Trends in Physical Properties
Chemistry: IB Diploma Higher Level, topic 3.6.2
Trends in Physical Properties
Physical properties in organic compounds, such as boiling point and solubility, depend on the intermolecular forces that occur between molecules.
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Intermolecular forces
- Physical properties in organic compounds, depend on the intermolecular forces that occur between molecules.
- Intermolecular forces vary when:
- changing the molar mass (number of electrons)
- changing the amount of branching
- changing the functional group
Boiling point (volatility)
- As the chain becomes longer in a homologous series the boiling point increases, as shown above.
Boiling point (volatility) cont.
- The increase in boiling point is attributed to stronger London dispersion forces between molecules.
- Compounds with high boiling points have low volatilities and ones with low boiling points have high volatilities.
- Branched structural isomers typically have lower boiling points as more branching results in a more spherical conformation and lower surface contact.
Boiling point (volatility) cont.
Boiling point (volatility) cont.
- The functional group present in the molecule also influences the boiling point.
- The main types of functional groups and their corresponding intermolecular forces are summarised in the table above.
Solubility
- Hydrocarbons are soluble in apolar (aliphatic) solvents and are insoluble in water.
- Ethanol is soluble in water as it can hydrogen bond with water.
- Increasing the carbon-chain length makes the alcohol become less soluble in water.
- Hexan-1-ol has limited solubility in water and decan-1-ol is practically insoluble in water.
Test yourself on Trends in Physical Properties
Which of these statements is NOT correct?
Which of these molecules has the highest boiling point?