Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following will impart highest specific impulse? (a) \(\mathrm{NH}_{3}+\mathrm{F}_{2}\) (b) \(\mathrm{H}_{2}+\mathrm{O}_{2}\) (c) Alcohol \(+\mathrm{O}_{2}\) (d) \(\mathrm{H}_{2}+\mathrm{F}_{2}\)

Step-by-Step Solution

Answer

(d) H_2 + F_2

Step 1: Understanding Specific Impulse

Specific impulse is a measure of how effectively a rocket propellant generates thrust, defined as the thrust per unit of propellant flow rate. It is usually measured in seconds. A higher specific impulse means better efficiency.

Step 2: Analysing the Options

Each option is a chemical propellant pair. We need to analyze their efficiency based on the exhaust velocity they can provide, given the energy released during the combustion is converted into kinetic energy of exhaust gases.

Step 3: Option (a): NH_3 + F_2

Ammonia (NH_3) and fluorine (F_2) react to produce NF_3, which has a moderately high specific impulse due to a relatively high energy release but heavier exhaust molecules compared to hydrogen-based fuels.

Step 4: Option (b): H_2 + O_2

Hydrogen (H_2) and oxygen (O_2) burn to form water vapor, releasing a large amount of energy. The exhaust (water vapor) is light, which usually results in a high specific impulse.

Step 5: Option (c): Alcohol + O_2

Alcohol (CH_3OH or similar) oxidizes with oxygen. Though it releases substantial energy, the exhaust gases are heavier than hydrogen combustion products, which might reduce efficiency slightly in terms of specific impulse.

Step 6: Option (d): H_2 + F_2

Hydrogen (H_2) and fluorine (F_2) react to produce hydrofluoric acid (HF) vapor. This reaction releases an extremely high amount of energy, and the exhaust is relatively light, offering a very high specific impulse.

Step 7: Conclusion

Upon analyzing all the options, H_2 with F_2 (option d) will impart the highest specific impulse due to the extremely high energy release and lighter exhaust gases.

Key Concepts

Rocket Propellants

Rocket propellants are the fuel and oxidizer used in rockets to produce thrust. The choice of propellant is crucial as it directly impacts the efficiency and performance of a rocket. Generally, propellants are categorized into several types, with liquid and solid being the most common.
Liquid propellants, like hydrogen and oxygen, are stored in a liquid state and mixed in the combustion chamber to produce thrust. Solid propellants, used in some rockets, are pre-mixed and stored as a single solid block.
Factors affecting the choice of propellant include:

• Specific impulse: The efficiency measure of a propellant, represented as thrust per unit of propellant weight.
• Storage conditions: Some fuels require cryogenic temperatures, while others can be stored at room temperature.
• Cost and availability: Determines the economic feasibility of a propellant.
• Safety: Involves the risk of handling and using certain propellants like fluorine.

Choosing the right propellant is essential for optimizing the performance of a rocket, ensuring it reaches its intended speed and altitude.

Chemical Reactions in Propulsion

Chemical reactions in propulsion are fundamental to the operation of rocket engines. These reactions occur when fuel and oxidizer are mixed and ignited in the combustion chamber, producing hot gases. These gases quickly expand, exiting the rocket engine and generating thrust.
The energy released during these reactions is crucial for determining the effectiveness of the propellant mixture. Key factors include:

• Energy density: The amount of energy released per unit mass of fuel. Higher energy density means more thrust.
• Reaction rate: A faster reaction rate results in quicker thrust production.
• Exhaust velocity: Higher exhaust velocities result from efficient conversion of energy to kinetic energy of gases, enhancing thrust.

In rocket propulsion, chemical reactions are designed to maximize energy release and transfer it efficiently to the rocket, propelling it forward. By carefully selecting and combining fuels, engineers can optimize these reactions for better rocket performance and specific impulse.

Combustion Energy Release

Combustion energy release is a critical aspect of rocket propulsion. In simple terms, it is the energy set free when a fuel burns with an oxidizer. This energy is released in the form of heat and converted to kinetic energy, accelerating the exhaust gases through the rocket nozzle to produce thrust.
Factors influencing combustion energy release include:

• Chemical composition of propellants: Different fuels and oxidizers release varying amounts of energy based on their molecular bonds.
• Combustion temperature: Higher temperatures generally mean more energy is carried by the gases.
• Thermal efficiency: The ability to convert heat into kinetic energy.
  

In the context of the specific impulse, maximizing combustion energy release directly enhances the efficiency of a rocket. For example, hydrogen and fluorine release a significant amount of energy compared to other propellants, resulting in higher specific impulse and more efficient propulsion. Understanding and harnessing combustion energy release is crucial for developing powerful and effective rocket engines.