What determines if a molecular substance is a solid, a liquid or a gas

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What determines if a molecular substance is a
solid, a liquid or a gas?

• clue think between molecules!

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intermolecular forces

• What happens to covalently bonded molecules as
  they change state?

When a solid melts or a liquid boils the energy
put in is being used to overcome the
INTERMOLECULAR forces.
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What is a dipole?

• Dipole a molecule (or part of a molecule) with
  a positive end and a negative end.
• E.g. HCl molecules have a dipole.
• When a molecule has a dipole it is polarised.

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Permanent Dipoles

• These occur when a two atoms sharing a covalent
  bond have substantially different
  electronegativity.
• HCl has a permanent dipole as Cl has a much
  higher electronegativity than H.
• Molecules with a permanent dipole are described
  as polar molecules.

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Bonded atoms have similar electronegativity.Elect
ron cloud evenly distributed no dipole
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At some instant, more of the electron cloud
happens to be at one end of the molecule than the
other, electron density is constantly changing.
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At some instant, more of the electron cloud
happens to be at one end of the molecule than the
other molecule has a dipole.
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Instantaneous dipoles

• If this happens near other molecules it can cause
  induced dipoles

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Induced dipoles

• The permanent dipole of a molecule can induce a
  dipole in an unpolarised molecule.
• A dipole can also be induced by the effects of an
  instantaneous dipole.

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Dipoles Intermolecular Forces

• All intermolecular forces arise from the
  attraction between dipoles.
• There are three types of attraction.
• permanent dipole-permanent dipole.
• permanent dipole-induced dipole.
• instantaneous dipole-induced dipole.

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Instantaneous dipole induced dipole

• Much weaker than other types of intermolecular
  forces.
• Occur between all molecules.
• They can be observed in most easily in noble
  gases and alkanes.

Xe
Xe
Xe
This atom has an instant dipole
The electrons are repelled and an induced dipole
occurs
This atom is unpolarised, at the moment!
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Factors affecting instantaneous-induced dipole
intermolecular forces?

• Atom or molecule size.
• Molecular shape.

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Atom or molecule size

• Larger molecules (or isolated atoms) will have
  more electrons, hence the strength of the dipole
  interaction with be greater.

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Molecular shape

• In straight chain alkanes there more contacts
  between the atoms of different molecules.
• Therefore more opportunities for induced dipoles
  to occur.
• This is the reason straight chain alkanes have
  higher boiling points than their branched isomers.

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Polar molecules?

• Decide if these molecules are polar or not.
• and WHY they might be.

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Bond polarity v. Molecular polarity

• A polar molecule is a molecule which has a
  permanent dipole.
• The spatial arrangement of polar covalent bonds
  is an important factor in determining if a
  molecule is polar or not.

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Permanent dipolespermanent dipoles

• Present in polar molecular substances
• Act as additional electrostatic forces of
  attraction between these molecules.
• Stronger than instantaneous dipole induced
  dipole interactions for molecules of equivalent
  size.

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Dipole - dipole interactions
Overall more attraction than repulsion, this is
one reason why the molecules stay as a liquid.

• There are also permanent dipole induced dipole
  forces AND instantaneous dipole induced dipole
  forces as well.

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Effects of permanent dipole interactions

• If we have two substances,
• one polar and one non-polar,
• whose molecules have the same mass
• (and so approximately the same number of
  electrons),
• the polar substance will have a higher boiling
  point than the non-polar substance.