Intermolecular Forces (DP IB Chemistry): Revision Note

Intermolecular forces

• Covalent molecular substances have no covalent bonds between their molecules

• Instead, they are held together by intermolecular forces, which are much weaker than covalent or ionic bonds

• These forces determine key physical properties such as:

  • Melting and boiling point

  • Volatility

  • Solubility

• There are four types of intermolecular force:

  • London (dispersion) forces

  • Dipole–dipole attractions

  • Dipole–induced dipole attractions

  • Hydrogen bonding

London (dispersion) forces

• Electrons in atoms and molecules are constantly moving

• At any moment, this motion can lead to an uneven distribution of electrons

  • This is a temporary dipole

  • Temporary dipoles are constantly appearing and disappearing

• This temporary dipole can induce a dipole in a neighbouring atom or molecule

  • This is a temporary induced dipole

  • This causes a weak attractive force between the atoms or molecules

• This attraction is known as a London (dispersion) force

• London (dispersion) forces are present between all atoms and molecules, but are usually very weak

  • They are the only intermolecular forces in nonpolar substances

• London (dispersion) forces tend to have strengths from 1 - 50 kJ mol^-1

• The strength of the London (dispersion) forces depends on:

  • The number of electrons in the atom or molecule

  • The surface area available for contact

Number of electrons

• The more electrons a molecule has, the greater the chance of an uneven distribution

• This increases the likelihood and strength of temporary dipoles

• As a result:

  • London (dispersion) forces become stronger

  • Melting and boiling points increase

• This trend is observed in the noble gases:

Surface area

• A larger surface area means more contact between molecules

• This increases the likelihood of temporary dipoles interacting

• London (dispersion) forces are stronger in molecules with extended or unbranched shapes

• This explains differences in boiling points between isomers with the same number of electrons but different shapes

Dipole-dipole attractions

• Some molecules have a permanent dipole due to a difference in electronegativity and an asymmetric shape

• These molecules experience dipole–dipole attractions

  • This is in addition to London (dispersion) forces

• Dipole-dipole attraction is between the δ⁺ end of one polar molecule and the δ⁻ end of a neighbouring molecule

The delta negative end of one polar molecule will be attracted towards the delta positive end of a neighbouring polar molecule

• Dipole–dipole attractions increase the strength of intermolecular forces

• As a result, compounds with permanent dipoles usually have higher boiling points than similar-sized nonpolar molecules

Comparing butane and propanone

• Butane and propanone have the same number of electrons

  • They experience similar London (dispersion) forces

  • Only propanone has a permanent dipole

• This means propanone experiences dipole–dipole attractions in addition to dispersion forces

• Therefore, more energy is needed to separate propanone molecules than butane molecules

  • This means that propanone has a higher boiling point than butane

Dipole-induced dipole attraction

• This type of attraction occurs when a polar molecule is placed near a nonpolar molecule

  • For example, hydrogen chloride (HCl)and chlorine (Cl₂)

• The permanent dipole of the polar molecule distorts the electron cloud of the nonpolar molecule

• This creates a temporary dipole in the nonpolar molecule, leading to a weak attractive force

• This force is called a dipole–induced dipole attraction

• It acts in addition to:

  • London (dispersion) forces between nonpolar molecules

  • Dipole–dipole forces between polar molecules

Hydrogen bonding

• Hydrogen bonding is the strongest type of intermolecular force

  • It is a special case of permanent dipole–dipole attraction

• For hydrogen bonding to occur, both of the following are needed:

  • A hydrogen atom covalently bonded to O, N, or F

  • A lone pair of electrons on an O, N, or F atom in a neighbouring molecule

• When hydrogen is bonded to one of these highly electronegative atoms:

  • The bond becomes strongly polarised

  • The hydrogen becomes very δ⁺ and is attracted to the lone pair on another molecule

• Hydrogen bonds are often represented by dotted or dashed lines

• The number of hydrogen bonds a molecule can form depends on:

  • The number of hydrogen atoms attached to O/N/F

  • The number of lone pairs available on O/N/F atoms

 Diagram to show hydrogen bonding in ammonia

Diagram to show hydrogen bonding in water

Summary: van der Waals and other intermolecular forces

• Van der Waals forces include:

  • London (dispersion) forces

  • Dipole–dipole attractions

  • Dipole–induced dipole attractions

• These are all intermolecular forces because they act between molecules, not within them

  • Intramolecular forces (e.g. covalent bonds) hold atoms together within a molecule