IMFs and Ion–Dipole Forces
i. The molecules are of the same chemical species.
ii. The molecules are of two different chemical species.
Quick Notes
- Intermolecular forces (IMFs) are forces between molecules, not within them.
- Types of IMFs (in increasing strength):
- London dispersion forces (LDFs) – in all molecules, strongest in large/nonpolar ones
- Dipole–dipole forces – between polar molecules
- Hydrogen bonding – special strong dipole–dipole interaction (H bonded to N, O, or F)
- Ion–dipole forces – between ions and polar molecules (strongest type of IMF)
- Larger molecules form stronger LDFs due to greater polarizability
- Hydrogen bonding and ion–dipole interactions are key in biological molecules
- Orientation and charge differences affect IMF strength and behavior
Full Notes
Intermolecular forces (IMFs) are the forces that exist between molecules and play a critical role in determining boiling point, melting point, solubility, and structure of substances, especially large biological molecules.
1. London Dispersion Forces (LDFs)
Attraction between temporary dipoles in molecules that occur due to random movement of electrons.
Present in all molecular substances, even noble gases and nonpolar molecules and are often the strongest IMFs in large, nonpolar molecules.
Factors affecting LDF strength:
- Number of electrons – more electrons means stronger dispersion forces
- Molecular size and shape – more surface contact between molecules means stronger interactions
- Polarizability – larger electron clouds distort more easily giving stronger forces
- Pi bonding also increases polarizability
Important: LDFs are a type of van der Waals force, but the terms are not interchangeable. Van der Waals forces include both dispersion and dipole interactions.
2. Dipole–Dipole Interactions
Occur between polar molecules with permanent dipoles, where molecules align so that positive ends attract negative ends of nearby dipoles.
Example: HCl molecules attract each other via permanent dipole–dipole forces
- Stronger than LDFs (if comparing molecules of similar size)
- Strength depends on:
- Magnitude of dipole moment
- Distance and orientation between dipoles
3. Dipole–Induced Dipole Forces
Occur when a polar molecule induces a temporary dipole in a nonpolar molecule, with attraction between oppositely charged dipoles.
- Strength increases with:
- Larger dipole on the polar molecule
- Greater polarizability of the nonpolar molecule
4. Ion–Dipole Forces
Attraction between a charged ion and oppositely charged end of a polar molecule.
Stronger than dipole–dipole forces and key in dissolving ionic compounds in water (e.g., Li⁺ attracted to O end of H₂O)
Strength depends on:
- Charge and size of the ion
- Magnitude and direction of the dipole
- Orientation matters – the negative end of a dipole is attracted to cations, the positive end to anions
5. Hydrogen Bonding
A special type of strong dipole–dipole interaction that occurs when H is bonded to N, O, or F and is attracted to N, O, or F in another molecule (or part of the same molecule).
- Highly directional and relatively strong
- Critical in biological structures (e.g., DNA base pairing, protein folding)
6. Noncovalent Interactions in Biomolecules
In large biological molecules, IMFs are responsible for:
- Maintaining structure (e.g., alpha helices, beta sheets in proteins)
- Interactions between molecules (e.g., enzyme–substrate binding)
See proteins and DNA for more information.
Such interactions include:
- Hydrogen bonding
- Dipole interactions
- Ion–dipole attractions
- Dispersion forces
Question: Rank the IMFs present in the following interactions from weakest to strongest:
- Interaction between two Cl₂ molecules
- Interaction between two HCl molecules
- Interaction between Na⁺ and H₂O
- Interaction between HF and HF
Answer: 1 (Cl₂–Cl₂, LDF) < 2 (HCl–HCl, dipole–dipole) < 4 (HF–HF, hydrogen bonding) < 3 (Na⁺–H₂O, ion–dipole)
If H is bonded to N, O, or F and interacting with another N, O, or F — it's hydrogen bonding! Remember you must have those conditions for H-bonding to occur.
Summary
Intermolecular forces vary in strength and arise from electrostatic attractions between molecules, ions, or parts of molecules. The strength and type of IMF depends on:
- Molecular structure
- Polarity
- Presence of charges
- Size and electron cloud polarizability
Understanding these forces helps explain differences in boiling point, solubility, molecular behavior, and biological function.
Key IMF types (in increasing strength):
- London dispersion: in all molecules; stronger in larger ones
- Dipole–induced dipole: between polar and nonpolar molecules
- Dipole–dipole: between polar molecules
- Hydrogen bonding: strong dipole–dipole when H is bonded to N, O, or F
- Ion–dipole: between ions and polar molecules; strongest IMF type