Problem 36 - Chemistry The Central Science In Si Units

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

Hydrogen bonding causes the high surface tension in hydrazine, hydrogen peroxide, and water.

Step 1: Draw Lewis Structure for Hydrazine

To draw the Lewis structure for hydrazine (\(\mathrm{H}_{2} \mathrm{NNH}_{2}\)), start by calculating the total number of valence electrons. Nitrogen has 5 valence electrons and hydrogen has 1; thus, hydrazine has \( 2 \times 5 + 2 \times 2 = 14 \) valence electrons. Connect the nitrogen atoms with a single bond and attach two hydrogen atoms to each nitrogen atom. Use the remaining electrons to complete the octets of the nitrogen atoms.

Step 2: Draw Lewis Structure for Hydrogen Peroxide

For hydrogen peroxide (\(\mathrm{HOOH}\)), calculate the total number of valence electrons: oxygen has 6, and hydrogen has 1, so there are \( 2 \times 6 + 2 \times 1 = 14 \) valence electrons needed. Connect the two oxygen atoms with a single bond and attach a hydrogen atom to each oxygen atom. Use the remaining electrons to fill in the lone pairs on the oxygen atoms for complete octet structures.

Step 3: Draw Lewis Structure for Water

In drawing the Lewis structure for water (\(\mathrm{H}_{2} \mathrm{O}\)), the total number of valence electrons is \(6 + 2 \times 1 = 8\). Connect the oxygen atom with two hydrogen atoms using single bonds and use four electrons to complete two lone pairs on the oxygen atom, satisfying its octet.

Step 4: Identify Common Structural Feature

The common structural property of hydrazine, hydrogen peroxide, and water is the presence of polar bonds and the ability to form hydrogen bonds. In each molecule, hydrogen is covalently bonded to more electronegative atoms (nitrogen or oxygen). This leads to the formation of polar molecules capable of participating in extensive hydrogen bonding.

Step 5: Reason for High Surface Tension

The high surface tension of these compounds is primarily due to hydrogen bonding. These molecules can form strong intermolecular hydrogen bonds, which significantly increase the cohesive forces at the liquid surface, leading to high surface tension compared to other similar-weight compounds.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. These electronegative atoms pull electrons toward themselves, creating a dipole.

• The hydrogen atom becomes slightly positive, while the electronegative atom gets a slightly negative charge.
• Because opposite charges attract, the hydrogen of one molecule can attract the electronegative atom of another molecule.
• This results in a strong type of dipole-dipole attraction known as hydrogen bonding.

Hydrogen bonding is stronger than most other types of dipole-dipole attractions, but it's not as strong as covalent or ionic bonds. Nevertheless, it plays a crucial role in determining the properties of compounds, such as their boiling and melting points.

This bonding is evident in molecules like water, hydrogen peroxide, and hydrazine, where the presence of hydrogen bonds leads to notably higher surface tension.

Polar Molecules

A molecule is considered polar when it has an uneven distribution of electrons. This occurs mostly because some atoms within the molecule do not share electrons equally, leading to a separation of charges within the molecule.

• Polar molecules have a positive region and a negative region, similar to the opposite poles of a magnet.
• Examples of polar molecules include water (2O), hydrogen peroxide (HOOH), and hydrazine (H2NNH2).

The polarity of these molecules arises because hydrogen bonds to more electronegative atoms such as oxygen and nitrogen.
In a polar molecule, this polarity leads to an uneven distribution of electric charge, which is central to the molecule's behavior and ability to interact strongly with other molecules, especially through hydrogen bonding.

This polar nature explains why hydrazine, hydrogen peroxide, and water can exhibit strong intermolecular forces leading to significant surface tension effects.

Surface Tension

Surface tension is essentially a measure of the cohesive forces acting at the surface of a liquid. It's the reason some insects can "walk" on water and why droplets form.

• The molecules at the surface of a liquid experience a net inward force of attraction as they are pulled by the molecules below.
• This inward pull causes the surface to contract, much like a stretched elastic sheet.

In the case of compounds such as water, hydrogen peroxide, and hydrazine, hydrogen bonding is responsible for a very high level of surface tension.
The strength of this hydrogen bonding results in a cohesive force that is substantial, making these liquids notably more "sticky" at their surfaces compared to others of similar molecular weight.

Therefore, the remarkable surface tension of these substances can be attributed to the extensive hydrogen bonding enabled by their polar nature and structural properties.

Valence Electrons

Valence electrons are the outermost electrons of an atom, and they play a key role in bond formation and the chemical properties of elements. They are the electrons available for bonding and can either be shared in covalent bonds or transferred in ionic bonds.

• Typically, atoms aim to have a complete outer electron shell, often represented by 8 electrons, known as the octet rule.
• In hydrazine (2NNH2), hydrogen peroxide (HOOH), and water (2O), all atoms achieve stable electronic configurations by sharing valence electrons.

Understanding valence electrons helps in drawing Lewis structures, where we denote each bond as a pair of shared electrons and confirm the octet rule is satisfied for every atom involved, contributing to the molecule’s stability.

Intermolecular Forces

Intermolecular forces are forces of attraction that occur between molecules. They are the forces that hold molecules together and determine many physical properties of substances.

• Types of intermolecular forces include dispersion forces, dipole-dipole interactions, and hydrogen bonds.
• Of these, hydrogen bonds are particularly strong for polar molecules like water, hydrogen peroxide, and hydrazine.

These forces are responsible for how molecules stick together, their boiling and melting points, and their solubility in different solvents.

For the molecules in focus like water and hydrogen peroxide, the intermolecular force of hydrogen bonding creates strong attractions between molecules, resulting in higher boiling points and significant surface tension compared to non-polar substances.