10.1 Intermolecular Forces

Molecules attract each other through different forces. These range from weak dispersion forces in nonpolar molecules to strong hydrogen bonds in water. Understanding these forces helps explain why substances behave differently.

Intermolecular forces impact physical properties like boiling points and viscosity. Stronger forces mean higher boiling points and more resistance to flow. This knowledge helps predict and explain the behavior of various substances in everyday life.

Types of Intermolecular Forces

Explain the three main types of intermolecular forces: dispersion forces, dipole-dipole attractions, and hydrogen bonding

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• Dispersion forces (London dispersion forces)
  • Weakest intermolecular force present in all molecules, including nonpolar ones ($\ce{H2}$, $\ce{Cl2}$, $\ce{CH4}$)
  • Caused by temporary, instantaneous dipoles resulting from the random movement of electrons creates brief attractive forces
  • Strength increases with increasing molecular size and surface area more electrons and larger surface area enhance dispersion forces
• Dipole-dipole attractions
  • Stronger than dispersion forces occur between polar molecules with permanent dipoles ($\ce{HCl}$, $\ce{SO2}$, $\ce{CHCl3}$)
  • Positive end of one dipole attracts the negative end of another dipole leading to net attractive force
  • Strength depends on the magnitude of the dipole moments larger dipole moments result in stronger attractions
• Hydrogen bonding
  • Strongest intermolecular force a special type of dipole-dipole attraction
  • Occurs when a hydrogen atom is bonded to a highly electronegative atom (N, O, or F) in molecules like $\ce{H2O}$, $\ce{NH3}$, $\ce{HF}$, alcohols, amines, and carboxylic acids
  • The small size of hydrogen allows for close approach and strong electrostatic attraction between the positive H and negative N, O, or F

Prediction of Intermolecular Forces

Predict the intermolecular forces present in substances based on molecular structure

• Nonpolar molecules
  • Only dispersion forces are present since there are no permanent dipoles
  • Examples include simple diatomic gases ($\ce{H2}$, $\ce{Cl2}$) and symmetrical hydrocarbons ($\ce{CH4}$)
• Polar molecules without H bonded to N, O, or F
  • Dispersion forces and dipole-dipole attractions are present due to permanent dipoles
  • Examples include polar molecules like $\ce{HCl}$, $\ce{SO2}$, and $\ce{CHCl3}$
• Polar molecules with H bonded to N, O, or F
  • Dispersion forces, dipole-dipole attractions, and hydrogen bonding are present
  • Examples include water ($\ce{H2O}$), ammonia ($\ce{NH3}$), hydrogen fluoride ($\ce{HF}$), alcohols, amines, and carboxylic acids

Impact on Physical Properties

Connect intermolecular forces to physical properties, especially melting and boiling points

• Stronger intermolecular forces lead to higher melting and boiling points
  1. More energy is required to overcome the attractive forces between molecules and break them apart
  2. Substances with only dispersion forces (noble gases, nonpolar hydrocarbons) tend to have lower melting and boiling points
• Substances with dipole-dipole attractions have higher melting and boiling points than those with only dispersion forces
  • Comparing $\ce{CH4}$ (dispersion forces only) and $\ce{CHCl3}$ (dispersion forces and dipole-dipole attractions) shows the impact of dipole-dipole forces
• Substances with hydrogen bonding have the highest melting and boiling points among molecules of similar size
  • Comparing $\ce{H2O}$ (hydrogen bonding) and $\ce{H2S}$ (dipole-dipole attractions) demonstrates the strength of hydrogen bonds
• Intermolecular forces also affect other physical properties
  • Viscosity: stronger forces lead to higher viscosity (resistance to flow)
  • Surface tension: stronger forces lead to higher surface tension (resistance to breaking the surface)
  • Vapor pressure: stronger forces lead to lower vapor pressure (less tendency to evaporate)

Effects of Intermolecular Forces on Liquid Properties

• Cohesion: the attraction between molecules of the same substance, which is stronger in liquids with stronger intermolecular forces
• Adhesion: the attraction between molecules of different substances, which can lead to wetting of surfaces
• Surface tension: the tendency of liquid surfaces to shrink into the minimum surface area possible, caused by the cohesive forces between molecules
• Capillary action: the ability of a liquid to flow against gravity in narrow spaces, resulting from the combined effects of cohesion and adhesion