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

Step-by-Step Solution

Answer

The reaction used to make a fuel cell is (c).

Step 1: Understand Fuels Cells

Fuel cells are devices that convert chemical energy from a fuel into electricity through a chemical reaction with oxygen (or another oxidizing agent). The most common fuel cell reaction involves hydrogen and oxygen as the reactants.

Step 2: Identify Hydrogen and Oxygen Reaction

The production of electricity in many fuel cells involves the reaction of hydrogen and oxygen to form water. This is a key indicator of a fuel cell reaction: \[ 2 \mathrm{H}_{2}(\mathrm{~g}) + \mathrm{O}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(l) \]

Step 3: Compare Given Reactions

Compare the given reactions with the typical fuel cell reaction. Only option (c), which is \( 2 \mathrm{H}_{2}(\mathrm{~g}) + \mathrm{O}_{2}(\mathrm{~g}) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(l) \), represents the hydrogen-oxygen reaction commonly used in fuel cells.

Step 4: Conclude the Correct Reaction

Given the comparison, option (c) is the reaction used in typical fuel cells, where hydrogen reacts with oxygen to form water, producing electricity through this reaction.

Key Concepts

Hydrogen-Oxygen Reaction

The hydrogen-oxygen reaction is central to the operation of many fuel cells. When hydrogen (\( \mathrm{H}_2 \) ) gas combines with oxygen (\( \mathrm{O}_2 \) ) gas, the chemical reaction produces water (\( \mathrm{H}_2 \mathrm{O} \) ). This reaction can be summarized by the equation:

\[2 \mathrm{H}_2(\mathrm{~g}) + \mathrm{O}_2(\mathrm{~g}) \longrightarrow 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})\]
This simple yet powerful reaction is widely used due to its efficiency and neatness. Not only does it create energy, it only leaves water as a byproduct, making it an environmentally friendly option.

The reaction within a fuel cell happens in a controlled environment, allowing the energy released to be harnessed as electricity. This reaction is exothermic, meaning it releases energy in the form of heat as well.

Electrochemical Cell Operation

An electrochemical cell is a device where chemical reactions occur and result in the flow of electrons, creating electricity. In a hydrogen fuel cell, which is a type of electrochemical cell, this process hinges on the hydrogen-oxygen reaction.

The cell consists of two electrodes – the anode and the cathode:

• Hydrogen gas arrives at the anode where it splits into protons and electrons.
• The electrons travel through an external circuit, generating an electric current.
• Meanwhile, protons traverse through an electrolyte to the cathode.
• Once at the cathode, protons, electrons, and oxygen unite to form water.

This smooth operation of splitting hydrogen and reuniting it with oxygen is what drives the practicality of fuel cells. All materials work together to continuously generate electricity as long as hydrogen and oxygen are supplied.

Chemical Energy Conversion

Fuel cells excel at chemical energy conversion, where they transform the chemical energy stored in hydrogen into usable electrical energy. This is fundamentally different from combustion engines, which burn fuel to produce heat and then convert that heat into mechanical energy.

Why are fuel cells so effective?

• Direct conversion: Chemical energy is directly converted into electrical energy with high efficiency.
• Minimal moving parts: This results in lower maintenance and reduced mechanical wear.
• Clean byproduct: The only waste produced is water, making them an eco-friendly alternative.

Fuel cells offer a direct, efficient way to convert chemical energy into electricity, making them an attractive option for sustainable energy solutions. As the demand for cleaner energy rises, the role of chemical energy conversion in fuel cells is becoming increasingly significant.