Yes, relative to inorganic solids, most organic solids have low
melting points. This is because in an inorganic solid there are
generally two strong factors favouring a high melting point.
First, ionic interactions are stronger in inorganic solids as these
are often made up of ions connected in a lattice as in table salt,
NaCl. Na donates an electron to Cl, making the pair Na+ and Cl-. At
short distances, the strenght of the attraction is very strong.
Second, some inorganic lattices are lattices of covalently bonded
molecules - that is that each molecules is bonded to two or more other
atoms, an example is quartz or glass, SiO2. There is a network of
bonds O-Si-O-Si connecting the atoms.
Now compare this to organic solids. Generally organic solids are made
up of discrete (separate) molecules (Salt or glass could be thought of
as being very large single molecules). When melting, the individual
molecules and the bonds within them do not break ( C-C, C-O, C-H, etc)
however the forces holding two molecules together are overpowered.
Generally these forces are dipole-dipole attractions or Van der Walls
forces, which compared to electrostatic forces in ionic solids is very
weak.
In short, in inorganic solids such as NaCl or SiO2, one needs to break
bonds to get the atoms moving freely (which is what happens when a
substance melts). Breaking bonds (ionic or covalent) takes quite a bit
of energy. Organic solids don't have bonds between the molecules but
instead rely on weaker forces to hold them together, thus it takes
less energy and less heat to get them to move freely of one another.
I hope that that helps answer your question. |
