Problem 137 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following can act as a propellant for rockets? (a) liquid hydrogen + liquid nitrogen (b) liquid nitrogen + liquid oxygen (c) liquid hydrogen + liquid oxygen (d) liquid oxygen + liquid argon

Step-by-Step Solution

Answer

(c) liquid hydrogen + liquid oxygen

Step 1: Understanding Rocket Propellants

Rockets require fuel (often a reducing agent) and an oxidizer to produce thrust through combustion. The most common combinations use liquid hydrogen (LH2) with liquid oxygen (LOX) because they react energetically to produce water vapor, generating a lot of thrust.

Step 2: Evaluating Each Option

We'll check if each pair can combust to give thrust: (a) Liquid hydrogen + liquid nitrogen: nitrogen is inert and won't react with hydrogen. (b) Liquid nitrogen + liquid oxygen: nitrogen is inert and will not combust. (c) Liquid hydrogen + liquid oxygen: combusts quickly, producing water vapor and high thrust. (d) Liquid oxygen + liquid argon: argon is inert and won't combust.

Step 3: Identifying the Correct Pair

The correct propellant combination is one that contains a fuel and an oxidizer which react to produce thrust. Based on step 2, option (c) - liquid hydrogen + liquid oxygen - is the viable combination.

Key Concepts

Liquid Hydrogen

Liquid hydrogen, often abbreviated as LH2, is one of the most common rocket fuels used today. It is favored for its high energy content and its ability to produce significant thrust when combusted. Hydrogen is the lightest and simplest element, making it a preferred choice for space propulsion systems that require high efficiency.

Why is it used?

• **High Efficiency**: Liquid hydrogen boasts a high specific impulse, which means rockets can achieve greater speeds and travel further on less fuel.
• **Clean Reaction**: When it burns with an oxidizer like liquid oxygen, the by-product is simply water vapor, making it an environmentally friendly option for rocket fuels.
• **Low Density**: Despite its energy efficiency, liquid hydrogen is very low density, requiring large storage tanks on rockets.

To keep hydrogen in its liquid state, it must be stored at extremely low temperatures of around -423°F (-253°C). This requirement for cryogenic storage poses challenges in handling and maintaining liquid hydrogen as a rocket fuel. Despite these challenges, the efficiency and clean-burning properties make it a standout choice.

Liquid Oxygen

Liquid oxygen, also known as LOX, is the most frequently utilized oxidizer in rocketry. Its role is crucial as it provides the oxygen necessary for the combustion of rocket fuels like liquid hydrogen. Oxygen supports the combustion process, releasing energy that propels rockets.

Why choose liquid oxygen?

• **High Reactivity**: LOX is extremely reactive and can efficiently oxidize fuels, producing the thrust needed to lift rockets.
• **Readily Available**: Oxygen is abundant and can be economically sourced from the Earth's atmosphere.
• **Low Temperature Storage**: It must be stored at very low temperatures (-297°F or -183°C), which makes it compatible with cryogenic fuels like liquid hydrogen.

Besides its high reactivity, liquid oxygen's ability to combine with various fuels, especially hydrogen, makes it a versatile component in rocket propellants. Although it requires careful handling due to its cryogenic nature and reactivity, its benefits in propulsion efficiency outweigh the logistical considerations.

Combustion Reactions

Combustion reactions in rocketry are the heart of generating thrust. In simple terms, combustion is a chemical reaction where a fuel burns with an oxidizer, releasing energy in the process. For rockets, this energy manifests as a powerful force that propels them into space.

How does it work?

• **Fuel and Oxidizer Interaction:** Most commonly, rockets use a fuel, like liquid hydrogen, in conjunction with an oxidizer, such as liquid oxygen.
• **Reaction Products:** The combustion of these substances typically produces water vapor and heat.
• **Thrust Generation:** The rapid expansion of reaction products, due to the heat and pressure developed, generates thrust. This force pushes the rocket in the opposite direction.

Combustion reactions are meticulously calculated to ensure efficient use of propellants, as every drop of fuel and oxidizer is valuable due to constraints in space missions. The selection of suitable propellants and understanding of combustion principles are critical in designing rockets that can safely and effectively transport payloads beyond Earth's atmosphere.