Problem 125 - Chemistry: An Atoms-Focused Approach

Question

Rocket Fuel Hydrazine, \(\mathrm{NH}_{2} \mathrm{NH}_{2},\) fueled the spacecraft that delivered the Mars rover Curiosity to the planet's surface in 2012 a. Draw the Lewis structure of hydrazine. b. What is the hybridization of its \(\mathrm{N}\) atoms? c. Is hydrazine a polar compound? Explain why or why not.

Step-by-Step Solution

Answer

Answer: The nitrogen atoms in hydrazine have sp hybridization. Hydrazine is a nonpolar compound because it does not have a net dipole moment.

Step 1: Find the total number of valence electrons in hydrazine

Hydrazine contains two nitrogen (N) and four hydrogen (H) atoms. Nitrogen has 5 valence electrons and hydrogen has 1 valence electron. So the total number of valence electrons in hydrazine is (2 * 5) + (4 * 1) = 14.

Step 2: Draw the basic structure of hydrazine

Connect the two nitrogen atoms together and connect each nitrogen atom to two hydrogen atoms. The structure should look like this: H-N-N-H.

Step 3: Distribute the remaining valence electrons

Each nitrogen atom should be surrounded by 8 electrons to achieve the full octet. We have already used 4 electrons to connect the atoms. The remaining 10 electrons should be distributed as lone pairs around the nitrogen atoms. Each nitrogen atom will have 3 lone pairs and 1 single bond. The Lewis structure of hydrazine looks like this: H | N - N |_ _|_

Step 4: Determine the hybridization of the nitrogen atoms in hydrazine

Each nitrogen atom in the hydrazine has one sigma (σ) bond and one lone pair around it. This means that it has two regions of electron density. The hybridization of an atom is determined by the number of regions of electron density around it. In this case, both N atoms have sp hybridization because there are two regions of electron density around each N atom (1 σ bond and 1 lone pair).

Step 5: Assess if hydrazine is a polar compound

A compound is considered polar if it has a net dipole moment. Each N-H bond in hydrazine exhibits some difference in electronegativity, making them polar bonds. However, due to the symmetric nature of the hydrazine molecule (both the N-H bonds are oriented opposite to each other), the dipole moment of these bonds cancels out. Thus, hydrazine is a nonpolar compound because it does not have a net dipole moment.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and they play a crucial role in chemical bonding. In hydrazine \( \mathrm{NH}_{2} \mathrm{NH}_{2} \), each nitrogen atom contributes 5 valence electrons and each hydrogen atom contributes 1 valence electron. This makes a total of 14 valence electrons for the entire molecule.
Understanding the concept of valence electrons is essential because they determine how atoms will bond to form a molecule. In Lewis structures, valence electrons are represented as dots around the element symbols. These structures help visualize both the distribution of electrons in a molecule and the arrangement of atoms.

• Key Points:
• Nitrogen has 5 valence electrons.
• Hydrogen has 1 valence electron.
• Hydrazine has a total of 14 valence electrons (2 nitrogen and 4 hydrogen).

Thus, understanding how to distribute these electrons is critical in constructing Lewis structures, and it provides insights into the stability and reactivity of the molecule.

Hybridization

Hybridization is a concept that describes the mixing of atomic orbitals to form new hybrid orbitals. These hybrid orbitals are used to bond atoms in a molecule. In the case of hydrazine \( \mathrm{NH}_{2} \mathrm{NH}_{2} \), each nitrogen is connected to another nitrogen and two hydrogen atoms.
The hybridization of each nitrogen atom in hydrazine is \( \text{sp} \). This is because each nitrogen atom has two regions of electron density: one for the sigma bond it forms with the other nitrogen and one for the lone pair of electrons it holds.

• Key Characteristics of sp Hybridization:
• Involves mixing one s orbital and one p orbital.
• Results in two hybrid orbitals.
• Examples include \( \sigma \) bonding and lone pairs.

Hybridization helps predict the geometry of the molecule. In this case, it suggests a linear arrangement between each nitrogen atom and its bonded atoms, allowing for a specific spatial distribution of electrons.

Polar Compounds

A polar compound is a type of chemical compound containing bonds that have unequal sharing of electrons, which results in a dipole moment. Hydrazine \( \mathrm{NH}_{2} \mathrm{NH}_{2} \) features N-H bonds, which are polar because of the difference in electronegativity between nitrogen and hydrogen.
The concept of polarity in a molecule is determined by both the individual bond polarities and the overall shape of the molecule. In hydrazine, the asymmetric arrangement of these polar N-H bonds contributes to the molecule's polarity.

• Key Points Regarding Polarity:
• A polar bond occurs when there is an electronegativity difference of more than 0.4.
• The molecule's shape plays a critical role in determining overall polarity.
• Molecules with symmetric shapes and evenly distributed polar bonds are generally nonpolar.

In summary, hydrazine is a polar molecule due to the non-cancellation of the polar N-H bonds, resulting in an overall net dipole moment.