Boiling Point Trends

A concise revision guide to boiling point trends explained by intermolecular forces: alkanes with chain length and branching, alcohols compared with alkanes, and the hydrogen halides HF to HI, for AQA A Level Chemistry.

Paper 1 and 2 AQA

3.1.3 Bonding

7405/1 and 7405/2

Written by:
Dr. Mohammed Al-Fatah

Chemistry specialist revision notes for A Level Chemistry.

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How Intermolecular Forces Set the Boiling point

A substance boils when its molecules gain enough energy to overcome the intermolecular forces between them, so the stronger the forces, the higher the boiling point. Every trend on this page is explained by asking which forces act between the molecules and what changes their strength: the number of electrons, the shape of the molecule, the presence of a permanent dipole and the presence of hydrogen bonds.

Students should always name the force, state what has changed and finish with the energy statement: “more energy is needed to separate the molecules”. A trend described without a force earns nothing.

Exam answer model: Identify the force → say why it is stronger or weaker → “so more (or less) energy is needed to overcome the forces between the molecules” → higher (or lower) boiling point.

Alkanes: Increasing Chain Length

Alkanes are non-polar, so the only force between their molecules is induced dipole-dipole forces. As the chain lengthens, each molecule has more electrons and a larger surface area of contact with its neighbours, so the induced dipole-dipole forces strengthen and the boiling point rises steadily. Methane, ethane, propane and butane are gases at room temperature; pentane to heptadecane are liquids; longer alkanes are waxy solids.

Alkane	Formula	Electrons	boiling point / °C
Methane	CH₄	10	-162
Ethane	C₂H₆	18	-89
Propane	C₃H₈	26	-42
Butane	C₄H₁₀	34	-1
Pentane	C₅H₁₂	42	36
Hexane	C₆H₁₄	50	69
Heptane	C₇H₁₆	58	98
Octane	C₈H₁₈	66	126

The boiling point of the alkanes rises with chain length because induced dipole-dipole forces strengthen; at a fixed formula, branching lowers it.

The rise is steepest at the start of the series, because adding one CH₂ group to methane increases its electron count by 80%, while adding one to heptane increases it by 14%. This is why the curve flattens as the chain lengthens.

Exam sentence: Hexane has a higher boiling point than butane because hexane molecules have more electrons and a larger surface area of contact, so the induced dipole-dipole forces between the molecules are stronger and more energy is needed to overcome them.

Alkanes: The Effect of Branching

Isomers have the same number of electrons, so any difference in their boiling points must come from shape. Branched alkanes are more compact and nearly spherical, so neighbouring molecules cannot approach each other as closely or over as large an area, the induced dipole-dipole forces are weaker and the boiling point is lower.

C₅H₁₂ isomer	Shape	boiling point / °C
Pentane	straight chain	36
2-Methylbutane	one branch	28
2,2-Dimethylpropane	four groups on one carbon, almost spherical	10

Straight chains can lie side by side along their whole length, like pencils in a box, giving a large area over which instantaneous dipoles can induce dipoles in the neighbour. A ball-shaped molecule touches its neighbours only at a few points.

Common mistake: Saying branched isomers have “fewer electrons” or “weaker bonds”. They have the same electrons and the same C-C and C-H bonds; only the contact area between molecules differs.

Alcohols Compared With Alkanes

Alcohols have much higher boiling points, and so a much lower volatility, than alkanes with a similar number of electrons, because the O-H group lets alcohol molecules form hydrogen bonds with each other, which alkanes cannot do. The fair comparison is between molecules of similar size, so that their induced dipole-dipole forces are similar and the difference can be attributed to the hydrogen bonds.

Compound	Formula	Electrons	Hydrogen bonding?	boiling point / °C
Propane	C₃H₈	26	no	-42
Ethanol	C₂H₅OH	26	yes	78
Butane	C₄H₁₀	34	no	-1
Propan-1-ol	C₃H₇OH	34	yes	97

Alcohols boil about 100 °C higher than alkanes with the same number of electrons because hydrogen bonds act between alcohol molecules in addition to induced dipole-dipole forces.

The 120 °C gap between propane and ethanol is far larger than the 100 °C gap that a permanent dipole produced between F₂ and HCl, which is one way to remember that hydrogen bonds are stronger than permanent dipole-dipole forces. Alcohols also have permanent dipole-dipole forces from the polar C-O and O-H bonds, but these are minor next to the hydrogen bonds.

Exam sentence: Ethanol has a higher boiling point than propane because ethanol molecules form hydrogen bonds between the O-H groups, which are stronger than the induced dipole-dipole forces between propane molecules, so more energy is needed to separate ethanol molecules.

The Hydrogen Halides HF to HI

The boiling points of the hydrogen halides do not fall smoothly down the group. HF is anomalously high because it forms hydrogen bonds; from HCl to HI the boiling point rises because the number of electrons increases and the induced dipole-dipole forces strengthen, even though the permanent dipole becomes smaller.

Hydrogen halide	Electrons	Electronegativity of halogen	Hydrogen bonding?	boiling point / °C
HF	10	4.0	yes	20
HCl	18	3.0	no	-85
HBr	36	2.8	no	-67
HI	54	2.5	no	-35

The HCl to HI trend is a useful test of understanding, because the dipole and the induced dipole-dipole forces pull in opposite directions. HCl has the largest permanent dipole of the three, yet it has the lowest boiling point, because the extra electrons in HBr and HI produce induced dipole-dipole forces that outweigh the shrinking permanent dipole-dipole forces. For molecules with a large number of electrons, induced dipole-dipole forces usually dominate.

The Group 4 to 7 hydride graph on the Anomalous Properties of Water page shows the HF anomaly alongside those of water and ammonia; the same drawing serves both pages.

Exam sentence: HF has a much higher boiling point than HCl because HF molecules form hydrogen bonds; HI has a higher boiling point than HCl because HI molecules have more electrons, so the induced dipole-dipole forces between them are stronger.

Putting the Trends Together

When two substances are compared, work through the forces in order of strength and stop at the first difference.

Question to ask	If the answer differs	Example
Does one form hydrogen bonds and the other not?	The hydrogen-bonded one boils much higher	ethanol 78 °C versus propane -42 °C
Does one have a permanent dipole and the other not?	The polar one boils higher	HCl -85 °C versus F₂ -188 °C
Which has more electrons?	The one with more electrons boils higher	HI -35 °C versus HCl -85 °C
Which is less branched?	The straighter molecule boils higher	pentane 36 °C versus 2,2-dimethylpropane 10 °C

Key idea: Hydrogen bonds outweigh dipoles, dipoles outweigh induced dipole-dipole forces for molecules of similar size, and induced dipole-dipole forces grow with electrons and contact area.

Common Exam Points

Explain the trend in boiling point of the alkanes

More electrons and larger surface contact, stronger induced dipole-dipole forces, more energy to separate molecules.

Explain why 2,2-dimethylpropane boils lower than pentane

Same electrons; more compact shape; smaller area of contact; weaker induced dipole-dipole forces.

Explain why alcohols are less volatile than alkanes

Hydrogen bonds between O-H groups, absent in alkanes, need more energy to overcome.

Explain the order HF > HI > HBr > HCl

HF: hydrogen bonds; HCl to HI: increasing electrons, stronger induced dipole-dipole forces, outweighing the falling dipole.

Check Your Understanding

Explain new boiling-point comparisons using the forces, not the examples in the notes.

FAQs

Use these quick answers to check the boiling point comparisons that AQA questions set.

Why does boiling point rise along the alkane series?

Each extra carbon adds electrons and surface area, so the induced dipole-dipole forces between molecules get stronger and more energy is needed to separate them. The rise flattens along the series because each extra CH₂ group is a smaller fraction of the total.

Why do branched alkanes boil lower than straight-chain isomers?

Isomers have the same number of electrons, so the difference is shape. A branched molecule is compact and touches its neighbours over a smaller area, so the induced dipole-dipole forces are weaker: pentane boils at 36 °C and 2,2-dimethylpropane at 10 °C.

Why are alcohols less volatile than alkanes?

The O-H group lets alcohol molecules hydrogen bond with each other, which alkanes cannot do. Ethanol and propane both have 26 electrons, yet ethanol boils 120 °C higher.

Why does HF boil higher than HCl, HBr and HI?

HF molecules form hydrogen bonds, which the other hydrogen halides cannot. From HCl to HI the trend then rises again as the number of electrons increases.

Which factor wins when they disagree?

Hydrogen bonding outweighs a permanent dipole, and a permanent dipole outweighs induced dipole-dipole forces for molecules of similar size. Once the electron count differs a great deal, induced dipole-dipole forces dominate, which is why HI boils higher than HCl.

Copyright and author footprint: This OLS revision page was written for Online Learning System by Dr. Mohammed Al-Fatah. It is designed for A Level Chemistry revision and should not be copied or redistributed without permission.