Alkanes and Fuels
3. Physical Properties of Alkanes
Learning outcomes
- I can describe the physical properties of alkanes.
- I can explain why alkanes are generally nonpolar molecules.
- I can identify trends in boiling points and melting points within the alkane series.
- I can explain how intermolecular forces affect alkane properties.
- I can relate alkane properties to their practical uses.
Introduction
Alkanes are saturated hydrocarbons containing only carbon and hydrogen atoms joined by single covalent bonds.
Although all alkanes have similar chemical structures, their physical properties change gradually as the molecules become larger.
Important physical properties include:
- physical state
- boiling point
- melting point
- density
- solubility
- viscosity
- volatility
These properties are strongly influenced by the intermolecular forces acting between alkane molecules.
Molecular Structure of Alkanes
Alkanes contain:
C–C single covalent bonds
and:
C–H single covalent bonds
Examples include:
Methane:
CH₄
Ethane:
C₂H₆
Propane:
C₃H₈
Butane:
C₄H₁₀
Pentane:
C₅H₁₂
As we move through the homologous series, each successive member differs from the previous member by:
CH₂
The molecules therefore become progressively larger.
Alkanes Are Generally Nonpolar
Alkanes are generally considered nonpolar molecules.
Carbon and hydrogen have relatively similar electronegativities, so C–H bonds are only weakly polar.
In addition, alkane molecules do not contain strongly polar functional groups.
As a result, alkanes generally have:
little or no permanent separation of electrical charge across the molecule.
This makes them very different from strongly polar substances such as water.
Polar and Nonpolar Molecules
A polar molecule has an uneven distribution of electrical charge.
Water is a familiar example.
A nonpolar molecule has a much more even overall distribution of charge.
Alkanes are generally nonpolar.
This difference has an important effect on properties such as:
solubility
and:
intermolecular attraction
Why Alkanes Do Not Mix Well with Water
Water is strongly polar.
Alkanes are nonpolar.
As a general rule:
"like dissolves like."
Polar substances tend to dissolve more readily in polar solvents.
Nonpolar substances tend to dissolve more readily in nonpolar solvents.
Therefore, alkanes are generally:
insoluble or only very slightly soluble in water.
This explains why many hydrocarbon liquids form a separate layer when mixed with water.
Intermolecular Forces
The atoms within an alkane molecule are held together by strong covalent bonds.
However, separate alkane molecules also attract one another.
These attractions are called:
intermolecular forces
For alkanes, the main intermolecular attractions are London dispersion forces.
These are also sometimes described as instantaneous dipole-induced dipole forces.
London Dispersion Forces
Electrons are constantly moving.
At any instant, the electrons in a molecule may become slightly unevenly distributed.
This creates a temporary:
instantaneous dipole
The temporary dipole can affect the electron distribution of a nearby molecule, producing an:
induced dipole
The two temporary dipoles then attract.
These attractions are London dispersion forces.
Temporary Does Not Mean Unimportant
Individual London dispersion forces are relatively weak.
However, many of them can act simultaneously between molecules.
Their combined effect can become significant, particularly for large molecules.
This explains an important trend:
larger alkane molecules generally experience stronger intermolecular attractions than smaller alkane molecules.
Molecular Size and Intermolecular Forces
Consider:
methane, CH₄
and:
octane, C₈H₁₈
Octane has:
- more carbon atoms
- more hydrogen atoms
- more electrons
- a larger electron cloud
- greater polarizability
Therefore, octane generally experiences stronger London dispersion forces than methane.
This difference affects properties such as boiling point and volatility.
Boiling Alkanes
For a liquid to boil, molecules must gain enough energy to separate from neighbouring molecules and enter the gas phase.
During boiling, the intermolecular attractions between molecules are overcome.
The covalent bonds inside the molecules are not normally broken.
This is an important distinction.
Boiling is a:
physical change
not a chemical reaction.
Boiling Point Trend
As the number of carbon atoms in an alkane increases:
boiling point generally increases.
For example, approximate normal boiling points are:
| Alkane | Formula | Approximate Boiling Point |
|---|---|---|
| Methane | CH₄ | −162°C |
| Ethane | C₂H₆ | −89°C |
| Propane | C₃H₈ | −42°C |
| Butane | C₄H₁₀ | −0.5°C |
| Pentane | C₅H₁₂ | 36°C |
| Hexane | C₆H₁₄ | 69°C |
| Heptane | C₇H₁₆ | 98°C |
| Octane | C₈H₁₈ | 126°C |
The overall trend is clear:
more carbon atoms → larger molecule → stronger dispersion forces → more energy needed → higher boiling point
Explaining the Boiling Point Trend
Suppose we compare pentane and octane.
Pentane:
C₅H₁₂
Octane:
C₈H₁₈
Octane contains more electrons and has a larger electron cloud.
Its electron cloud is more easily polarized.
Therefore:
London dispersion forces are stronger between octane molecules.
More thermal energy is required to separate the molecules.
Therefore:
octane has a higher boiling point than pentane.
Chain Length and Boiling Point
A useful general relationship is:
Increasing carbon-chain length → increasing boiling point
This trend is one of the most important physical patterns in the alkane homologous series.
Branching and Boiling Point
Molecular shape can also affect boiling point.
Consider two molecules with the same molecular formula.
A straight-chain molecule generally has a larger area of contact with neighbouring molecules than a highly branched molecule.
Greater contact can allow stronger overall dispersion attractions.
Therefore, among many alkane isomers:
greater branching generally lowers the boiling point.
For example, straight-chain pentane has a higher boiling point than its more highly branched isomers.
Melting Points
The melting points of alkanes also tend to increase overall as molecular size increases.
However, the melting-point trend is less smooth than the boiling-point trend.
Why?
Melting depends not only on intermolecular forces but also on:
how efficiently molecules pack together in a solid crystal.
Molecular symmetry and shape can therefore have important effects.
Why the Melting Trend Is Irregular
Two alkanes of similar molecular size can have noticeably different melting points because their molecules may pack differently in the solid state.
Some structures fit together more efficiently.
Better packing can produce stronger overall attractions within the solid.
Therefore:
melting point does not rise perfectly smoothly with carbon-chain length.
This is why boiling-point trends are often easier to predict than exact melting-point trends.
Physical State at Room Temperature
The changing intermolecular forces also affect whether an alkane is a gas, liquid, or solid at room temperature.
In general:
small alkanes → gases
medium-chain alkanes → liquids
large alkanes → solids or waxy materials
At typical room conditions:
Methane, ethane, propane, and butane are gases.
Pentane and many intermediate alkanes are liquids.
Long-chain alkanes can be waxy solids.
Why Physical State Changes
Small alkane molecules have relatively weak intermolecular forces.
Therefore, relatively little energy is required to separate them.
This contributes to low boiling points.
As molecular size increases:
- electron clouds become larger
- polarizability increases
- dispersion forces become stronger
- boiling points increase
Eventually, molecules have sufficiently strong intermolecular attractions to remain liquid or solid at room temperature.
Volatility
Volatility describes how easily a substance vaporizes.
A highly volatile liquid evaporates readily.
Generally:
smaller alkanes are more volatile
and:
larger alkanes are less volatile
This is related to boiling point.
Lower boiling point usually means:
greater volatility
Higher boiling point usually means:
lower volatility
Volatility and Intermolecular Forces
Smaller molecules have weaker London dispersion forces.
Therefore, molecules can escape from the liquid more easily.
As alkane molecules become larger:
dispersion forces increase
so:
volatility generally decreases
This relationship is important when considering fuels and solvents.
Viscosity
Viscosity describes a fluid's resistance to flowing.
A low-viscosity liquid flows easily.
A high-viscosity liquid flows more slowly.
As alkane chain length increases:
viscosity generally increases.
Longer molecules experience stronger intermolecular attractions and can also interact and entangle more extensively.
Therefore, long-chain hydrocarbon liquids tend to flow less easily than short-chain hydrocarbons.
Density
Liquid alkanes generally have densities lower than water.
This means that many liquid hydrocarbons float on water.
For example, if a liquid alkane is mixed with water, two layers often form.
The hydrocarbon layer is commonly above the water layer.
This behavior is caused by both:
- poor solubility between the substances
- differences in density
Solubility in Water
Alkanes are generally:
insoluble in water
because:
alkanes are nonpolar
while:
water is polar
Water molecules strongly attract one another through hydrogen bonding.
An alkane cannot provide similarly strong interactions with water.
Therefore, mixing is energetically unfavorable and the substances tend to separate.
Solubility in Nonpolar Solvents
Alkanes mix more readily with many other nonpolar substances.
This again reflects the general idea:
like dissolves like
This property helps explain why hydrocarbons can dissolve certain oils, greases, and other nonpolar materials.
Electrical Conductivity
Alkanes are generally poor electrical conductors.
They do not contain:
- freely moving ions
- delocalized electrons that can move throughout the substance
Therefore, pure alkanes normally do not conduct electricity effectively.
This is different from metals and ionic solutions.
Colour and Odour
Pure lower alkanes are generally colourless.
Some pure alkanes have little or relatively mild odour, although hydrocarbon products may contain other substances that produce noticeable smells.
For example, fuel gases may have odorants deliberately added so that leaks can be detected.
The smell of a commercial fuel should therefore not automatically be assumed to be the smell of the pure alkane itself.
Summary of Major Trends
As the number of carbon atoms increases:
molecular size increases
number of electrons increases
London dispersion forces strengthen
boiling point increases
volatility decreases
viscosity generally increases
physical state tends to change from gas → liquid → solid
Melting point also increases overall, but the trend is less regular.
A Cause-and-Effect Chain
An important piece of chemical reasoning is:
Longer carbon chain
↓
Larger molecule and more electrons
↓
Greater polarizability
↓
Stronger London dispersion forces
↓
More energy required to separate molecules
↓
Higher boiling point
This reasoning is more useful than simply memorizing that boiling points increase.
Comparing Methane and Hexane
Methane:
CH₄
Hexane:
C₆H₁₄
Methane is much smaller and contains fewer electrons.
Therefore, methane has weaker dispersion forces.
Methane has a boiling point of approximately:
−162°C
Hexane boils at approximately:
69°C
At ordinary room conditions:
methane is a gas
while:
hexane is a liquid
The difference can be explained largely by the strength of their intermolecular forces.
Comparing Pentane and Octane
Pentane:
C₅H₁₂
Octane:
C₈H₁₈
Octane is larger.
Therefore:
- octane has stronger London dispersion forces
- octane has a higher boiling point
- octane is less volatile
- octane generally has greater viscosity
Comparing Straight and Branched Alkanes
Suppose two alkane molecules have the same molecular formula.
The straight-chain molecule often has:
greater surface contact
with neighbouring molecules.
The branched molecule is often more compact.
Therefore, branching can reduce the strength of intermolecular attractions between molecules.
Result:
more highly branched isomer → generally lower boiling point
for comparable alkane isomers.
Practical Use: Gaseous Alkanes
Small alkanes have low boiling points and are gases under ordinary conditions.
Examples include:
methane
ethane
propane
butane
Methane is widely used as a fuel.
Propane and butane can be liquefied under pressure, making them convenient to store and transport in suitable cylinders.
Their physical properties therefore contribute directly to their practical uses.
Practical Use: Liquid Hydrocarbons
Medium-sized alkanes are often liquids at ordinary temperatures.
Liquid hydrocarbons can be useful because they are:
- relatively easy to transport
- able to flow through pipes
- able to be stored in tanks
- useful as components of fuels
- useful as nonpolar solvents in appropriate applications
Their boiling ranges and volatility strongly influence how they are used.
Practical Use: Fuels
Fuel must often vaporize or mix appropriately with air before efficient combustion can occur.
More volatile hydrocarbons vaporize more easily.
Less volatile hydrocarbons require higher temperatures to vaporize.
This helps explain why different petroleum fractions are suitable for different types of fuel.
Practical Use: Lubricants
Longer-chain hydrocarbons tend to have:
higher viscosity
and:
lower volatility
These properties can make them useful as components of lubricating materials.
A lubricant needs to remain between moving surfaces rather than evaporating immediately.
Longer hydrocarbon molecules are therefore better suited to many lubrication applications than very small volatile hydrocarbons.
Practical Use: Paraffin Wax
Very long-chain alkanes can be solids at room temperature.
Mixtures of these hydrocarbons are found in materials such as:
paraffin wax
Paraffin wax is used in applications such as:
- candles
- coatings
- waterproofing
- polishes
Its solid or waxy nature results from stronger intermolecular attractions between its relatively large hydrocarbon molecules.
Petroleum and Alkane Properties
Crude oil contains a complex mixture of hydrocarbons.
These hydrocarbons have different boiling points.
This difference allows them to be separated into groups called fractions using fractional distillation.
Shorter-chain hydrocarbons generally:
- have lower boiling points
- are more volatile
- are less viscous
Longer-chain hydrocarbons generally:
- have higher boiling points
- are less volatile
- are more viscous
These differences help determine how petroleum fractions are separated and used.
Intermolecular Forces vs Covalent Bonds
A common misunderstanding is that boiling breaks covalent bonds.
It does not.
Consider liquid hexane.
Within each molecule:
C–C and C–H covalent bonds remain intact.
During boiling:
intermolecular attractions between separate hexane molecules are overcome.
The molecules themselves remain hexane molecules.
This distinction between intramolecular bonds and intermolecular forces is extremely important.
Intermolecular vs Intramolecular
Intramolecular means:
within a molecule
Examples in alkanes:
- C–C covalent bonds
- C–H covalent bonds
Intermolecular means:
between molecules
Example in alkanes:
- London dispersion forces
Physical changes such as boiling mainly involve overcoming intermolecular forces.
Chemical reactions involve changes in chemical bonding.
Worked Example 1: Boiling Point
Which would probably have the higher boiling point?
C₄H₁₀
or:
C₈H₁₈
Answer:
C₈H₁₈
Reason:
It is larger, contains more electrons, and experiences stronger London dispersion forces.
Therefore, more energy is required to separate its molecules.
Worked Example 2: Volatility
Which is likely to be more volatile?
pentane
or:
decane
Answer:
pentane
Pentane is smaller and has weaker intermolecular attractions.
Its molecules escape into the gas phase more easily.
Worked Example 3: Water Solubility
Would hexane be expected to dissolve readily in water?
No.
Hexane is nonpolar.
Water is polar.
Therefore, hexane has very low solubility in water.
Worked Example 4: Physical State
Why is methane a gas while many larger alkanes are liquids?
Methane is a very small molecule with weak London dispersion forces.
Larger alkanes experience stronger intermolecular forces.
Therefore, more energy is required to separate larger alkane molecules.
Worked Example 5: Viscosity
Which would generally be more viscous?
hexane
or:
a much longer-chain alkane
Answer:
the longer-chain alkane
Longer molecules experience stronger intermolecular attractions and greater molecular interaction.
Worked Example 6: Branching
Two compounds have the molecular formula:
C₅H₁₂
One is straight-chain pentane.
The other is highly branched.
Which generally has the lower boiling point?
The more highly branched isomer.
Its compact shape generally reduces effective surface contact between molecules.
Worked Example 7: Boiling
What bonds are broken when liquid octane boils?
Under normal boiling:
the C–C and C–H covalent bonds are not broken.
Instead, intermolecular forces between octane molecules are overcome.
Worked Example 8: Predicting a Trend
Arrange these in order of increasing boiling point:
propane, hexane, nonane
Increasing molecular size:
propane < hexane < nonane
Therefore, expected boiling point order:
propane < hexane < nonane
Worked Example 9: Practical Application
Why are long-chain hydrocarbons more useful than methane as components of lubricants?
Long-chain hydrocarbons:
- are less volatile
- have higher boiling points
- generally have higher viscosity
Methane is a gas under ordinary conditions and would not remain as a lubricating liquid between moving surfaces.
Worked Example 10: Explaining a Trend
A student states:
"Octane has a higher boiling point than butane because octane has stronger covalent bonds."
This explanation is incorrect.
The important difference is not that octane has fundamentally stronger C–C bonds.
Instead:
Octane is a larger molecule with more electrons.
Therefore, it experiences stronger London dispersion forces between molecules.
More energy is required to separate those molecules.
Therefore, octane has a higher boiling point.
Reading an Alkane Property Graph
A graph of boiling point against number of carbon atoms generally rises as the carbon number increases.
When interpreting such a graph:
Step 1: Identify the variable on each axis.
Step 2: Look for the overall trend.
Step 3: Compare specific compounds.
Step 4: Describe the trend.
Step 5: Explain the trend using molecular structure and intermolecular forces.
A strong scientific explanation does more than say:
"Boiling point increases."
It explains why.
Explaining Trends Scientifically
A useful structure for an explanation is:
Observation → Molecular Change → Force Change → Property Change
For example:
Observation: Boiling point increases down the alkane series.
Molecular change: Molecules become larger and contain more electrons.
Force change: London dispersion forces become stronger.
Property change: More energy is required to separate the molecules.
Conclusion: Boiling point increases.
This reasoning can be used for many questions about alkane properties.
Practical Properties Summary
Smaller alkanes generally have:
- lower boiling points
- greater volatility
- lower viscosity
- weaker intermolecular attractions
Larger alkanes generally have:
- higher boiling points
- lower volatility
- greater viscosity
- stronger intermolecular attractions
All alkanes are generally:
- nonpolar
- poorly soluble in water
- soluble in many nonpolar substances
- poor electrical conductors
Common Mistakes
Mistake 1: Saying alkanes have no intermolecular forces
Alkanes experience:
London dispersion forces
Mistake 2: Confusing intermolecular forces with covalent bonds
Covalent bonds act:
within molecules
London dispersion forces act:
between molecules
Mistake 3: Saying covalent bonds break when an alkane boils
Boiling overcomes intermolecular forces.
The molecules themselves remain intact.
Mistake 4: Saying larger alkanes have lower boiling points
The general trend is:
larger alkane → higher boiling point
Mistake 5: Saying alkanes dissolve well in water
Alkanes are generally nonpolar, while water is polar.
They therefore have very low water solubility.
Mistake 6: Assuming melting points increase perfectly smoothly
Melting point is influenced by molecular packing and symmetry as well as intermolecular forces.
The trend is therefore less regular than the boiling-point trend.
Mistake 7: Assuming all alkanes are liquids
Small alkanes can be gases.
Intermediate alkanes are often liquids.
Long-chain alkanes can be solids.
Mistake 8: Saying branching changes the molecular formula
Structural isomers can have the same molecular formula but different arrangements of atoms.
Branching can change physical properties without changing the molecular formula.
Did You Know?
The physical properties of hydrocarbons are one reason crude oil can be separated into useful fractions.
Hydrocarbon molecules with different sizes have different boiling points.
These differences allow petroleum refineries to separate complex mixtures into fractions used for:
- fuels
- solvents
- lubricants
- waxes
- chemical feedstocks
A change at the molecular level — such as increasing carbon-chain length — can therefore have major practical consequences.
Key Terms
- Physical property: Characteristic that can be observed or measured without changing a substance into a different substance.
- Nonpolar: Having little or no permanent separation of electrical charge across a molecule.
- Intermolecular force: Attraction acting between separate molecules.
- London dispersion force: Intermolecular attraction caused by temporary fluctuations in electron distribution.
- Instantaneous dipole: Temporary uneven distribution of charge in a particle.
- Induced dipole: Temporary dipole produced by the influence of a nearby charge distribution.
- Polarizability: Ease with which an electron cloud can be distorted.
- Boiling point: Temperature at which a liquid boils at a specified pressure.
- Melting point: Temperature at which a solid becomes a liquid at a specified pressure.
- Volatility: Tendency of a substance to vaporize.
- Viscosity: Resistance of a fluid to flowing.
- Solubility: Amount or ability of a substance to dissolve in another substance.
- Density: Mass per unit volume.
- Intramolecular: Acting within a molecule.
- Homologous series: Family of related organic compounds with similar chemical properties and a common general formula.
- Structural isomer: Compound with the same molecular formula as another compound but a different arrangement of atoms.
Key Trends
As alkane carbon-chain length increases:
Molecular size ↑
Number of electrons ↑
Polarizability ↑
London dispersion forces ↑
Boiling point ↑
Volatility ↓
Viscosity generally ↑
Physical state tends toward gas → liquid → solid
Melting point:
generally increases overall, but irregularly
Water solubility:
remains very low
Key Takeaways
- Alkanes are generally nonpolar molecules.
- Their main intermolecular attractions are London dispersion forces.
- London dispersion forces arise from temporary changes in electron distribution.
- Larger alkane molecules have more electrons and more polarizable electron clouds.
- London dispersion forces generally become stronger as alkane molecules become larger.
- Stronger intermolecular forces require more energy to overcome.
- Therefore, boiling points generally increase as carbon-chain length increases.
- Melting points increase overall but show a less regular pattern because molecular packing is important.
- Smaller alkanes tend to be gases, intermediate alkanes liquids, and sufficiently long-chain alkanes solids or waxy materials at room temperature.
- Volatility generally decreases as chain length increases.
- Viscosity generally increases as chain length increases.
- Alkanes are generally insoluble in water because alkanes are nonpolar while water is polar.
- Many liquid alkanes are less dense than water and form a separate layer above it.
- Branching generally lowers boiling point among comparable alkane isomers because compact molecules tend to have less effective intermolecular contact.
- Boiling does not break the covalent bonds inside alkane molecules; it overcomes intermolecular forces between molecules.
- Alkane physical properties help determine their practical uses as gases, liquid fuels, solvents, lubricants, and waxes.
- Understanding intermolecular forces allows us to explain and predict the physical properties of members of the alkane homologous series.