Question

Hydrazine \(\left(\mathrm{H}_{2} \mathrm{NNH}_{2}\right)\), hydrogen peroxide (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 common structural property of hydrazine, hydrogen peroxide, and water is the presence of hydrogen bonding. In hydrazine, hydrogen bonds occur between nitrogen and hydrogen; in hydrogen peroxide, it occurs between oxygen and hydrogen; and in water, it occurs between oxygen and hydrogen. This hydrogen bonding leads to strong intermolecular forces between the molecules, resulting in high surface tension.

Step-by-step solution

Drawing Lewis Structure of Hydrazine

Hydrazine (\(\mathrm{H}_{2} \mathrm{NNH}_{2}\)) has 8 valence electrons: 1 from each hydrogen, and 5 from each nitrogen. To draw its Lewis structure: 1. Place the two nitrogen atoms in the center and connect them with a single bond. 2. Add two hydrogen atoms to each nitrogen and connect them with single bonds. 3. Add lone pair electrons to the nitrogen atoms, such that both nitrogen atoms have complete octets. The Lewis structure of hydrazine is thus: H - N - N - H | | H H

Drawing Lewis Structure of Hydrogen Peroxide

Hydrogen peroxide (HOOH) has 12 valence electrons: 1 from each hydrogen and 6 from each oxygen. To draw its Lewis structure: 1. Connect two oxygen atoms with a single bond. 2. Attach one hydrogen atom to each oxygen atom with single bonds. 3. Add lone pairs to oxygen atoms, ensuring that both have complete octets. The Lewis structure of hydrogen peroxide is: H - O - O - H | |

Drawing Lewis Structure of Water

The water molecule (\(\mathrm{H}_{2} \mathrm{O}\)) has 8 valence electrons: 1 from each hydrogen and 6 from the oxygen. To draw its Lewis structure: 1. Place the oxygen atom in the center. 2. Connect the two hydrogen atoms to the oxygen atom with single bonds. 3. Add lone pairs to the oxygen atom, ensuring it has a complete octet. The Lewis structure of water is: O | \ H H

Identifying Common Structural Property

We can see that in all three molecules, there is a presence of hydrogen bonding. In hydrazine, the hydrogen bonding is between nitrogen and hydrogen; in hydrogen peroxide, it is between oxygen and hydrogen; and in water, it is also between oxygen and hydrogen.

Explaining High Surface Tensions

The common structural property, hydrogen bonding, accounts for high surface tension because hydrogen bonds are strong intermolecular forces that create a strong attraction between molecules. This strong attraction between the molecules leads to high surface tension, as it requires more energy to separate the molecules at the surface.

Key concepts

Lewis Structures

Lewis structures are diagrams that represent the bonds between atoms in a molecule, as well as any lone pairs of electrons. They help show how atoms are arranged and bonded together.
Each line represents a bond, which usually consists of two electrons.
The main goal of drawing Lewis structures is to ensure that each atom satisfies its valence, following the octet rule for most elements. The steps involve:

• Counting total valence electrons available for bonding.
• Arranging atoms and forming bonds to satisfy the octet rule.
• Adding lone pairs where necessary to complete the octets.

In compounds like hydrazine, hydrogen peroxide, and water, Lewis structures reveal that atoms are bonded in specific formations, such as single or double bonds, with strong hydrogen bonds affecting their physical properties.

Hydrogen Bonding

Hydrogen bonding is a unique type of dipole-dipole attraction that occurs among molecules containing hydrogen atoms bonded to highly electronegative atoms, such as nitrogen or oxygen.
This type of bonding greatly affects the physical properties of substances. Key features of hydrogen bonding include:

• It occurs only with hydrogen bonded to nitrogen, oxygen, or fluorine due to their high electronegativity.
• These bonds are generally stronger than other dipole-dipole forces but weaker than covalent and ionic bonds.

In hydrazine, hydrogen bonds occur between nitrogen and hydrogen, in hydrogen peroxide and water, between oxygen and hydrogen.
This strong intermolecular force plays a crucial role in the high surface tensions observed in these substances.

Surface Tension

Surface tension is a physical phenomenon where the surface of a liquid is under tension, making it behave as if it's covered with a stretched elastic membrane. This effect is primarily due to intermolecular forces within the liquid. For example, in water, strong hydrogen bonds pull the molecules tightly together, forming a surface that requires considerable energy to break. Factors affecting surface tension include:

• The strength of the intermolecular forces: Higher intermolecular forces, like hydrogen bonds, increase surface tension.
• Temperature: Generally, as temperature increases, surface tension decreases.

In substances like water, hydrogen peroxide, and hydrazine, their high surface tension is attributed to the strong hydrogen bonds that necessitate higher energy to deform or break the surface.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial to forming bonds with other atoms. These electrons determine the chemical properties and reactivity of an element. Key points about valence electrons include:

• They are responsible for the formation of chemical bonds, including covalent, ionic, and metallic bonds.
• The number of valence electrons influences an atom’s bonding behavior, usually striving to achieve a stable electron configuration like a noble gas.

When drawing Lewis structures or assessing molecular behavior, the count and arrangement of valence electrons are essential. For example, in hydrazine ( ext{H}_{2} ext{NNH}_{2} ), hydrogen peroxide ( ext{HOOH} ), and water ( ext{H}_{2} ext{O} ), knowing the valence electrons helps understand bonding and molecular geometry, impacting properties like surface tension.