Physical Properties of Alkanes
Master the non-polar world of hydrocarbons. Decode the exact rules governing boiling points, surface area, and the infamous melting point alternation effect.
Module Focus: The Power of Weak Forces
Alkanes are almost completely non-polar. Because the electronegativity difference between Carbon (2.5) and Hydrogen (2.1) is tiny, they do not possess dipole moments or hydrogen bonding. The ONLY intermolecular forces holding alkane molecules together are weak London Dispersion Forces (van der Waals forces). Therefore, every physical property depends entirely on the size, mass, and surface area of the molecule.
1. Physical State & Density
- C₁ - C₄ Gases (Methane to Butane)
- C₅ - C₁₇ Liquids (Pentane to Heptadecane)
- C₁₈+ Waxy Solids
All alkanes are lighter than water.
Density increases with increasing molecular mass but reaches a limiting maximum value of about 0.8 g/cm³ (water is 1.0 g/cm³). Therefore, oil/alkanes will always float on water.
2. Boiling Point Trends
Boiling point depends on the strength of intermolecular van der Waals forces. These forces increase with molecular mass and surface area.
- Rule 1: For a homologous series, boiling point increases steadily with increasing molecular mass (approx. $20-30\text{K}$ per $-CH_2-$ group added).
- Rule 2: For isomeric alkanes, boiling point DECREASES with branching.
Branching makes the molecule more compact and spherical. A sphere has the minimum surface area for a given volume, which reduces the area of contact between molecules, weakening van der Waals forces.
3. Melting Point & The Alternation Effect
Melting point depends not just on mass, but crucially on how well the molecules pack into a solid crystal lattice. Unlike boiling points, melting points of alkanes do not show a smooth gradation.
Alkanes with an EVEN number of carbon atoms have significantly higher melting points than the alkanes with an ODD number of carbon atoms immediately below and often even above them.
In an extended zig-zag chain, if the number of carbons is even, the terminal methyl groups lie on opposite sides. If odd, they lie on the same side, disrupting tight packing.
4. Solubility
The universal rule of solubility is "Like dissolves like".
- Alkanes are strictly non-polar. Therefore, they are insoluble in polar solvents like water.
- They are highly soluble in non-polar solvents like benzene, ether, chloroform, and carbon tetrachloride ($CCl_4$).
- This property is why grease and oil stains (which are higher alkanes) cannot be washed away with plain water, but easily dissolve in petrol or dry-cleaning solvents.
NEET Grand Test: Physical Properties
15 High-Yield Questions testing the Alternation effect, branching anomalies, and van der Waals principles.
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