Problem 22 - Chemistry The Central Science In Si Units

Question

Hydrazine \(\left(\mathrm{N}_{2} \mathrm{H}_{4}\right)\) and dinitrogen tetroxide \(\left(\mathrm{N}_{2} \mathrm{O}_{4}\right)\) form a self-igniting mixture that has been used as a rocket propellant. The reaction products are \(\mathrm{N}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\). (a) Write a balanced chemical equation for this reaction. (b) What is being oxidized, and what is being reduced? (c) Which substance serves as the reducing agent and which as the oxidizing agent?

Step-by-Step Solution

Answer

The balanced equation is \(\mathrm{N}_2\mathrm{H}_4 + \mathrm{N}_2\mathrm{O}_4 \rightarrow 3\mathrm{N}_2 + 4\mathrm{H}_2\mathrm{O}\). Nitrogen in \(\mathrm{N}_2\mathrm{H}_4\) is oxidized, and in \(\mathrm{N}_2\mathrm{O}_4\), it is reduced. \(\mathrm{N}_2\mathrm{H}_4\) is the reducing agent, and \(\mathrm{N}_2\mathrm{O}_4\) is the oxidizing agent.

Step 1: Write the Unbalanced Equation

To start, identify the reactants and products from the problem statement. The reactants are hydrazine \(\mathrm{N}_2\mathrm{H}_4\) and dinitrogen tetroxide \(\mathrm{N}_2\mathrm{O}_4\), and the products are nitrogen \(\mathrm{N}_2\) and water \(\mathrm{H}_2\mathrm{O}\). Write the unbalanced chemical equation: \[ \mathrm{N}_2\mathrm{H}_4 + \mathrm{N}_2\mathrm{O}_4 \rightarrow \mathrm{N}_2 + \mathrm{H}_2\mathrm{O} \]

Step 2: Balance the Chemical Equation

Start by balancing the number of nitrogen atoms. Both sides already have four nitrogen atoms. Balance the hydrogen atoms next: 4 hydrogen atoms in hydrazine will form 2 water molecules. Finally, balance the oxygen atoms: 2 \(\mathrm{H}_2\mathrm{O}\) have 2 oxygen atoms, matching the 4 oxygen atoms from \(\mathrm{N}_2\mathrm{O}_4\). The balanced equation is: \[ \mathrm{N}_2\mathrm{H}_4 + \mathrm{N}_2\mathrm{O}_4 \rightarrow 3\mathrm{N}_2 + 4\mathrm{H}_2\mathrm{O} \]

Step 3: Determine Oxidation States

Assess the oxidation states in the reaction. In \(\mathrm{N}_2\mathrm{H}_4\), nitrogen typically has an oxidation state of -2, and hydrogen is +1. In \(\mathrm{N}_2\mathrm{O}_4\), nitrogen is +4, and oxygen is -2. In the products \(\mathrm{N}_2\) and \(\mathrm{H}_2\mathrm{O}\), nitrogen is 0, and hydrogen/oxygen remain +1/-2.

Step 4: Identify Oxidized and Reduced Elements

Compare initial and final oxidation states: Nitrogen in \(\mathrm{N}_2\mathrm{H}_4\) changes from -2 to 0 (oxidation), and nitrogen in \(\mathrm{N}_2\mathrm{O}_4\) changes from +4 to 0 (reduction). Thus, in \(\mathrm{N}_2\mathrm{H}_4\), nitrogen is being oxidized, and in \(\mathrm{N}_2\mathrm{O}_4\), nitrogen is being reduced.

Step 5: Identify Reducing and Oxidizing Agents

The substance being oxidized is the reducing agent, and the one being reduced is the oxidizing agent. Therefore, \(\mathrm{N}_2\mathrm{H}_4\), which donates electrons when nitrogen is oxidized, is the reducing agent, and \(\mathrm{N}_2\mathrm{O}_4\), which accepts electrons when nitrogen is reduced, is the oxidizing agent.

Key Concepts

Chemical Equation Balancing

Balancing chemical equations is essential for understanding how substances transform during a chemical reaction. In this context, balancing involves making sure that the number of atoms for each element is the same on both sides of the equation. This process respects the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction. To begin, write down the unbalanced equation noting all reactants and products: \[ \mathrm{N}_2\mathrm{H}_4 + \mathrm{N}_2\mathrm{O}_4 \rightarrow \mathrm{N}_2 + \mathrm{H}_2\mathrm{O} \] Next, focus on one element at a time to achieve balance. Start with nitrogen, then move to hydrogen, and finally oxygen. For nitrogen, there are four atoms on both sides, so it's initially balanced. Adjust hydrogen by ensuring the four hydrogens in \(\mathrm{N}_2\mathrm{H}_4\) create two water molecules:

• \(\mathrm{N}_2\mathrm{H}_4 \rightarrow 2\, \mathrm{H}_2\mathrm{O}\)

This leaves two oxygen atoms from \(\mathrm{H}_2\mathrm{O}\) balanced with the four from \(\mathrm{N}_2\mathrm{O}_4\). Thus, the equation becomes: \[ \mathrm{N}_2\mathrm{H}_4 + \mathrm{N}_2\mathrm{O}_4 \rightarrow 3\, \mathrm{N}_2 + 4\, \mathrm{H}_2\mathrm{O} \] Balancing equations ensures the correct stoichiometric relationships allowing us to quantify reactants and products involved.

Oxidation and Reduction

Oxidation and reduction (redox) reactions are processes where the oxidation states of atoms change through the transfer of electrons. An increase in oxidation state signifies oxidation, whereas a decrease signifies reduction. In the given reaction between hydrazine and dinitrogen tetroxide, we must determine the changes in oxidation states to distinguish between oxidized and reduced elements.

• Nitrogen in \(\mathrm{N}_2\mathrm{H}_4\) initially has an oxidation state of \(-2\). In \(\mathrm{N}_2\), this changes to 0, indicating that nitrogen is oxidized.
• Simultaneously, nitrogen in \(\mathrm{N}_2\mathrm{O}_4\) has a starting oxidation state of \(+4\), which also shifts to 0. Here, nitrogen is reduced.

In a redox reaction, the electron donor gets oxidized and is termed the reducing agent, while the electron acceptor gets reduced and is called the oxidizing agent. Therefore, in this scenario:

• \(\mathrm{N}_2\mathrm{H}_4\) is the reducing agent.
• \(\mathrm{N}_2\mathrm{O}_4\) is the oxidizing agent.

Understanding these processes is crucial for analyzing chemical reactions in both laboratory and industrial settings.

Rocket Propellants

Rocket propellants are substances used in rockets to produce thrust via the expulsion of gases. These propellants consist of various chemical compounds that undergo rapid combustion. Among them, a mixture of hydrazine \(\mathrm{N}_2\mathrm{H}_4\) and dinitrogen tetroxide \(\mathrm{N}_2\mathrm{O}_4\) is well-known for its self-igniting properties. "Hypergolic" fuels ignite spontaneously upon contact with each other and do not require an external ignition source. This characteristic makes them highly efficient and reliable in rocket technology.

• Hydrazine acts as the fuel, while dinitrogen tetroxide functions as the oxidizer.
• This combination is often used in situations demanding immediate and precise engine response, such as in space missions for launch, in-orbit maneuvers, or landing operations.

Despite their efficiency, these compounds are toxic and require careful handling. Their application in rocketry exemplifies how chemical knowledge is harnessed to achieve technological advancements. The chemistry of these reactions is crucial for ensuring the safe and efficient design of propulsion systems. Providing thrust reliably and accurately is essential for the success of rocket missions.