Question

Pick out the property which is not relevant, In a molecule, hydrogen bond results in (1) molecule association (2) abnormal boiling points (3) high solubility value (4) high Van der Waal's forces

Short answer

High Van der Waals forces.

Step-by-step solution

Understand Hydrogen Bonds

Hydrogen bonds are weak bonds that form between a hydrogen atom in one molecule and an electronegative atom (such as oxygen or nitrogen) in another molecule. These bonds significantly affect the physical properties of substances.

Evaluate Each Property

Evaluate how hydrogen bonds affect each listed property: (1) Molecule association: Hydrogen bonds often cause molecules to stick together. (2) Abnormal boiling points: Substances with hydrogen bonds usually have higher boiling points. (3) High solubility value: Hydrogen bonds can increase the solubility of substances in water.

Identify the Irrelevant Property

High Van der Waals forces are not directly related to hydrogen bonding. Van der Waals forces are weak intermolecular forces that occur in all molecules, regardless of hydrogen bonding.

Key concepts

Molecule Association

Molecule association occurs when molecules have a tendency to stick together due to intermolecular forces. One key player in molecule association is hydrogen bonding. Hydrogen bonds form between a hydrogen atom in one molecule and an electronegative atom like oxygen or nitrogen in another molecule. These weak yet significant bonds draw molecules closely together.
For example:

• In water (H₂O), hydrogen bonds form between the hydrogen atom of one water molecule and the oxygen atom of another.
• This hydrogen bonding leads to the high cohesiveness and surface tension observed in water.

Molecule association is crucial in many biological processes, such as the structure of DNA, where hydrogen bonding helps hold the two strands of the double helix together. The consistent association due to hydrogen bonds can also affect the viscosity and phase behavior of liquids.

Abnormal Boiling Points

Substances with hydrogen bonds typically exhibit abnormal boiling points. The term 'abnormal' here refers to boiling points that are higher than what would be expected based purely on molecular weight and other types of van der Waals forces.
For instance:

• Water, with its extensive hydrogen bonding network, boils at 100°C, much higher than other group 16 hydrides like H₂S.
• Ammonia (NH₃) also has a higher boiling point due to hydrogen bonding compared to other group 15 hydrides like PH₃.

The reason behind this is that additional energy is required to break the hydrogen bonds between molecules before the phase transition from liquid to gas can occur. This leads to the need for a higher temperature, thereby resulting in a higher boiling point.

Solubility

Hydrogen bonding plays a pivotal role in the solubility of substances, especially in water. Solubility refers to how well a solute can dissolve in a solvent, forming a homogeneous solution. Hydrogen bonds enhance solubility when the solute forms hydrogen bonds with the solvent.
Some examples include:

• Alcohols, where the hydroxyl group (-OH) forms hydrogen bonds with water, making them highly soluble.
• Carboxylic acids, which can also form hydrogen bonds with water molecules.

These interactions make polar and hydrogen-bonding substances much more soluble in water, a polar solvent. Hydrophilic (water-loving) substances dissolve readily in water due to the ability to form hydrogen bonds, unlike hydrophobic (water-fearing) substances, which do not.

Van der Waals Forces

Van der Waals forces are weak intermolecular interactions that occur between all molecules, irrespective of hydrogen bonds. These forces are divided into three types: dipole-dipole interactions, London dispersion forces, and dipole-induced dipole interactions.
Here are some key points:

• Dipole-dipole interactions happen between polar molecules.
• London dispersion forces are present in all molecules, including nonpolar ones, and result from momentary changes in electron density.
• The strength of van der Waals forces increases with larger, more polarizable molecules.

While they are weaker compared to hydrogen bonds, van der Waals forces are universal and contribute to the overall intermolecular interaction within substances. In contexts like boiling points or solubility, hydrogen bonds often overshadow van der Waals forces, but the latter still play a crucial role, particularly in nonpolar compounds.