Learning Objective: To investigate which intermolecular forces are predominant in a liquid state and to study the impact of molecular weight, molecular shape, and polarity on the boiling point of liquids.
Skill Level - Intermediate
Prerequisites:
Chapter: Physical Properties
Sub-topic: Boiling Point
Author's Note: Liquids boil to gases, some much faster than others. If liquid molecules are held firmly by intermolecular attractive forces, then their evaporation isn't rapid but a slow process requiring higher than normal temperatures. Boiling point investigation is helpful in purity assessment, safety considerations while storing and transporting, and investigating an unknown compound and its nature.
This section evaluates the active intermolecular interactions based on the molecule's functional group and studies how the structural differences in shapes, sizes (molecular weight), and presence of polar groups can affect a substance's boiling point.
Physical properties - Boiling Point
The liquids, unlike solids, have no definite shape since the particles are held farther in liquids than in the solids due to their weaker intermolecular attractive force.
The liquid particles gain heat energy while raising the temperature, translating to increased kinetic motion. The particles begin to overcome the remaining intermolecular attractive force and move away, eventually going into the gaseous state. In the gaseous state, the particles are farthest, having the weakest intermolecular attractive forces and the highest kinetic energy.
Therefore, the boiling point is a typical characteristic of the liquids and represents the transition temperature at which liquids change to the gaseous phase.
The boiling point of a liquid is high or low depending on the strength of the intermolecular forces holding the particles. Therefore, the greater the intermolecular forces, the higher the boiling point.
The trend in Intermolecular Forces and their effect on the Boiling Point
The commonly seen intermolecular forces operating in the liquids are Hydrogen bonding and Vander Waal forces...
Role of Molecular Weight (Size)
When a similar class of nonpolar compounds like hydrocarbons is compared for their sizes, it is observed that the larger molecules have a ....
Molecular Shape
Long linear molecules have electrons spread over a larger area than compact spherical molecules. The linearity allows molecules to....
Polarity
Comparing two similar-sized molecules with relative atomic weights such that their electron numbers are almost the same, the only difference being their polarity, it is seen that the polar molecule have ....
Boiling Point - Amino acids, Carbohydrates, and Fluoro compounds
Amino Acids
Bifunctional compounds that are zwitter ions have very strong intermolecular forces due to their ....
Carbohydrates
Carbohydrates have multiple hydroxy groups, due to which....
Fluoro Compounds
Fluorine has the highest electronegativity in the periodic table. However, its small size makes it....
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