AQA AS Chemistry

Revision Notes

1.5.3 Types of Forces Between Molecules

Intramolecular Forces vs. Intermolecular Forces

Intramolecular forces

  • Intramolecular forces are forces within a molecule
  • Ionic bonding is the electrostatic attraction between positive (cations) and negative (anions) ions in an ionic crystal lattice
    • These ions are formed by transferring the electrons from one species to the other
  • Covalent bonds are formed when the outer electrons of two atoms are shared
  • Metallic bonding is the electrostatic attraction of positively charged metal ions and their delocalised electrons in a metal lattice

Chemical Bonding Intramolecular Forces, downloadable AS & A Level Chemistry revision notes

Intramolecular forces

Intermolecular forces

  • Intermolecular forces are forces between molecules and are also called Van der Waals’ forces
  • Permanent dipole – permanent dipole are the attractive forces between two neighbouring molecules with a permanent dipole
  • Hydrogen bonds are a special type of permanent dipole – permanent dipole forces
  • Instantaneous dipole – induced dipole (London dispersion) forces are the attractive forces between a temporary dipole and a neighbouring molecule with an induced dipole

Chemical Bonding Intermolecular Forces (1), downloadable AS & A Level Chemistry revision notes Chemical Bonding Intermolecular Forces (2), downloadable AS & A Level Chemistry revision notes Chemical Bonding Intermolecular Forces (3), downloadable AS & A Level Chemistry revision notes

Intermolecular forces

  • In general, the intramolecular forces are stronger than intermolecular forces
  • The strengths of the types of bond or force are as follows:

 

Chemical Bonding Strengths of different types of bonds, downloadable AS & A Level Chemistry revision notes

The varying strengths of different types of bonds

Polar Bonds

  • Electronegativity is the ability of an atom to draw a pair of electrons towards itself in a covalent bond
  • Electronegativity increases across a Period and decreases going down a Group

Polarity

  • When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar

Chemical Bonding Nonpolar Diatomic Molecule, downloadable AS & A Level Chemistry revision notes

The two chlorine atoms have similar electronegativities so the bonding electrons are shared equally between the two atoms

  • When two atoms in a covalent bond have different electronegativities the covalent bond is polar and the electrons will be drawn towards the more electronegative atom
  • As a result of this:
    • The negative charge centre and positive charge centre do not coincide with each other
    • This means that the electron distribution is asymmetric
    • The less electronegative atom gets a partial charge of δ+ (delta positive)
    • The more electronegative atom gets a partial charge of δ- (delta negative)
  • The greater the difference in electronegativity the more polar the bond becomes

Chemical Bonding Polar Diatomic Molecule, downloadable AS & A Level Chemistry revision notes

Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive

Assigning polarity to molecules

  • To determine whether a molecule with more than two atoms is polar, the following things have to be taken into consideration:
    • The polarity of each bond
    • How the bonds are arranged in the molecule
  • Some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such way that the individual dipole moments cancel each other out

Chemical Bonding Polar Molecule, downloadable AS & A Level Chemistry revision notes

There are four polar covalent bonds in CH3Cl which do not cancel each other out causing CH3Cl to be a polar molecule; the overall dipole is towards the electronegative chlorine atom

Chemical Bonding Nonpolar Molecule, downloadable AS & A Level Chemistry revision notes

Though CCl4 has four polar covalent bonds, the individual dipole moments cancel each other out causing CCl4 to be a nonpolar molecule

Induced Dipole-Dipole Forces

  • Covalent bonds are strong intramolecular forces
  • Molecules also contain weaker intermolecular forces which are forces between molecules
  • These intermolecular forces are called van der Waals’ forces
  • There are two types of van der Waals’ forces:
    • Instantaneous (temporary) dipole – induced dipole forces also called London dispersion forces
    • Permanent dipole – permanent dipole forces

Chemical Bonding Inter and Intramolecular Forces, downloadable AS & A Level Chemistry revision notes

The polar covalent bonds between O and H atoms are intramolecular forces and the permanent dipole – permanent dipole forces between the molecules are intermolecular forces as they are a type of van der Waals’ force

Instantaneous dipole – induced dipole (id – id)

  • Instantaneous dipole – induced dipole forces or London dispersion forces exist between all atoms or molecules
  • The electron charge cloud in non-polar molecules or atoms are constantly moving
  • During this movement, the electron charge cloud can be more on one side of the atom or molecule than the other
  • This causes a temporary dipole to arise
  • This temporary dipole can induce a dipole on neighbouring molecules
  • When this happens, the δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other
  • Because the electron clouds are moving constantly, the dipoles are only temporary

Chemical Bonding Id-Id Forces, downloadable AS & A Level Chemistry revision notes

Id-id (London dispersion) forces between two non-polar molecules

  • Id – id forces increase with:
    • Increasing number of electrons (and atomic number) in the molecule
    • Increasing the places where the molecules come close together

 

Chemical Bonding Enthalpy and Boiling Point, downloadable AS & A Level Chemistry revision notes

Going down the Group, the id-id forces increase due to the increased number of electrons in the atoms

Chemical Bonding Contact Points, downloadable AS & A Level Chemistry revision notes

The increased number of contact points in pentane means that it has more id-id forces and therefore a higher boiling point

Permanent dipole – permanent dipole (pd – pd)

  • Polar molecules have permanent dipoles
  • The molecule will always have a negatively and positively charged end
  • Forces between two molecules that have permanent dipoles are called permanent dipole – permanent dipole forces
  • The δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other

Chemical Bonding Permanent Dipole - Permanent Dipole, downloadable AS & A Level Chemistry revision notes

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

  • For small molecules with the same number of electrons, pd – pd forces are stronger than id – id
    • Butane and propanone have the same number of electrons
    • Butane is a nonpolar molecule and will have id – id forces
    • Propanone is a polar molecule and will have pd – pd forces
    • Therefore, more energy is required to break the intermolecular forces between propanone molecules than between butane molecules
    • So, propanone has a higher boiling point than butane

Chemical Bonding Pd - Pd vs Id - Id, downloadable AS & A Level Chemistry revision notes

Pd-pd forces are stronger than id-id forces in smaller molecules with an equal number of electrons

Exam Tip

Remember this difference: intramolecular forces are forces within a molecule, whereas intermolecular forces are forces between a molecule.

Hydrogen Bonds

Hydrogen bonding

  • Hydrogen bonding is the strongest form of intermolecular bonding
    • Intermolecular bonds are bonds between molecules
    • Hydrogen bonding is a type of permanent dipole – permanent dipole bonding
  • For hydrogen bonding to take place the following is needed:
    • A species which has an O or N (very electronegative) atom with an available lone pair of electrons
    • A species with an -OH or -NH group
  • When hydrogen is covalently bonded to an electronegative atom, such as O or N, the bond becomes very highly polarised
  • The H becomes so δ+ charged that it can form a bond with the lone pair of an O or N atom in another molecule

Chemical Bonding Polarisation, downloadable AS & A Level Chemistry revision notes

The electronegative atoms O or N have a stronger pull on the electrons in the covalent bond with hydrogen, causing the bond to become polarised

  • For hydrogen bonding to take place, the angle between the -OH/-NH and the hydrogen bond is 180o
  • The number of hydrogen bonds depends on:
    • The number of hydrogen atoms attached to O or N in the molecule
    • The number of lone pairs on the O or N

 

Chemical Bonding Ammonia H-Bonds, downloadable AS & A Level Chemistry revision notes

Ammonia can form a maximum of one hydrogen bond per molecule

 

Chemical Bonding Water H-Bonds, downloadable AS & A Level Chemistry revision notes

Water can form a maximum of two hydrogen bonds per molecule

Properties of water

  • Hydrogen bonding in water, causes it to have anomalous properties such as high melting and boiling points, high surface tension and anomalous density of ice compared to water

High melting & boiling points

  • Water has high melting and boiling points which is caused by the strong intermolecular forces of hydrogen bonding between the molecules
  • In ice (solid H2O) and water (liquid H2O) the molecules are tightly held together by hydrogen bonds
  • A lot of energy is therefore required to break the water molecules apart and melt or boil them

 

Chemical Bonding Melting and Boiling Points of Water, downloadable AS & A Level Chemistry revision notes

Hydrogen bonds are strong intermolecular forces which are difficult to break causing water to have high melting and boiling points

  • The graph below compares the enthalpy of vaporisation (energy required to boil a substance) of different hydrides
  • The enthalpy changes increase going from H2S to H2Te due to the increased number of electrons in the Group 16 elements
  • This causes an increased instantaneous dipole – induced dipole forces as the molecules become larger
  • Based on this, H2O would have a much lower enthalpy change (around 17 kJ mol-1)
  • However, the enthalpy change of vaporisation is almost 3 times larger which is caused by the hydrogen bonds present in water but not in the other hydrides

Chemical Bonding Trends Boiling Points Hydrides, downloadable AS & A Level Chemistry revision notes

The high enthalpy change of evaporation of water suggests that instantaneous dipole-induced dipole forces are not the only forces present in the molecule – there are also those of the strong hydrogen bonds, which cause the high boiling points

Density

  • Solids are denser than their liquids as the particles in solids are more closely packed together than in their liquid state
  • In ice however, the water molecules are packed in a 3D hydrogen-bonded network in a rigid lattice
  • Each oxygen atom is surrounded by hydrogen atoms
  • This way of packing the molecules in a solid and the relatively long bond lengths of the hydrogen bonds means that the water molecules are slightly further apart than in the liquid form
  • Therefore, ice has a lower density than liquid water

Chemical Bonding Density Water, downloadable AS & A Level Chemistry revision notes

The ‘more open’ structure of molecules in ice causes it to have a lower density than liquid water

Exam Tip

Ice floats on water because of ice’s lower density.

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