Physical Properties of Alkanes
Trends in Boiling Point, Melting Point & Solubility | Hydrocarbons Class 11
1. Physical State & Nature
Alkanes are non-polar molecules held together by weak Van der Waals forces. Their state depends on molecular mass:
- $C_1$ to $C_4$: Gases (Methane, Ethane, Propane, Butane).
- $C_5$ to $C_{17}$: Liquids.
- $C_{18}$ onwards: Waxy Solids.
They are colourless and odourless.
2. Boiling Point Trends
The boiling point of alkanes depends on the strength of Van der Waals forces, which in turn depends on:
A. Molecular Mass
As molecular mass increases, the size and surface area increase, leading to stronger Van der Waals forces. Thus, Boiling Point increases regularly with the number of carbon atoms.
B. Branching (Isomeric Alkanes)
Order for Pentane ($C_5H_{12}$):
n-Pentane (309 K) > Isopentane (301 K) > Neopentane (282.5 K)
3. Melting Point Trends
Melting point depends on how closely molecules pack in the crystal lattice. Generally, it increases with molecular mass, but there is an irregularity.
The Alternation Effect
Alkanes with an even number of carbon atoms have higher melting points than the next lower odd-numbered alkane.
Example: MP of Propane ($C_3$) $\ll$ MP of Ethane ($C_2$).
4. Solubility & Density
Solubility
Alkanes are non-polar. Following the principle "Like dissolves Like":
- Insoluble in Water (Polar).
- Soluble in Organic Solvents (Non-polar) like Benzene, Ether, $CCl_4$.
Density
The density of alkanes increases with molecular mass but becomes constant at about $0.8 g/cm^3$.
Important: Alkanes are lighter than water and float on it.
Practice Quiz
Test your knowledge on Physical Properties.
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