Intermolecular Forces

• What kind of attraction exists between molecules due to intermolecular forces?

  Weak attractions
• What are intermolecular forces?

  Attractive forces that exist between molecules
• Are intermolecular forces stronger or weaker than covalent bonds?

  Weaker
• What are the three main types of intermolecular force, listed in order of increasing strength?

  • Induced dipole-dipole forces
  • Permanent dipole-dipole forces
  • Hydrogen bonding
• What are the key characteristics of induced dipole-dipole forces, permanent dipole-dipole forces and hydrogen bonding?

  • Induced dipole-dipole forces: weak, found in all molecules and noble gases
  • Permanent dipole-dipole forces: moderate, found in polar molecules
  • Hydrogen bonding: strong, found in polar molecules with H-F, H-O or H-N bonds
• Between what types of molecules do induced dipole-dipole forces occur?

  All molecules
• What are other names for induced dipole-dipole forces?

  London dispersion forces or Van der Waals’ forces
• What causes induced dipole-dipole forces?

  Temporary fluctuations in electron distribution
• How do induced dipole-dipole forces arise?

  • Electrons in atoms are constantly moving, which can lead to more electrons on one side of the atom than the other, creating a temporary dipole.
  • This temporary dipole can induce an opposite dipole in a neighbouring atom, causing a weak electrostatic attraction.
  • This induced dipole can then induce further dipoles in other nearby particles.
  • The overall effect is a net attraction between the atoms or molecules.
• What factors affect the strength of induced dipole-dipole forces?

  • Size: Larger atoms and molecules have stronger forces.
  • Surface area: Molecules with a larger surface area have stronger forces.
• How does the strength of induced dipole-dipole forces affect boiling points?

  Stronger forces lead to higher boiling points
• What trend is observed in the boiling points of group 4 hydrides?

  Increase down the group
• What causes the trend in boiling points of group 4 hydrides?

  Increasing molecular size and number of electrons
• What is the relationship between induced dipole-dipole forces and boiling points in group 4 hydrides?

  Stronger forces lead to higher boiling points
• How do induced dipole-dipole forces contribute to molecular lattice structures?

  • Induced dipole-dipole forces can be strong enough to hold molecules together in a lattice structure.
  • Example: Solid iodine (I2) molecules are held together by covalent bonds, then these I2 molecules are attracted to each other by weak induced dipole-dipole forces, forming a molecular lattice.
• Why do noble gases exist as liquids and solids at very low temperatures?

  Due to weak induced dipole-dipole forces
• What kind of molecules experience permanent dipole-dipole forces?

  Polar molecules
• What causes permanent dipoles in polar molecules?

  Unequal sharing of electrons
• What are permanent dipole-dipole forces?

  Electrostatic attractions between partial charges on molecules
• How do permanent dipole-dipole forces arise?

  • Permanent dipole-dipole forces are electrostatic attractions between the partial positive end of one polar molecule and the partial negative end of another.
  • Example: In gaseous hydrogen chloride (HCl), the H-Cl bond is polar due to the greater electronegativity of chlorine compared to hydrogen.
  • The hydrogen atom bears a partial positive charge (δ+) and the chlorine a partial negative charge (δ-).
  • HCl molecules align so the δ+ hydrogen of one molecule is attracted to the δ- chlorine of a neighbouring molecule.
• What types of intermolecular forces exist in polar molecules?

  Permanent and induced dipole-dipole forces
• How do the overall intermolecular forces compare between polar and non-polar molecules of similar size?

  Polar molecules have stronger forces
• What is the relationship between intermolecular forces and boiling points?

  Stronger forces lead to higher boiling points
• Why does methanal have a higher boiling point than ethane, even though they have similar molecular masses?

  • Methanal is polar and experiences both permanent and induced dipole-dipole forces.
  • Ethane is non-polar and only experiences induced dipole-dipole forces.
  • More energy is required to overcome the stronger intermolecular forces in methanal.
• What is hydrogen bonding?

  A special type of permanent dipole-dipole force
• When does hydrogen bonding occur?

  When hydrogen is bonded to F, O, or N
• What are the requirements for hydrogen bonding to occur?

  • The molecule must contain a hydrogen atom covalently bonded to either fluorine (F), oxygen (O), or nitrogen (N).
  • There must be a lone pair of electrons on the F, O, or N atom of an adjacent molecule available to interact with the hydrogen.
• How do hydrogen bonds form?

  • The H-F, H-O, and H-N bonds are highly polar due to the large electronegativity differences between hydrogen and these elements.
  • This leads to a significant partial positive charge (δ+) on the hydrogen atom and a partial negative charge (δ-) on the F, O, or N atom.
  • The small size of the hydrogen atom allows it to get close to the lone pair of electrons on an adjacent F, O, or N atom.
  • The positively charged hydrogen is strongly attracted to the negatively charged lone pair, forming a hydrogen bond between the molecules.
• In what types of molecules does hydrogen bonding occur?

  Molecules containing N-H, O-H, and H-F bonds
• What are the impacts of hydrogen bonding on properties?

  • Greater solubility in water (for substances that can form hydrogen bonds with water)
  • Higher melting and boiling points compared to similar-sized molecules that cannot hydrogen bond.
• How does hydrogen bonding affect the boiling point of water compared to other group 6 hydrides?

  Water has a significantly higher boiling point
• What intermolecular force explains increasing boiling points from H2S to H2Te?

  Induced dipole-dipole forces
• What properties of molecules cause increasing boiling points from H2S to H2Te?

  Increasing molecular mass and size
• How does hydrogen bonding explain the anomalous properties of water and ice?

  • Ice is denser than water: In solid ice, water molecules are arranged in a 3D lattice held together by hydrogen bonds. Upon melting, some of these hydrogen bonds break. Since hydrogen bonds are relatively long compared to covalent bonds, this causes ice to be less dense than liquid water.
  • Water and ice have high melting and boiling points: Water has relatively high melting and boiling points compared to other molecules of similar size. This is due to the strong hydrogen bonds between H2O molecules, which require more energy to break.
• Why is ice less dense than liquid water?

  Hydrogen bonds are long and form a 3D lattice
• Why do water and ice have relatively high melting and boiling points?

  Due to strong hydrogen bonds
• What influences the physical properties of simple molecular substances?

  Type and strength of intermolecular forces
• Why do covalent compounds often have low melting and boiling points?

  Weak intermolecular forces require little energy
• How does solubility in water relate to intermolecular forces?

  Polar molecules with hydrogen bonding are soluble
• Which type of molecule is hydrophobic and insoluble in water?

  Non-polar molecules