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Week 4 Chemie
Intermoleculaire krachten en oplossingen
Hoofdstuk 8 Gases, liquids and solids
What determines whether a substance is a gas, a liquid or a solid at a given temperature? Why do
molecular compounds have lower melting points than ionic compounds? To answer these we need to
look into the nature of intermolecular forces - the forces that act between different molecules rather
than within an individual molecule. In gases the intermolecular forces are negligible, so the gas
molecules act independently of one another. In liquids and solids, however, intermolecular forces are
strong enough to hold the molecules in close contact. As a general rule, the stronger the intermolecular
forces in a substance, the more difficult it is to separate the molecules, and higher the melting and
boiling points. There are 3 major types of intermolecular forces :
VAN DER WAALS FORCES
London dispersion forces (2-10 kJ/mol) : the short lived attractive force due to the constant
motion of electrons within molecules. All molecules, regardless the structure, experience this force.
London dispersion forces are the only intermolecular forces available for nonpolar molecules
(relatively weak but they increase with molecular massa and amount of surface area available for
interaction). Averaged over time, the distribution of electrons throughout the molecule is uniform,
but at any given instant there may be more electrons at one end of the molecule than the other. At
that instant, the molecule has a short lived polarity. Electrons in neighboring molecules are
attracted to the positive end of the polarized molecule, resulting in a polarization of the neighbor
and creation of an attractive London dispersion force that holds the molecules together.
Dipole - Dipole forces (4 kJ/mol) : the attractive force between positive and negative ends of polar
molecules.
Hydrogen bonds (waterstofbruggen) & (8-40 kJ/mol) : the attraction between a hydrogen atom
bonded to an electronegative atom (N,F,O) and another nearby electronegative N or O atom. While
O and N atoms bonded to C can act as H-bond acceptors, F atoms bonded to C rarely act as H-
bond acceptors. Hydrogen bonding is just a special kind of dipole dipole interaction. The O-H, N-H
and F-H bonds are highly polar, with a partial positive charge on the hydrogen and a partial
negative charge on the electronegative atom. In addition, the hydrogen atom has no inner-shell
electrons to act as a shield around its nucleus, and it is small, so it can be approached closely. As a
result, the dipole dipole attractions involving positively polarized hydrogens are unusually strong.
Week 4 Chemie 1
Week 4 Chemie
Intermoleculaire krachten en oplossingen
Hoofdstuk 8 Gases, liquids and solids
What determines whether a substance is a gas, a liquid or a solid at a given temperature? Why do
molecular compounds have lower melting points than ionic compounds? To answer these we need to
look into the nature of intermolecular forces - the forces that act between different molecules rather
than within an individual molecule. In gases the intermolecular forces are negligible, so the gas
molecules act independently of one another. In liquids and solids, however, intermolecular forces are
strong enough to hold the molecules in close contact. As a general rule, the stronger the intermolecular
forces in a substance, the more difficult it is to separate the molecules, and higher the melting and
boiling points. There are 3 major types of intermolecular forces :
VAN DER WAALS FORCES
London dispersion forces (2-10 kJ/mol) : the short lived attractive force due to the constant
motion of electrons within molecules. All molecules, regardless the structure, experience this force.
London dispersion forces are the only intermolecular forces available for nonpolar molecules
(relatively weak but they increase with molecular massa and amount of surface area available for
interaction). Averaged over time, the distribution of electrons throughout the molecule is uniform,
but at any given instant there may be more electrons at one end of the molecule than the other. At
that instant, the molecule has a short lived polarity. Electrons in neighboring molecules are
attracted to the positive end of the polarized molecule, resulting in a polarization of the neighbor
and creation of an attractive London dispersion force that holds the molecules together.
Dipole - Dipole forces (4 kJ/mol) : the attractive force between positive and negative ends of polar
molecules.
Hydrogen bonds (waterstofbruggen) & (8-40 kJ/mol) : the attraction between a hydrogen atom
bonded to an electronegative atom (N,F,O) and another nearby electronegative N or O atom. While
O and N atoms bonded to C can act as H-bond acceptors, F atoms bonded to C rarely act as H-
bond acceptors. Hydrogen bonding is just a special kind of dipole dipole interaction. The O-H, N-H
and F-H bonds are highly polar, with a partial positive charge on the hydrogen and a partial
negative charge on the electronegative atom. In addition, the hydrogen atom has no inner-shell
electrons to act as a shield around its nucleus, and it is small, so it can be approached closely. As a
result, the dipole dipole attractions involving positively polarized hydrogens are unusually strong.
Week 4 Chemie 1
