Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The pair that cannot be used as a rocket propellant is (b): Liquid O2 + Liquid Ar.

Step 1: Understand Rocket Propellants

Rocket propellants must react energetically to produce thrust. Typically, propellants consist of a fuel (that combusts) and an oxidizer (that facilitates the combustion). Certain combinations of chemicals are known to be effective rocket propellants.

Step 2: Examine the Given Pairs

Review each pair to determine if it includes an oxidizer and a fuel. This is necessary for the pair to potentially be used as a rocket propellant.

Step 3: Analyze Pair (a): Liquid H2 + Liquid O2

This pair consists of hydrogen acting as a fuel and oxygen as an oxidizer. They react energetically, producing water and significant thrust, making them a credible rocket propellant pair.

Step 4: Analyze Pair (b): Liquid O2 + Liquid Ar

This pair includes liquid oxygen, an oxidizer, but argon is an inert gas and does not combust. Thus, liquid argon cannot act as a fuel, making this pair ineffective as a rocket propellant.

Step 5: Analyze Pair (c): Liquid H2 + Liquid N2

In this pair, hydrogen could act as a fuel, but molecular nitrogen is quite stable and does not serve as an oxidizer. Both components must support a chemical reaction, which this pair does not allow.

Step 6: Analyze Pair (d): Liquid O2 + Liquid N2H4

Liquid oxygen acts as an oxidizer and hydrazine (N2H4) serves as a fuel. This pair can react energetically, making it suitable for use as a rocket propellant.

Step 7: Identify the Unsupported Pair

From the analysis, pair (b): Liquid O2 + Liquid Ar does not contain a fuel/oxidizer combination that supports combustion, making it unsuitable for use as a rocket propellant.

Key Concepts

Oxidizer and Fuel Combinations

In the context of rocket propellants, the combination of fuel and oxidizer is crucial for successful propulsion. The fuel serves as the substance that burns, while the oxidizer enables this burning in the absence of atmospheric oxygen.

Think of oxidizers like a special ingredient that helps fire catch on in space. Without an oxidizer, the fuel cannot combust properly, as it requires oxygen to burn. Active examples of these combinations include:

• Liquid hydrogen (H₂) paired with liquid oxygen (O₂), where H₂ acts as a fuel and O₂ as an oxidizer.
• Hydrazine (N₂H₄) combined with liquid oxygen, where hydrazine serves as a powerful fuel.

These combinations are defined by their reactive properties and ability to produce thrust, thus propelling the rocket. Conversely, pairings such as liquid oxygen and argon fail as propellants because argon is inert and does not combust.

Energetic Chemical Reactions

Chemical reactions in rockets are not just any reactions; they need to be highly energetic. When the fuel and oxidizer meet, they should ignite in a controlled explosion, producing gas that propels the rocket forward. This translates the reaction's energy into the motion of the rocket.

For example, liquid hydrogen and liquid oxygen combine to form water after ignition, releasing massive energy and thrust in the process. The reaction can be expressed as: \[ 2H_2 + O_2 \rightarrow 2H_2O + Energy \]

In comparison, a pair like liquid hydrogen and liquid nitrogen does not produce an energetic reaction. Nitrogen is stable and inert, which means it does not contribute to the energy needed for propulsion. Hence, identifying suitable reagents for energetic chemical reactions is imperative in rocket science.

Rocket Propellant Suitability

Suitability of a rocket propellant is determined by the efficiency and safety of the reaction it produces. A suitable rocket propellant both ignites readily and releases substantial energy without risks of instability or explosion during handling. Factors influencing this suitability are:

• The temperature and pressure conditions under which the reaction takes place.
• The ease of storing and handling propellant mixtures.
• The weight and energy output of the reaction.

Liquid hydrogen and liquid oxygen form a classic propellant pair due to their high efficiency and energy output. In contrast, using inert gases like argon with an oxidizer results in no significant energy release, rendering them unsuitable for propulsion.

Hydrazine as Fuel

Hydrazine (N₂H₄) is a widely used rocket fuel known for its high-energy content and ability to combust efficiently with available oxidizers. It is a hypergolic fuel, meaning it ignites spontaneously upon contact with an oxidizer like liquid oxygen, making it desirable for rocket propulsion systems.

Despite its benefits, hydrazine is highly toxic and requires careful handling and storage. Its ability to rapidly release gas upon ignition translates into significant thrust, paving the way for its use in engines for spacecraft and missiles.

When combined with liquid oxygen, hydrazine demonstrates a powerful reaction, making it highly suitable as a rocket propellant due to both its reactivity and the volume of energy it produces.