3. Intermolecular Forces

Bonds Within Molecules and Forces Between Molecules

To understand the properties of substances, it is important to distinguish between chemical bonds and intermolecular forces.

A chemical bond holds atoms together within a substance. Examples include:

• Covalent bonds
• Ionic bonds
• Metallic bonds

An intermolecular force is an attraction between separate molecules.

For example, in liquid water:

• Strong covalent O–H bonds hold each water molecule together.
• Intermolecular attractions occur between neighboring water molecules.

These are very different interactions.

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Intramolecular and Intermolecular

The word intramolecular means:

within a molecule

The word intermolecular means:

between molecules

For water:

Intramolecular: O–H covalent bonds

Intermolecular: attractions between one H₂O molecule and another H₂O molecule

This distinction is especially important when explaining melting and boiling.

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Why Intermolecular Forces Matter

Intermolecular forces influence many physical properties, including:

• Melting point
• Boiling point
• Evaporation rate
• Viscosity
• Surface tension
• Solubility
• Physical state at room temperature

In general:

stronger intermolecular forces → more energy required to separate molecules

This often produces:

• Higher boiling points
• Lower evaporation rates
• Greater viscosity
• Greater surface tension

The three important types we will consider are London dispersion forces, dipole-dipole attractions, and hydrogen bonding.

London Dispersion Forces

London dispersion forces are weak intermolecular attractions caused by temporary changes in electron distribution.

Electrons are constantly moving.

At one instant, electrons may become slightly more concentrated on one side of a molecule.

This creates a temporary:

• Slightly negative region
• Slightly positive region

This temporary dipole can cause a neighboring molecule to develop another temporary dipole.

The opposite partial charges then attract.

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London Forces Occur in All Molecules

London dispersion forces occur between:

• Nonpolar molecules
• Polar molecules
• Individual atoms such as noble gases

Therefore:

all atoms and molecules can experience London dispersion forces.

For nonpolar molecules, London forces may be the only intermolecular attraction present.

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Example: Methane

Methane:

CH₄

is nonpolar.

Therefore, methane molecules do not experience permanent dipole-dipole attractions.

However, temporary changes in their electron clouds create London dispersion forces.

These forces are relatively weak, helping explain why methane has a very low boiling point and exists as a gas under ordinary room conditions.

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What Makes London Forces Stronger?

London dispersion forces generally become stronger as:

• The number of electrons increases.
• The electron cloud becomes larger and more easily distorted.
• Molecular size increases.
• The area of contact between molecules increases.

This helps explain why larger molecules often have higher boiling points than smaller molecules of a similar type.

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Example: The Halogens

Consider:

F₂

Cl₂

Br₂

I₂

All are nonpolar molecules.

As molecular size and number of electrons increase, London dispersion forces become stronger.

This contributes to the familiar change in physical state:

• F₂ – gas
• Cl₂ – gas
• Br₂ – liquid
• I₂ – solid

The larger molecules experience stronger intermolecular attractions.

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Dipole-Dipole Forces

Dipole-dipole forces occur between polar molecules.

A polar molecule has:

• A partially positive region, δ+
• A partially negative region, δ−

The δ+ region of one molecule attracts the δ− region of another.

These attractions are usually stronger than London dispersion forces between similarly sized molecules.

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Example: Hydrogen Chloride

Hydrogen chloride:

HCl

is polar.

Chlorine attracts the bonding electrons more strongly than hydrogen.

Therefore:

Hδ+ — Clδ−

The positive end of one HCl molecule can attract the negative end of another.

These are:

dipole-dipole attractions

HCl also experiences London dispersion forces because London forces occur in all molecules.

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Molecules Can Have More Than One Type of Attraction

It is important not to think that each substance has only one type of intermolecular force.

For example, polar molecules experience:

London dispersion forces

and:

dipole-dipole attractions

Some molecules also experience hydrogen bonding.

When we classify intermolecular forces, we often focus on the strongest important type present.

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Hydrogen Bonding

Hydrogen bonding is a particularly strong type of intermolecular attraction.

It occurs when hydrogen is covalently bonded to a strongly electronegative atom, especially:

• Nitrogen
• Oxygen
• Fluorine

Common examples include:

H₂O

NH₃

HF

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Hydrogen Bonding in Water

Water is polar.

The oxygen atom has a partial negative charge:

Oδ−

The hydrogen atoms have partial positive charges:

Hδ+

The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of another.

This attraction is a:

hydrogen bond

A hydrogen bond acts between molecules.

It is not the same as the covalent O–H bond inside a water molecule.

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Comparing the Three Main Intermolecular Forces

For molecules of broadly comparable size, a useful general comparison is:

London dispersion < dipole-dipole < hydrogen bonding

However, this is a general trend, not an absolute ranking for every substance.

Large molecules can have very strong London dispersion forces because they contain many electrons and have highly polarizable electron clouds.

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Intermolecular Forces and Boiling

For a liquid to boil, its molecules must separate from one another and enter the gas phase.

This requires energy to overcome intermolecular attractions.

Therefore, substances with stronger intermolecular forces often have:

higher boiling points

The covalent bonds inside the molecules normally remain intact during boiling.

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Boiling Does Not Break Molecules Apart

Consider water boiling:

H₂O(l) → H₂O(g)

Liquid water contains H₂O molecules.

Water vapor also contains H₂O molecules.

During boiling:

intermolecular attractions are overcome

but the O–H covalent bonds remain intact.

The molecules separate from one another; the molecules themselves are not normally broken apart.

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Melting and Intermolecular Forces

A similar idea applies when many molecular solids melt.

In a solid, particles are held in relatively fixed positions.

As energy is supplied:

• Particle motion increases.
• Intermolecular attractions are increasingly overcome.
• Molecules become able to move past one another.

The substance becomes liquid.

Again, melting generally does not require breaking the covalent bonds inside individual molecules.

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Intermolecular Forces and Evaporation

Evaporation occurs when molecules at the surface of a liquid gain enough energy to escape into the gas phase.

If intermolecular forces are weak:

molecules escape more easily

and evaporation tends to occur faster.

If intermolecular forces are strong:

more energy is required

and evaporation tends to be slower.

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Everyday Example: Different Liquids Evaporate Differently

Different liquids can evaporate at different rates because their molecules experience different intermolecular attractions.

This is why some volatile liquids disappear quickly when exposed to air while others remain much longer.

Temperature also matters.

At higher temperatures, molecules have greater average kinetic energy, so evaporation generally occurs faster.

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Intermolecular Forces and Vapor Pressure

Molecules escaping from a liquid produce vapor above its surface.

Liquids with weaker intermolecular forces generally allow molecules to escape more easily.

They therefore tend to have:

higher vapor pressures

at the same temperature.

Stronger intermolecular forces generally produce:

lower vapor pressures

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Intermolecular Forces and Viscosity

Viscosity describes a fluid's resistance to flowing.

A liquid with high viscosity flows relatively slowly.

Examples include:

• Honey
• Syrup
• Some oils

A liquid with low viscosity flows more easily.

Strong intermolecular attractions can contribute to greater viscosity because molecules resist moving past one another.

Molecular size and shape also affect viscosity.

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Intermolecular Forces and Surface Tension

Molecules inside a liquid experience attractions from neighboring molecules in many directions.

Molecules at the surface do not have molecules above them.

This creates a net inward attraction and produces surface tension.

Water has relatively high surface tension partly because of hydrogen bonding.

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Water Droplets

Water often forms rounded droplets.

Hydrogen bonding causes water molecules to attract one another strongly.

The molecules tend to remain together, and surface tension tends to reduce the surface area.

This contributes to the rounded shape of small water droplets.

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Water Striders

Some small insects can move across the surface of water.

They are supported partly by water's surface tension.

Hydrogen bonding between water molecules contributes to this unusually strong surface behavior.

This is a good example of a microscopic molecular interaction producing a visible everyday effect.

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Cohesion

Cohesion is attraction between molecules of the same substance.

Water molecules show strong cohesion because they can form hydrogen bonds with one another.

Cohesion contributes to:

• Water droplets
• Surface tension
• Continuous columns of water in plant xylem

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Adhesion

Adhesion is attraction between different substances.

Water can be attracted to polar surfaces.

For example, water may adhere to the walls of narrow glass tubes.

The balance between:

cohesion

and:

adhesion

helps explain phenomena such as capillary action.

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Capillary Action

Water can rise through very narrow tubes.

This occurs because:

• Water molecules adhere to the tube surface.
• Water molecules cohere to one another.
• Additional molecules are pulled along.

Capillary action contributes to water movement through narrow spaces, including structures in plants.

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Intermolecular Forces and Solubility

Whether one substance dissolves in another depends partly on the intermolecular attractions involved.

A useful general guideline is:

like dissolves like

Polar substances often dissolve more readily in polar solvents.

Nonpolar substances often dissolve more readily in nonpolar solvents.

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Why Sugar Dissolves in Water

Sugar molecules contain several polar O–H groups.

These groups can interact strongly with water molecules, including through hydrogen bonding.

Water molecules can surround sugar molecules and separate them from one another.

As a result, many sugars dissolve readily in water.

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Why Oil Does Not Mix Well with Water

Water is strongly polar.

Most oils are largely nonpolar.

Water molecules interact strongly with one another through hydrogen bonding, while oil molecules interact mainly through London dispersion forces.

Water and oil therefore do not interact favorably enough to mix completely under ordinary conditions.

They tend to form separate layers.

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Soap and Intermolecular Forces

Soap molecules contain:

• A water-attracting polar or ionic head.
• A nonpolar hydrocarbon tail.

The nonpolar tail interacts with oils and grease.

The polar head interacts strongly with water.

Soap therefore helps disperse grease in water so that it can be washed away.

This is an everyday application of molecular polarity and intermolecular interactions.

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Comparing Similar Molecules

Suppose two molecules have similar sizes.

One is nonpolar and experiences mainly London dispersion forces.

The other is polar and experiences London dispersion plus dipole-dipole attractions.

The polar substance will often have stronger overall intermolecular attractions.

It may therefore have:

• A higher boiling point.
• A lower evaporation rate.
• A higher viscosity.

However, molecular size and shape must also be considered.

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Molecular Size Matters

Consider two nonpolar molecules.

If one molecule is much larger and contains many more electrons, its electron cloud is generally more polarizable.

This produces stronger temporary dipoles.

Therefore:

larger molecule → stronger London dispersion forces

in many comparable substances.

This explains why London dispersion forces can become very important in large molecules.

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Molecular Shape Matters Too

Molecules with large areas of contact can sometimes experience stronger dispersion attractions than compact molecules of similar mass.

Long molecules can often make more surface contact with neighboring molecules.

Therefore, intermolecular forces depend not only on:

polarity

but also on:

size and shape

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Comparing Intermolecular Attractions

Remember that actual strength depends on the molecules being compared.

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Bonds Versus Intermolecular Forces

Consider liquid water.

To convert:

H₂O(l) → H₂O(g)

we mainly overcome:

intermolecular hydrogen bonding

To chemically decompose water into hydrogen and oxygen, we must break:

covalent O–H bonds

Breaking covalent bonds requires much more energy than simply separating water molecules during boiling.

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Physical Change Versus Chemical Change

Intermolecular forces help explain the difference between many physical and chemical changes.

Physical change

Examples:

• Melting
• Freezing
• Boiling
• Condensation

The molecules themselves usually remain chemically unchanged.

Chemical change

Chemical bonds are broken and/or formed, producing different substances.

This distinction is fundamental in chemistry.

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Real-World Connection: Sweating

Sweating helps cool the body because water evaporates from the skin.

For water molecules to evaporate, they must gain enough energy to overcome intermolecular attractions.

That energy is taken from the surroundings, including the skin.

The remaining surface therefore loses thermal energy.

This contributes to cooling.

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Real-World Connection: Cooking

Intermolecular forces influence:

• Boiling of water.
• Evaporation.
• Dissolving ingredients.
• Mixing oils and water.
• Texture and viscosity.

When water boils during cooking, the water molecules separate from one another but remain H₂O molecules.

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Real-World Connection: Plants

Water's intermolecular attractions are important in plants.

Hydrogen bonding contributes to strong cohesion between water molecules.

Together with adhesion and other processes, this helps maintain columns of water within xylem vessels.

This contributes to water transport from roots toward leaves.

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Real-World Connection: Cleaning

Cleaning often involves interactions among:

• Water.
• Oils.
• Detergents.
• Surfaces.

Detergents help bridge the difference between polar water and nonpolar grease.

Understanding intermolecular forces therefore helps explain why soap and detergent make water more effective at removing oily substances.

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Predicting Physical Properties

Suppose Substance A and Substance B have similar molecular sizes.

Substance A has mainly London dispersion forces.

Substance B forms hydrogen bonds.

We would generally predict that Substance B has:

• Stronger intermolecular attractions.
• A higher boiling point.
• Slower evaporation.

This type of reasoning allows molecular structure to be connected to observable physical properties.

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Worked Comparison: CH₄ and H₂O

Methane, CH₄

• Nonpolar.
• Mainly London dispersion forces.
• Weak intermolecular attractions.
• Gas at room temperature.

Water, H₂O

• Polar.
• Hydrogen bonding.
• Much stronger intermolecular attractions.
• Liquid at room temperature.

The difference in intermolecular forces helps explain their very different physical properties.

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Worked Comparison: HCl and H₂O

HCl

• Polar.
• London dispersion forces.
• Dipole-dipole attractions.

H₂O

• Polar.
• London dispersion forces.
• Dipole-dipole interactions.
• Hydrogen bonding.

Water therefore has especially strong intermolecular attractions compared with many similarly small molecules.

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Common Mistakes

Saying Intermolecular Forces Are Bonds Inside Molecules

Intermolecular forces act between separate molecules.

Covalent bonds hold atoms together within molecules.

Saying Boiling Breaks Covalent Bonds

Boiling generally overcomes intermolecular attractions.

The molecules themselves remain intact.

Saying Only Nonpolar Molecules Have London Forces

All atoms and molecules experience London dispersion forces.

Saying Polar Molecules Only Have Dipole-Dipole Forces

Polar molecules also experience London dispersion forces.

Saying Every Molecule Containing Hydrogen Forms Hydrogen Bonds

Hydrogen bonding generally requires hydrogen to be directly bonded to N, O, or F.

CH₄ does not form conventional hydrogen bonds even though it contains hydrogen.

Assuming London Forces Are Always Negligible

They can become very strong in large molecules with many electrons.

Assuming Intermolecular Force Is the Only Factor Affecting Boiling Point

Molecular size, shape and the combination of intermolecular interactions must also be considered.

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Check Your Understanding

1. What is the difference between an intramolecular bond and an intermolecular force?

2. Name three important types of intermolecular attraction.

3. What causes London dispersion forces?

4. Do nonpolar molecules experience intermolecular forces? Explain.

5. Why do larger molecules often have stronger London dispersion forces?

6. What type of molecule experiences dipole-dipole attraction?

7. Explain how dipole-dipole attractions form.

8. Which atoms are commonly involved in hydrogen bonding?

9. Explain hydrogen bonding between water molecules.

10. Why does boiling water not normally break O–H covalent bonds?

11. Explain why stronger intermolecular forces generally increase boiling point.

12. How do intermolecular forces affect evaporation?

13. Define viscosity.

14. Explain how intermolecular forces can influence viscosity.

15. What is surface tension?

16. Explain why water has relatively high surface tension.

17. Distinguish between cohesion and adhesion.

18. Explain how intermolecular forces contribute to capillary action.

19. Why do oil and water tend to separate?

20. Explain how soap helps water remove grease.

21. Compare the intermolecular forces in CH₄ and H₂O.

22. Why can London dispersion forces become significant in very large molecules?

23. Predict which would generally evaporate faster: a liquid with weak intermolecular forces or one with strong intermolecular forces. Explain.

24. Explain how intermolecular forces contribute to cooling during sweating.

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Key Terms

• Intermolecular force – attraction between separate molecules.
• Intramolecular bond – chemical bond acting within a molecule.
• London dispersion force – attraction resulting from temporary fluctuations in electron distribution.
• Temporary dipole – temporary uneven distribution of charge.
• Dipole-dipole force – attraction between permanent partial charges of polar molecules.
• Hydrogen bond – relatively strong intermolecular attraction involving hydrogen bonded to N, O, or F.
• Polarity – uneven distribution of electrical charge.
• Boiling point – temperature at which a liquid boils at a given pressure.
• Evaporation – escape of particles from the surface of a liquid into the gas phase.
• Vapor pressure – pressure produced by vapor above a liquid in a closed system.
• Viscosity – resistance of a fluid to flowing.
• Surface tension – tendency of a liquid surface to resist being stretched or disrupted.
• Cohesion – attraction between molecules of the same substance.
• Adhesion – attraction between different substances.
• Capillary action – movement of liquid through narrow spaces due partly to cohesive and adhesive forces.

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Key Takeaways

• Chemical bonds hold atoms together within substances, while intermolecular forces act between separate molecules.
• Intermolecular forces are generally weaker than covalent bonds.
• Physical changes such as boiling usually overcome intermolecular attractions rather than breaking molecules apart.
• London dispersion forces occur in all atoms and molecules.
• London forces result from temporary changes in electron distribution.
• Larger, more polarizable electron clouds generally produce stronger London dispersion forces.
• Polar molecules can experience dipole-dipole attractions.
• Hydrogen bonding is a particularly strong intermolecular attraction found in suitable molecules containing H bonded to N, O, or F.
• Water forms extensive hydrogen bonds.
• For similarly sized small molecules, a useful general trend is London dispersion < dipole-dipole < hydrogen bonding.
• Actual intermolecular attraction also depends on molecular size and shape.
• Stronger intermolecular forces generally lead to higher boiling points.
• Stronger attractions generally reduce evaporation.
• Intermolecular forces can influence viscosity and surface tension.
• Hydrogen bonding contributes to water's unusually high surface tension.
• Cohesion describes attraction between like molecules, while adhesion describes attraction between different substances.
• Intermolecular forces help explain capillary action.
• Polarity and intermolecular attractions strongly influence solubility.
• Intermolecular forces help explain everyday phenomena including water droplets, evaporation, sweating, cleaning, cooking and water transport in plants.
• Understanding intermolecular forces connects microscopic molecular interactions to the physical properties we can observe and measure.