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) \(\mathrm{Cd}(\mathrm{s})+2 \mathrm{Ni}(\mathrm{OH})_{3}(\mathrm{~s}) \longrightarrow \mathrm{CdO}(\mathrm{s})\) \(+2 \mathrm{Ni}(\mathrm{OH})_{2}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})\) (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) \(2 \mathrm{Fe}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{~g})+4 \mathrm{H}^{+} \longrightarrow 2 \mathrm{Fe}^{2+}(\mathrm{aq})\) \(+2 \mathrm{H}_{2} \mathrm{O}\) (l)

Step-by-Step Solution

Answer

The reaction used in fuel cells is (c): \(2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(l) \).

Step 1: Understanding the Question

We need to identify which chemical reaction from the given options serves as the fundamental process in a fuel cell. A fuel cell typically converts chemical energy into electrical energy through a redox reaction involving hydrogen and oxygen.

Step 2: Analyzing Reaction (a)

Reaction (a) describes a chemical process involving cadmium and nickel hydroxide. This reaction is typical in nickel-cadmium batteries, not fuel cells. Fuel cells require a direct redox reaction between hydrogen and oxygen to produce water.

Step 3: Analyzing Reaction (b)

Reaction (b) involves lead and lead dioxide reacting with sulfuric acid to produce lead sulfate and water. This corresponds to the lead-acid battery reaction, which is not used in fuel cells.

Step 4: Analyzing Reaction (c)

Reaction (c) describes hydrogen gas reacting with oxygen gas to produce water. This is the core reaction of a hydrogen fuel cell, where hydrogen and oxygen react to produce electricity, heat, and water.

Step 5: Analyzing Reaction (d)

Reaction (d) involves iron reacting with oxygen to produce iron ions and water. This reaction does not involve hydrogen and is not suitable for a fuel cell.

Step 6: Conclusion

Based on the analysis, Reaction (c) is a typical fuel cell reaction where hydrogen and oxygen react to form water, generating electrical energy in the process.

Key Concepts

Electrochemical Reactions

In the world of chemistry, electrochemical reactions play a crucial role. These are processes where chemical reactions produce electrical currents or use electrical currents to drive chemical reactions. They involve two primary reactions: oxidation and reduction. Both occur simultaneously in a device called an electrochemical cell.

Electrochemical reactions are central to many technologies, including batteries and fuel cells. They work by converting chemical energy into electrical energy, which is then used to do work, like powering a device or allowing an electron flow through a circuit.

When understanding electrochemical reactions, it's essential to break them down:

• **Oxidation:** This is the loss of electrons from a molecule, atom, or ion. During oxidation, the substance loses energy because it lets go of electrons.


• **Reduction:** This involves gaining electrons. The substance that gains these electrons undergoes reduction and increases its energy.


• **Redox Reactions:** These are reactions where both oxidation and reduction occur simultaneously. In these reactions, electrons transfer between substances, leading to changes in their oxidation states.

These reactions are fascinating as they form the foundation for creating power in devices such as fuel cells.

Hydrogen Fuel Cell

A hydrogen fuel cell is a remarkable kind of fuel cell that offers a clean way of producing power. It essentially involves electrochemical reactions between hydrogen and oxygen to produce electricity, water, and a small amount of heat. The reaction is simple yet effective.

Fuel cells differ from conventional combustion-based technologies in that they directly convert the chemical energy of hydrogen into electricity without burning hydrogen. Here's how they work:

• Hydrogen gas enters the fuel cell at the anode side, where it splits into protons and electrons.


• The electrons travel through an external circuit, providing electrical power before they return to the fuel cell.


• Protons pass through a membrane to the cathode side of the fuel cell.


• At the cathode, protons, electrons, and oxygen combine to form water, which is the only emission from this reaction.

The process is highly efficient and produces water as a byproduct, making hydrogen fuel cells an environmentally friendly option compared to fossil fuels. While hydrogen fuel cells are efficient and clean, challenges remain. Hydrogen storage, infrastructure for widespread use, and cost are the primary hurdles to overcome for broader adoption of this technology. But as technology advances, hydrogen fuel cells have the potential to become a mainstream energy source.

Redox Reactions

Among the various types of chemical reactions, redox reactions stand out due to their fundamental role in energy transformations. A redox reaction is a combination of two processes: reduction and oxidation. It drives much of the changes in the chemical world due to the movement of electrons from one reactant to another.

In the context of fuel cells, redox reactions are particularly significant. They are the backbone reactions that allow fuel cells to generate electricity. The neutral equation for a basic understanding of a redox reaction is:

• One substance (oxidizing agent) gets reduced by gaining electrons.


• Another substance (reducing agent) gets oxidized by losing electrons.

In a hydrogen fuel cell, **hydrogen acts as the reducing agent**. It loses electrons (is oxidized) at the anode and generates electricity. **Oxygen serves as the oxidizing agent**, gaining those electrons at the cathode, which results in the formation of water. Understanding redox reactions involves breaking down the movement of electrons and tracking oxidation states. A grasp of these reactions is fundamental not only for studying fuel cells but also for many biological and industrial processes where electron transfer is crucial. This knowledge is essential, whether dealing with the electricity generation in batteries or the metabolism happening in our bodies.