Problem 106 - Chemistry The Science in Context

Question

Before settling on hydrogen as the fuel of choice for space vehicles, several other fuels were explored, including hydrazine, \(\mathrm{N}_{2} \mathrm{H}_{4},\) and pentaboranc, \(\mathrm{B}_{5} \mathrm{H}_{11} .\) Both are gases under conditions in space. a. Write balanced chemical equations for the combustion of hydrazine and pentaborane given that the products in addition to water are \(\mathrm{NO}_{2}(g)\) and \(\mathrm{B}_{2} \mathrm{O}_{3}(s),\) respectively. b. What ratios of partial pressures of hydrazine to oxygen and pentaborane to oxygen are needed for these reactions?

Step-by-Step Solution

Answer

Answer: The ratios of partial pressures required for the combustion of hydrazine to oxygen are 2:3, and for the combustion of pentaborane to oxygen are 1:3.

Step 1: a. Balanced chemical equations for the combustion of hydrazine and pentaborane

To balance the chemical equations for the combustion of hydrazine and pentaborane, we have to make sure that the number of atoms on both sides of the equation is equal. For hydrazine: N2H4 (g) + O2 (g) -> NO2 (g) + H2O (g) Balanced equation: 2 N2H4 (g) + 3 O2 (g) -> 4 NO2 (g) + 4 H2O (g) For pentaborane: B5H11 (g) + O2 (g) -> B2O3 (s) + H2O (g) Balanced equation: 2 B5H11 (g) + 6 O2 (g) -> 5 B2O3 (s) + 11 H2O (g)

Step 2: b. Ratios of partial pressures of hydrazine to oxygen and pentaborane to oxygen

Now that we have the balanced equations, we can find the ratios of partial pressures needed for each reaction. For hydrazine (N2H4) to oxygen (O2) ratio: From the balanced equation, we have: 2 N2H4 (g) + 3 O2 (g) -> 4 NO2 (g) + 4 H2O (g) So, the needed ratio of partial pressures is: N2H4: O2 = 2:3 For pentaborane (B5H11) to oxygen (O2) ratio: From the balanced equation, we have: 2 B5H11 (g) + 6 O2 (g) -> 5 B2O3 (s) + 11 H2O (g) So, the needed ratio of partial pressures is: B5H11: O2 = 2:6 = 1:3 In conclusion, the ratios of partial pressures for the combustion of hydrazine to oxygen are 2:3, and for the combustion of pentaborane to oxygen are 1:3.

Key Concepts

Combustion Reactions

In chemistry, a combustion reaction is a type of chemical reaction where a substance reacts with oxygen gas (\(O_2\)) to release energy in the form of heat and light. Combustion reactions are crucial in various applications, from powering vehicles to everyday cooking.
These reactions typically involve a hydrocarbon or similar fuel, and produce common products like carbon dioxide (\(CO_2\)) and water (\(H_2O\)). In scientific contexts, understanding these reactions helps in developing efficient fuel systems.
The key components of a combustion reaction include:

• Fuel: The substance that burns, such as hydrazine (\(N_2H_4\)) or pentaborane (\(B_5H_{11}\)).
• Oxygen: The oxidizing agent that allows combustion to occur.
• Products: Often involving compounds like water, carbon dioxide or other oxides.
• Energy Release: Often observed as flames or heat.

During combustion, chemical bonds in the reactants break, and new bonds form in the products, releasing energy as a result. In our case, hydrazine and pentaborane react with oxygen to form specific products: \(NO_2\) and \(B_2O_3\), respectively.

Balancing Chemical Equations

Balancing chemical equations is like solving a puzzle by ensuring that the number of atoms for each element is the same on both sides of the equation. This process requires careful attention to the quantities of each reactant and product.
To balance a chemical equation:

• Identify each element present in the reactants and products.
• Count the number of atoms for each element in both reactants and products.
• Use coefficients to equalize the number of atoms of each element on both sides of the equation.
• Ensure that all coefficients are in the smallest whole number ratio.

As illustrated by the exercise, the combustion of hydrazine (\(N_2H_4\)) requires a balance involving two molecules of hydrazine and three molecules of oxygen to yield four molecules of \(NO_2\) and four molecules of water. Similarly, balancing pentaborane (\(B_5H_{11}\)) involves using two molecules of pentaborane and six molecules of oxygen to produce five molecules of \(B_2O_3\) and eleven molecules of water.
This balance ensures the law of conservation of mass is satisfied in a chemical reaction.

Space Propulsion Fuels

Space propulsion fuels are specialized substances used to power rockets and spacecraft. These fuels need to exhibit high energy efficiency and stability under extreme conditions. Hydrazine, a commonly used fuel, provides several advantages:

• Stability and ease of handling in liquid form.
• High energy content and specific impulse, crucial for effective propulsion.

Pentaborane, although explored as a potential fuel, presented challenges with toxicity and handling safety, leading to its limited use. Instead, hydrogen has become the popular choice, due to:

• High energy efficiency, with the ability to produce potent thrust.
• Environmental friendliness, producing only water as an exhaust.

Space fuels require specific considerations, including:

• Compatibility with other rocket components.
• Efficiency in fuel-to-thrust conversion to minimize weight and maximize payload.

The focus on different materials, such as hydrazine and pentaborane, reflects the ongoing search for the optimal balance between performance and safety in space travel.