Overview
The lesson explains ion–dipole and ion–induced dipole interactions, contrasts permanent vs. temporary dipoles, and applies concepts to examples and practice problems.
Ion–Dipole Interactions
- Definition: Attraction between an ion and a polar molecule (a permanent dipole).
- Water example with NaCl:
- Oxygen (partial negative) attracts Na+; hydrogen (partial positive) attracts Cl−.
- Water is polar; thus a permanent dipole with two charge regions.
- Occurrence: Whenever an ionic compound mixes with a polar substance.
- Key idea: Opposite charges attract; partial charges on polar molecules align with ions.
Ion–Induced Dipole Interactions
- Definition: Attraction between an ion and a nonpolar molecule whose electron cloud is distorted by the ion.
- Mechanism:
- Ion’s electric field distorts electron cloud of nearby nonpolar species.
- Creates a temporary dipole with partial charges that vanish when ion is removed.
- Temporary vs. permanent:
- Induced dipole is temporary; exists only near the ion.
- Polar molecules are permanent dipoles (e.g., water).
- Example with Zn2+ and H2:
- H2 is nonpolar; electrons redistribute toward Zn2+.
- Hydrogen nearer Zn2+ becomes partially negative; the other partially positive.
- Resulting attraction is an ion–induced dipole interaction.
Polarity Rules and Examples
- Single-element molecules (H2, I2, O2) are nonpolar.
- Hydrocarbons (only C and H) are nonpolar.
- CO2: Nonpolar overall; linear geometry causes bond dipoles to cancel.
- SO2: Polar due to bent shape; oxygen atoms carry partial negative, sulfur partial positive.
- CO: Polar; single dipole moment does not cancel.
Structured Summary of Interaction Types
| Interaction Type | Participants | Dipole Nature | Example Scenario | Key Feature |
|---|
| Ion–dipole | Ion + polar molecule | Permanent dipole | Na+ with water oxygen; Cl− with water hydrogens | Strong alignment of ion with partial charges |
| Ion–induced dipole | Ion + nonpolar molecule/atom | Temporary induced dipole | Zn2+ with H2; P3− with O2 | Ion distorts electron cloud to create dipole |
Practice Problems Reviewed
- Problem 1: Which molecule forms an ion–dipole with Ni2+?
- Polar requirement: Ion + polar molecule.
- H2, I2: Nonpolar (single-element molecules).
- Hydrocarbon (C and H only): Nonpolar.
- CO2: Nonpolar overall (dipoles cancel).
- SO2: Polar (bent); oxygen ends partially negative.
- Answer: SO2 (option D).
- Problem 2: Which molecule forms an ion–induced dipole with P3−?
- Requirement: Ion + nonpolar molecule.
- H2O, HF, NH3: Polar; contain hydrogen bonding, highly polar.
- CO: Polar; one dipole moment.
- O2: Nonpolar (single-element molecule).
- With P3− near O2: Electrons repel; nearer O becomes partially positive, farther O partially negative.
- Answer: O2.
Key Terms & Definitions
- Polar molecule: Molecule with permanent separation of partial charges; a permanent dipole.
- Nonpolar molecule: Molecule with even electron distribution; no permanent dipole.
- Dipole: Object with two regions of charge within the same entity.
- Ion–dipole interaction: Attraction between an ion and a permanent dipole.
- Ion–induced dipole interaction: Attraction between an ion and a temporary dipole created by electron cloud distortion.
- Induced dipole: Temporary dipole formed when an external charge distorts electron distribution.
- Polarization: Distortion of electron cloud creating partial charges.
Action Items / Next Steps
- Identify molecular polarity using shape and bond dipoles; predict interaction types with ions.
- Apply rules: Ion + polar → ion–dipole; Ion + nonpolar → ion–induced dipole.
- Practice drawing charge distributions for induced dipoles near cations and anions.