van der Waals Forces — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on van der Waals forces, a systematic approach is essential:

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Identify Molecular Polarity: — The first step for any molecule is to determine if it's polar or nonpolar. This involves understanding bond polarity (electronegativity differences) and molecular geometry (VSEPR theory) to see if bond dipoles cancel out. For example, $ext{H}_2\text{O}$ is polar, $ext{CO}_2$ is nonpolar.
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Identify All Present IMFs:

* Always LDFs: Remember that London Dispersion Forces are present in *all* molecules. * Check for DDFs: If the molecule is polar, it will also have dipole-dipole forces. * Check for Hydrogen Bonding: If the molecule has H bonded directly to N, O, or F, it will exhibit hydrogen bonding (a special, strong type of DDF). * Check for DIDF: If you have a mixture of a polar and a nonpolar molecule, dipole-induced dipole forces will be present.

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Compare Strengths: — Understand the general order of strength: Hydrogen Bonding > Dipole-Dipole > London Dispersion. However, be mindful that for very large molecules, LDFs can become cumulatively stronger than DDFs in smaller polar molecules, and sometimes even stronger than hydrogen bonding in very small molecules.
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Factors Affecting LDFs: — For nonpolar molecules or when comparing LDFs, remember that strength increases with:

* Molar Mass/Number of Electrons: More electrons, more polarizable, stronger LDFs. * Surface Area/Shape: More extended shapes (e.g., n-alkanes) have larger surface areas for contact, leading to stronger LDFs than compact, branched isomers.

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Relate to Physical Properties: — Directly link the strength of IMFs to physical properties:

* Stronger IMFs $implies$ Higher boiling point, higher melting point, higher viscosity, higher surface tension, lower vapor pressure. * Solubility: 'Like dissolves like' – polar solvents dissolve polar solutes (due to DDFs, H-bonding), nonpolar solvents dissolve nonpolar solutes (due to LDFs).

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Trap Options: — Be wary of common traps, such as confusing bond polarity with molecular polarity (e.g., $ext{CCl}_4$ is nonpolar despite polar C-Cl bonds) or misclassifying hydrogen bonding as a van der Waals force.