions, where oxygen gas gains electrons (O_2 + 4H^+ + 4e^- 52; 2H_2O).

Fuel cells are built with electrodes (anode and cathode) where these reactions take place. At the anode, hydrogen is oxidized, releasing electrons that flow through an external circuit, providing electrical power. Oxygen is reduced at the cathode, allowing it to accept electrons and complete the circuit by forming water as a by-product.

This entire process is meticulously managed within the architecture of a fuel cell to ensure maximum energy retrieval from the reaction with minimal waste, highlighting the practical application of electrochemical principles in sustainable energy solutions.