Intermolecular forces


Induced dipole-dipole interactions (London forces)

  • London forces are weak intermolecular forces that exist between all molecules, whether polar or non-polar. 
  • Movement of electrons produces a changing dipole in a molecule.
  • At any instant, an instantaneous dipole will exist, but its position is constantly shifting. 
  • The instantaneous dipole induces a dipole on a neighbouring molecule.
  • the induced dipole induces further dipoles on neighbouring molecules, which then attract one another. 
  • Induced dipoles are only temporary. 
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Strength of induced dipole-dipole interactions

The more electrons in each molecule:

  • the larger the instantaneous and induced dipoles.
  • the greater the induced dipole-dipole interactions
  • the stronger the attractive forces between molecules
  • Larger numbers of electrons means larger induced dipoles so more energy is needed to overcome the intermolecular forces so the boiling point increases. 
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Permanent dipole-dipole interactions

  • Act between the permanent dipoles in different polar molecules. 
  • Extra energy is needed to break the additional permanent dipole-dipole interactions. 
  • This means the boiling point of molecules with permanent dipole-dipole inetarctions is higher. 
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Simple molecular substances

  • A simple molecular substance is made up of simple molecules - small units containing a definite number of atoms with a definite molecular structure such as neon, hydrogen, water, and carbon dioxide. 
  • In the soild state, simple molecules form a regular structure called a simple molecular lattice.
  • The molecules are held in place by weak intermolecular forces. 
  • The atoms within each molecule are strongly bonded together by covalent bonds. 
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Boiling and melting points

  • In simple molecular lattice, the weak intermolecular forces can be broken even by the energy present at low temperatures. 
  • Simple molecular substances have low melting and boiling points. 
  • When a simple molecular lattice is broken apart during melting only the weak intermolecular forces break. The covalent bonds are strong and do not break. 
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Solubility of non-polar simple molecular substance

  • When a simple molecular compound is added to a non-polar solvent, such as hexane, intermolecular forces form between the molecules and the solvent. 
  • The interactions weaken the intermolecular forces in the simple molecular lattice. The intermolecular forces break and the compound dissolves. 
  • Non-polar simple molecular substances tend to be soluble in non-polar substances. 
  • When a simple molecular substance is added to a polar solvent there is little interaction between the molecules in the lattice and the solvent molecules. 
  • The intermolecular bonding within the polar solvent is too strong to be broken. 
  • Simple molecular substances tend to be insouble in polar solvents. 
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Solubility of polar simple molecular substances

  • Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other. 
  • Solubility depends upon the strength of the dipole and can be hard to predict. 
  • Some compounds, such as ethanol, contain both polar and non-polar parts in their structure and can dissolve in both polar and non-polar solvents. 
  • Some biological molecules have hydrophobic and hydrophilic parts. The hydrophilic part will be polar and contain elecytronegative atoms that can interact with water. The hydrophobic part will be non-polar and comprised of a carbon chain. 
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Electrical conductivity

  • There are no mobile charged particles in simple molecular structures. 
  • With no charged particles that can move, there is nothing to complete an electrical circuit. 
  • Simple molecular structures are non-conductors of electricity. 
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