Question

Hydrazine \(\left(\mathrm{H}_{2} \mathrm{NNH}_{2}\right)\), hydrogen peroxide \((\mathrm{HOOH})\), and water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) all have exceptionally high surface tensions compared with other substances of comparable molecular weights. (a) Draw the Lewis structures for these three compounds. (b) What structural property do these substances have in common, and how might that account for the high surface tensions?

Short answer

The Lewis structures for hydrazine (H2NNH2), hydrogen peroxide (HOOH), and water (H2O) are: H2NNH2: ``` H - N - N - H | | H H ``` HOOH: ``` H - O - O - H | | | ``` H2O: ``` O | | H H ``` The common structural property among these molecules is their ability to form hydrogen bonds due to the presence of highly electronegative atoms (N in H2NNH2, O in HOOH and H2O) bonded to hydrogen atoms. This results in stronger intermolecular forces, leading to higher surface tensions compared to other substances with similar molecular weights.

Step-by-step solution

Draw Lewis structures for H2NNH2, HOOH, and H2O

To draw the Lewis structures, we need to count the valence electrons for each atom and distribute them among bonds and lone pairs. Hydrazine (H2NNH2): - N has 5 valence electrons - H has 1 valence electron Total valence electrons: 2(5) + 4(1) = 10 + 4 = 14. Lewis structure for H2NNH2: ``` H - N - N - H | | H H ``` Hydrogen peroxide (HOOH): - O has 6 valence electrons - H has 1 valence electron Total valence electrons: 2(6) + 2(1) = 12 + 2 = 14. Lewis structure for HOOH: ``` H - O - O - H | | | ``` Water (H2O): - O has 6 valence electrons - H has 1 valence electron Total valence electrons: 6 + 2(1) = 6 + 2 = 8. Lewis structure for H2O: ``` O | | H H ```

Identify the structural property and discuss its relationship to surface tension

All three molecules, H2NNH2, HOOH, and H2O, exhibit a common structural property: they can form hydrogen bonds. In each molecule, there is a highly electronegative atom (N in H2NNH2, O in HOOH and H2O) bonded to a hydrogen atom, making the molecule polar and capable of hydrogen bonding. Hydrogen bonding leads to stronger intermolecular forces between molecules. High surface tension is a consequence of strong intermolecular forces that make it more difficult for molecules to escape the surface and enter the gas or vapor phase. In summary, all three molecules can form hydrogen bonds, which results in stronger intermolecular forces and, consequently, higher surface tensions compared to other substances of similar molecular weights.

Key concepts

Lewis Structures

Understanding Lewis structures is fundamental in chemistry as they provide a simple way to represent molecules and understand their bonding. A Lewis structure shows how valence electrons are distributed among atoms in a molecule.
To draw a Lewis structure, start by counting all the valence electrons for each atom involved. Distribute these electrons to form bonds between atoms and satisfy the octet rule, which states that atoms tend to have eight electrons in their outer shell.
For hydrazine (\( ext{H₄} \text{N₂}\)), nitrogen has five valence electrons while hydrogen has one. You create a bridge of electrons that form bonds between N atoms and H atoms. Similarly, hydrogen peroxide (\( ext{HOOH}\)) follows a similar procedure with oxygen's 6 valence electrons bonding with 1 of hydrogen.
Finally, water (\( ext{H₂O}\)), with oxygen's own six valence electrons used to bond with hydrogen atoms, making it a molecule with strong polar bonds.
Lewis structures not only illustrate the connectivity of the atoms but also help in predicting the molecule's shape, reactivity, and properties like surface tension.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction between molecules, which occurs when a hydrogen atom covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine is near another electronegative atom.
This bond is much stronger than a typical dipole-dipole interaction because of the high electronegativity difference creating a significant dipole.

• In \( ext{H₂NNH₂}\) (hydrazine), each nitrogen atom in the structure shares electrons with hydrogen atoms, allowing for potential hydrogen bonding due to nitrogen's electronegativity.
• \( ext{HOOH}\) (hydrogen peroxide) features oxygen atoms that engage in extensive hydrogen bonding, heightening its intermolecular attractions.
• In \( ext{H₂O}\) (water), the classic hydrogen bonds occur because each water molecule can form bonds with up to four others, leading to the unique properties of water, including its high surface tension.

Hydrogen bonds are essential for determining the characteristics of many compounds, including their phase at room temperature and their interactions in biological systems such as DNA structure and protein folding.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles like atoms, molecules, or ions. They are crucial in determining the physical properties of substances.
Hydrogen bonding is one such force, but there are others:

• London Dispersion Forces (LDF): These are weak forces arising from temporary dipoles induced in atoms or molecules, significant in larger and non-polar molecules.
• Dipole-Dipole Forces: These occur in molecules with permanent dipoles, and are weaker than hydrogen bonds but stronger than London forces.

For \( ext{H₂NNH₂}\), \( ext{HOOH}\), and \( ext{H₂O}\), hydrogen bonding is the prevailing intermolecular force, leading to their exceptionally high surface tension compared to other molecules of similar weight.
These strong intermolecular forces make it energetically more difficult for the molecules to separate from one another, affecting the melting and boiling points of these substances and their behavior in various states of matter. Understanding the different types of intermolecular forces helps to comprehend the complexity of molecular interactions and solubility.