Intermolecular forces and solvent evidence
Introduced| English | Meaning |
|---|---|
| London forces | Attractive interactions caused by temporary fluctuations in electron distribution that induce dipoles in neighbouring particles. |
| dipole/ˈdaɪpəʊl/ | A pair or separation of opposite electric charges, producing a positive and a negative end. |
| hydrogen bonding/ˈhaɪdrədʒn ˈbɒndɪŋ/ | An attraction involving a hydrogen atom bonded to an electronegative atom and an electron-rich site on another atom. |
| solubility/ˌsɒljuːˈbɪlɪti/ | The maximum amount of a solute that can dissolve in a specified amount of solvent at a given temperature and pressure. |
Why can a smaller molecule boil later?
- Ethanol can boil at a higher temperature than a similar-size alkane. A molecular formula alone does not show all the attractions.
- Predict which attractions are present before comparing the energy needed to separate molecules.
Build the attraction model
- All molecules have London forces 伦敦力 from instantaneous and induced dipoles 偶极. Polar molecules also have permanent dipole attractions.
- Hydrogen bonding 氢键 needs H bonded to N, O or F and a suitable lone pair on a neighbouring molecule.
Which pure substance can hydrogen-bond to its own molecules?
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Methanol has O–H donors and oxygen lone pairs. The ether has no suitable H donor; methane has neither.
Name the attraction involving H bonded to N, O or F and a suitable neighbouring lone pair.
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A hydrogen bond is an intermolecular attraction, distinct from the covalent bond to that H.
Explain the boiling evidence
- Longer alkane chains usually have stronger London forces. Branching can reduce contact between isomers with the same electron count.
- Ethanol also hydrogen-bonds. Boiling overcomes attractions between molecules; it does not break all their internal covalent bonds.
Compare pentane with its compact branched isomer. Both have the same formula; the branched isomer usually boils at a lower temperature because intermolecular contact is reduced.
All molecular substances have London forces.
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Their electron clouds can develop instantaneous dipoles that induce dipoles in neighbours.
Test a solvent prediction
- Water hydrates ions and hydrogen-bonds with small alcohols. A longer non-polar carbon chain usually lowers an alcohol’s water solubility 溶解度.
- An ether can accept hydrogen bonds from water but lacks an O–H donor for a pure-ether hydrogen-bond network.
Why can a halogenoalkane be polar but poorly water-soluble?
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Solubility depends on the balance of attractions broken and formed, not on a single polar bond.
Check the model limits
- Ice has an open hydrogen-bonded arrangement and lower density than liquid water near the melting point. Its molecules are not larger.
- A polar C–X bond does not alone ensure water solubility. New attractions must compensate for those broken in the solvent and solute.
“Boiling ethanol breaks its O–H covalent bonds” is false. The change mainly separates intact molecules by overcoming intermolecular attractions.
Identify particles, attractions and the physical change before explaining a temperature or solubility trend.
Which two statements explain floating ice near its melting point?
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The open arrangement spaces molecules further apart, lowering density. Individual water molecules keep their identity.