Problem 36 - Chemistry: The Central Science in SI Units, Global Edition

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?

Step-by-Step Solution

Answer

The common structural property of hydrazine, hydrogen peroxide, and water is that they all involve hydrogen bonded to highly electronegative atoms: nitrogen in hydrazine, oxygen in hydrogen peroxide, and oxygen in water. The high surface tension in these substances can be attributed to the strong hydrogen bonding intermolecular forces resulting from the presence of highly electronegative atoms (N and O) covalently bonded to hydrogen atoms in their respective molecular structures.

Step 1: Draw the Lewis structures

First, we need to draw the Lewis structures for each of these compounds: 1. Hydrazine: \(\mathrm{H}_{2} \mathrm{NNH}_{2}\) 2. Hydrogen peroxide: \(\mathrm{HOOH}\) 3. Water: \(\mathrm{H}_{2} \mathrm{O}\) To draw the Lewis structures, we must consider each atom's valence electrons, placing them as shared pairs to form bonds or as lone pairs around atoms.

Step 2: Identify common structural property

Next, we look at the Lewis structures we drew previously and identify the common structural property among hydrazine, hydrogen peroxide, and water. The common structural property in these compounds is that they all involve hydrogen bonded to highly electronegative atoms: nitrogen in hydrazine, oxygen in hydrogen peroxide, and oxygen in water.

Step 3: Relate structural property to high surface tension

Finally, we need to explain how the identified common structural property might account for the high surface tension of these substances. Surface tension is the result of cohesive forces between molecules in a liquid. Hydrogen bonding is a strong type of intermolecular force, which occurs in these compounds due to the presence of highly electronegative atoms (as identified in step 2) covalently bonded to hydrogen atoms. The strong hydrogen bonding intermolecular forces result in a higher surface tension in these substances compared to others with similar molecular weights that do not have hydrogen bonding. In conclusion, the high surface tension of hydrazine, hydrogen peroxide, and water can be attributed to the presence of hydrogen bonds between the highly electronegative atoms (N and O) and hydrogen atoms in their respective molecular structures.

Key Concepts

Lewis structures

Lewis structures are a way to represent molecules and their bonds using dots and lines. These representations help us understand how atoms in a molecule are arranged and how they share electrons. In a Lewis structure, each dot represents an electron, and a line represents a pair of shared electrons, that is, a covalent bond.

To draw the Lewis structure:

• Determine the total number of valence electrons available by adding up the valence electrons of all the atoms in the molecule.
• Arrange the atoms, generally placing the least electronegative atom in the center.
• Connect the atoms with single bonds initially.
• Distribute the remaining electrons as lone pairs to satisfy the octet rule (or duet rule for hydrogen).
• If necessary, adjust bonds to form double or triple bonds to ensure all atoms have a complete outer shell.

For example, in hydrazine \[ \text{H}_2\text{NNH}_2 \]the nitrogen atoms are in the center, each bonded to each other and to the hydrogen atoms, with the remaining lone pair electrons placed around the nitrogen to fulfill the octet rule. The Lewis structures help reveal the potential for certain types of chemical behavior, such as hydrogen bonding.

surface tension

Surface tension is a fascinating property that occurs at the interface between a liquid and a gas. It relates to the cohesive forces among the molecules in the liquid. These forces tend to minimize the surface area of the liquid, making it behave as though its surface were covered with a stretched elastic membrane. This leads to phenomena like water droplets forming beads on surfaces rather than spreading out.

The high surface tension in substances like water, hydrazine, and hydrogen peroxide can be prominently attributed to hydrogen bonding. Because these substances consist of molecules with strong attractions towards one another (thanks to hydrogen bonds), they have an increased force working to keep each liquid molecule connected tightly with others. This strong cohesive force results in a higher resistance to external forces that try to break the liquid apart, hence the high surface tension.

A practical example of surface tension is when lightweight objects, like a paper clip, can float on water even though they are denser than water. This happens because the surface tension of the water creates an upward force that counters the weight of the object.

intermolecular forces

Intermolecular forces are the forces of attraction and repulsion between molecules. These forces are crucial for understanding the physical properties of substances, such as boiling points, melting points, and solubilities. Several types of intermolecular forces can act between molecules, such as dipole-dipole interactions, London dispersion forces, and hydrogen bonds.

Hydrogen Bonds
Among the strongest of these forces are hydrogen bonds, which occur specifically when hydrogen is covalently bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. These bonds form due to the significant difference in electronegativity between hydrogen and these atoms, creating a dipole where the hydrogen becomes slightly positive, and the electronegative atom becomes slightly negative. The attraction between these dipoles leads to hydrogen bonds.

For example, in water (H₂O), each oxygen atom forms hydrogen bonds with the hydrogen atoms of nearby water molecules. This network of hydrogen bonds contributes to water's high boiling point and surface tension, as well as its unique properties as a solvent.

Understanding this type of force explains why substances like water, hydrazine, and hydrogen peroxide, which rely heavily on hydrogen bonding, exhibit unique physical properties relative to their molecular weight. The presence of hydrogen bonds makes these substances much "stickier," resulting in more energy required to separate the molecules, hence influencing their various macroscopic properties.