Wax, Covalent Bonds, and Melting

name         Jennifer
status       student
grade        9-12
location     Canada

Question -   Candle wax is composed partly of C18H38 (They are 
subscripts^-^) The covalent bonds between the atoms in these 
molecules are extremely strong. Yet wax is soft and has a low 
melting point. Why?
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You are correct, the bonds between the ATOMS in a single 
MOLECULE of candle wax -- the C-C bonds and the C-H bonds -- are 
very strong (both between 330 and 360 kJ/mol) depending upon the 
particular molecule. In contrast, there is no chemical bond between 
one MOLECULE of candle wax and another MOLECULE of candle wax. 
There is a weak attractive force between one MOLECULE and another 
MOLECULE (about 1 kJ/mol). This is due to the movement in the 
electrons in one molecule  movement of electrons in neighboring 
molecules (the name for this attraction is "van der Waals forces"). 
Although the molecules are tangled, the attraction between them is weak.
   A fair analogy is a bowl of hot wet spaghetti. Each strand is 
strong, but the strands can slide by one another rather easily. 
This is why waxes are soft and have low melting temperatures. The 
wax molecules can slide by one another. Some waxes are 
harder/softer than others depending upon the length of the carbon 
chain (longer is harder) and branches in the carbon chain (the more 
branching is harder up to a point).

Vince Calder
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Jennifer,

Even though the covalent bonds are strong between the atoms of any 
SINGLE molecule of wax, it is the bonding interactions between many 
molecules that result in the bulk properties of the 
compound.  C18H38 is a hydrocarbon and most lower molecular weight 
hydrocarbons are in fact liquids.  In fact C18 is right on the verge 
of being a liquid.  If you take away a couple carbons to get into 
the C12-C15 range, you have kerosene fuel and in the average 
molecular formula for diesel fuel is C12H26.  You can see that the 
addition of just a few more carbons can change the bulk properties 
of the compound from a liquid to a solid, in this case a waxy 
solid.  The reason for the waxy feel is because the molecule is 
composed of only hydrgen and carbon.  Hydrocarbons are very greasy 
and repel water, hence why you wax your car!

Matt Voss
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Jennifer,

While you are correct that the C-C bonds are quite strong, the 
cohesiveness of a solid is due more to how well the molecules bind 
to each other. The cohesion strength of molecules is due to many 
factors: the strength of the intermolecular forces, how well the 
molecules fit into each other, the mobility of the components within 
the molecule, etc. It so happens that wax has a very weak 
intermolecular attractive force (London Forces) and so it does not 
hold on to other molecules very well. Also, because it is a 
relatively short and flexible chain (think of short bits of thread) 
it does not fit well with other molecules like itself (as opposed to 
really long chains that can entangle other chains, or rigid 
structures like cubes or squares that can stack on top of each 
other). As such, wax tend to be soft and amorphous.

Greg (Roberto Gregorius)
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