Problem 86 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following pair cannot be used as a Rocket Propellant? (a) Liquid \(\mathrm{H}_{2}+\) Liquid \(\mathrm{O}_{2}\) (b) Liquid \(\mathrm{O}_{2}+\) Liquid \(\mathrm{Ar}\) (c) Liquid \(\mathrm{H}_{2}+\) Liquid \(\mathrm{N}_{2}\) (d) Liquid \(\mathrm{O}_{2}^{2}+\) Liquid \(\mathrm{N}_{2} \mathrm{H}_{4}\)

Step-by-Step Solution

Answer

Option (b), Liquid O₂ + Liquid Ar, cannot be used as a rocket propellant.

Step 1: Understanding Rocket Propellants

Rocket propellants require a combination of a fuel and an oxidizer. The fuel burns with the oxidizer to produce thrust. Common fuels include hydrogen, and common oxidizers include oxygen.

Step 2: Analyzing Each Option

- Option (a): Liquid H₂ as a fuel and Liquid O₂ as oxidizer is a classic rocket propellant combination. - Option (b): Liquid O₂ as an oxidizer and Liquid Ar is inert, hence they cannot react or produce thrust. - Option (c): Liquid H₂ as a fuel and Liquid N₂ (inert) cannot produce thrust, as there's no oxidizer for H₂. - Option (d): Liquid O₂ and Liquid N₂H₄ is used in some rocket propulsion systems due to its energy release.

Step 3: Determining Invalid Combinations

Identify pairs where one or both substances cannot react to produce thrust, such as combinations with inert gases like argon or nitrogen.

Step 4: Conclusion Based on Analysis

The options analyzed show that option (b) and (c) involve inert elements with no capacity to release energy when combined, but only one cannot be a rocket propellant.

Key Concepts

Fuel and Oxidizer Combination

In rocketry, a successful launch depends on the effective combination of a fuel and an oxidizer. These components are essential to create the high-energy chemical reactions necessary for propulsion. The role of the fuel is to combust, providing necessary energy, while the oxidizer ensures this combustion can occur even in the vacuum of space.

• Fuel: A substance that burns to release energy. Common examples include hydrogen (\(\text{H}_2\)).
• Oxidizer: A substance that supplies oxygen or another oxidizing agent. Oxygen (\(\text{O}_2\)) is frequently used for this purpose due to its high availability and reactivity.

Together, the right mix of fuel and oxidizer results in the release of energy in the form of thrust, propelling rockets. For example, liquid hydrogen and liquid oxygen are often paired in rocket engines because they produce significant energy when combined, allowing efficient thrust potentials and high specific impulse, which is a measure of thrust compared to fuel consumption. Innovative propellant combinations continue to be a significant focus in aerospace engineering, aiming to improve safety and efficiency.

Inert Gases

Not all gases react with fuels to produce energy in rocketry. Some are inert, meaning they do not chemically interact in ways that release energy. In the context of rocket science, this can make them ill-suited as part of a propellant system.

• Argon (\(\text{Ar}\)): A noble gas known for its lack of reactivity due to a full outer electron shell. This chemical stability makes it inert.
• Nitrogen (\(\text{N}_2\)): Another example of an inert gas, particularly in its molecular form, which does not readily participate in combustion.

Because of their inert nature, these gases are unable to act as oxidizers in a propellant system, as they do not promote the combustion of fuel. As a result, pairings like Liquid \(\text{O}_2\) and Liquid \(\text{Ar}\) or Liquid \(\text{H}_2\) and Liquid \(\text{N}_2\) are ineffective for rocketry. In some scenarios, inert gases might be used not for energy release, but for purposes such as pressurizing systems.

Chemical Reactions in Rocketry

Rockets generate thrust through rapid chemical reactions, converting potential chemical energy into kinetic energy. Understanding these reactions is crucial in developing efficient propellant systems. A key factor is the exothermic nature of most rocket combustions, meaning they release considerable amounts of heat and gas expansion upon combustion.

• Combustion Reaction: The chemical process of burning fuel in the presence of an oxidizer, producing hot gases that expand rapidly and exit the rocket nozzle to produce thrust.
• Exothermic Process: Combustion reactions in rockets are inherently exothermic, converting stored chemical energy into powerful kinetic output.

These reactions require precise conditions to maximize efficiency. The balance between fuel and oxidizer must be maintained to ensure complete combustion, avoiding wasted material and unburned fuel that might reduce efficiency. Engineers meticulously adjust ratios to achieve optimal output, taking into account various conditions like altitude and desired thrust. This attention to chemical detail in propulsion design is what pushes modern rockets to achieve incredible speeds and reach great distances.