What NEET Asks
- Questions primarily focus on trends in boiling point, melting point, and density with increasing molecular weight and branching.
- Solubility characteristics of alkanes are also frequently tested.
- Expect direct conceptual questions or comparative analysis MCQs.
Key Points
- Alkanes are nonpolar molecules due to similar electronegativities of C and H and symmetrical structure.
- Intermolecular forces are weak London Dispersion Forces (LDFs) or van der Waals forces.
- The first four alkanes (C1-C4) are gases, C5-C17 are liquids, and C18 onwards are solids at room temperature.
- Boiling point increases with increasing molecular weight (number of carbon atoms) due to stronger LDFs.
- Boiling point decreases with increasing branching due to reduced surface area for intermolecular interaction.
- Melting point generally increases with molecular weight, but also depends on packing efficiency in the solid state.
- Alkanes are insoluble in water (polar solvent) but soluble in nonpolar organic solvents.
Must-Know Formula / Reaction
There's no specific 'formula' for physical properties. The key concept is:
- Strength of Intermolecular Forces (LDFs) ∝ Molecular Weight ∝ Surface Area
- LDFs: London Dispersion Forces, the only intermolecular forces in nonpolar alkanes.
- Molecular Weight: Sum of atomic masses; more mass means more electrons, leading to stronger LDFs.
- Surface Area: Greater contact area allows for more extensive LDFs.
Common Mistakes
- Students often confuse the effect of branching on boiling point vs. melting point. Branching decreases boiling point but can increase melting point for symmetrical molecules due to better packing.
- Don't assume alkanes are insoluble in all solvents; they are soluble in nonpolar organic solvents like CCl₄.
- Forgetting that alkanes are less dense than water is a common oversight.
Rapid Revision
Alkanes are nonpolar, held by weak LDFs. BP & MP generally increase with MW. Branching decreases BP by reducing surface area. Insoluble in water, soluble in organic solvents. Density is less than water.