Worked example 1
Propane (44 g/mol, nonpolar) boils at -42 °C and acetaldehyde (44 g/mol, polar) at 20. Explain.
Try it first: Note what is the same before looking for what is different.
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Types of Solids and IMFSIntro to IMFSContent level: Core 18 min
Dipole-Dipole and Ion-Dipole Forces
What you'll be able to do: Identify dipole-dipole and ion-dipole interactions and rank them against dispersion.
Best after: London Dispersion Forces
Introduction
Once a molecule has a permanent dipole, its neighbours no longer rely on fleeting fluctuations. Permanent charges line up, and the attraction gets stronger and more predictable.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Decide whether a molecule has a permanent dipole
- Describe dipole-dipole alignment between neighbouring molecules
- Explain ion-dipole attraction and hydration of ions
- Rank dispersion, dipole-dipole and ion-dipole by typical strength
Lesson
Permanent dipoles align
A polar molecule has a positive end and a negative end that do not move. Neighbours orient so that opposite ends face each other. The attraction is stronger than dispersion for molecules of similar size, typically a few kJ/mol up to about 25 kJ/mol.
Comparing at equal size
Dipole-dipole only decides the outcome when the molecules being compared have similar electron counts. Propane and acetaldehyde have similar masses, but acetaldehyde is polar and boils about 62 degrees higher.
Compare size first. If sizes are close, polarity decides.
Ion-dipole: charge meets dipole
A full ionic charge attracts the partial charge on a polar molecule far more strongly than one partial charge attracts another. When NaCl dissolves, the partial negative oxygen of water surrounds Na⁺ and the partial positive hydrogens surround Cl⁻.
Ion-dipole attraction is what releases the energy that pays for breaking the ionic lattice.
The working strength order
For molecules of comparable size: dispersion < dipole-dipole < hydrogen bonding < ion-dipole < ionic bonding. Treat the list as a guide, since a very large nonpolar molecule can outweigh a small polar one.
Consider ion size as well as charge
Ion-dipole attraction depends on an ion's charge and the distance at which a polar molecule can approach it. At the same charge, a smaller ion has greater charge density and can attract nearby water molecules more strongly. Hydration enthalpy also depends on the number and arrangement of water molecules, so a simple size rule is a controlled comparison, not a complete prediction of solubility.
Compare like charges first; then consider ion size and distance.
Key ideas
Definition
Dipole-dipole force
Attraction between the permanent dipoles of neighbouring polar molecules.
Definition
Ion-dipole force
Attraction between an ion and the oppositely charged end of a polar molecule.
Rule
Strength guide
Dispersion < dipole-dipole < hydrogen bonding < ion-dipole at comparable size.
Rule
Hydration
Dissolving an ionic solid in water replaces lattice attractions with ion-dipole attractions.
Worked examples
Worked example 2
Describe the interactions when MgCl₂ dissolves in water and why Mg²⁺ is hydrated more strongly than Na⁺.
Try it first: Draw one water molecule near a cation and label its partial charges.
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Worked example 3
Compare the strength of local ion-dipole attractions when Li⁺ and K⁺ each become hydrated.
Try it first: Both ions have charge +1, so isolate their size difference.
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Common mistakes
Assuming any molecule with polar bonds shows dipole-dipole forces.
Why it's wrong: Symmetric molecules such as CCl₄ have cancelling bond dipoles and no net dipole.
Check instead: Check the geometry and confirm a net dipole before claiming dipole-dipole.
Using dipole-dipole to compare molecules of very different sizes.
Why it's wrong: Dispersion scales with electron count and can dominate entirely.
Check instead: Compare electron counts first, then polarity.
Calling the water to ion attraction a dipole-dipole force.
Why it's wrong: One partner carries a full charge, not a partial one.
Check instead: Name it ion-dipole whenever an ion is involved.
After the lesson · Question bank
Independent practice
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What you should now know
Polar molecules attract each other head to tail through dipole-dipole forces, which add to the dispersion always present. When an ion meets a polar molecule the attraction is stronger still, and ion-dipole forces are what make ionic compounds dissolve in water.
- Polar molecules attract through permanent dipole alignment
- Dipole-dipole adds to dispersion, never replaces it
- Ion-dipole forces explain dissolving of ionic solids
- Charge density controls the strength of hydration
Sources and further reading
- Chemistry 2e, Section 10.1: Intermolecular ForcesOpenStax · Rice University · 10.1View source
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