What is the binding energy of sugar crystals in eV per molecule?
Granulated sugar (sucrose) looks and feels a lot like salt NaCl. Salt forms
crystals with strong ionic bonds. They crush similarly with a spoon.
Relatively large crystals can be formed of either substance. There is the
rock candy formed on cords. There is the coarse salt or even larger crystals
formed from salt.
Both crystals are tough. Breaking either is not easy. A hammer is the
approach took when I was a kid. Melting points indicate that salt is bound
more strongly. The absolute melting temperature of salt is 1077 deg K. Sugar
melts at about 450 deg K. This indicates to me that the binding energy for
sugar is bit less than half that for salt per molecule.
So my question is: What holds sugar crystals together?
Repeating Decimal <[Only registered users see links. ]> wrote in message news:<[Only registered users see links. ]>...
Sugar has a lot of hydroxyl groups which will enter into strong hydrogen bonds.
On Sun, 26 Oct 2003 18:47:48 GMT, Repeating Decimal
<[Only registered users see links. ]> wrote:
The energy for an H-bond is on the order of 3-5 kcal/mol --1/20 or so
the energy for common covalent bonds.
Sugar (sucrose) has 11 oxygen atoms, 8 of which are in -OH groups.
Each O could potentially form 2 H-bonds, and each H of an -OH could
form one. How many of these are actually formed, I donít know. We
would need to look at the details of a crystal structure. But it is
likely that it is "many", so that the total energy of H-bonding is of
the same order as a typical covalent bond.
Those are secondary issues for pure cellulose. The main thing holding
cellulose together in general is H-bonding.
One can quibble about exactly what that term means in this case. But
"crystallinity" of cellulose is one of its characteristics, with
cotton being extremely crystalline.
A simpler example of the power of H-bonds... water and methane have
approx the same molecular mass. The boiling point of water is +100 deg
C. For methane it is -160 deg C. Difference is due to H-bonding in
in article [Only registered users see links. ], Bob at [Only registered users see links. ] wrote on 10/26/03 12:36 PM:
Using a value of 4 kcal/mo, the energy per molecule is 0.17 eV if I did my
arithmetic correctly. I presume, that is what holds the crystal togwther and
is in addition to the enthalpy of formation of the sugar molecules
In one place, I saw the enthalpy of formation of NaCl listed as 411 kJ/mol.
Again, I my arithmetic is correct, this amounts to 98 kcal/mol.
I guess I'll quibble. I would not call cellulose in any form
"crystalline." Crystallinity implies a highly ordered, regular,
reproducible array of elements (molecules or ions), which is lacking
in polymeric solids. Among other things, this highly ordered array
leads to a well-defined melting point, since the bonds to be broken
are well-defined and reproducible.
In contrast, polymers such as cellulose are like big bunches of
spaghetti, held together by mass entanglement. Orientations and
interactions between elements vary throughout the mass. They do not
have characteristic melting points.
On Mon, 27 Oct 2003 03:16:47 GMT, Steve Turner
<[Only registered users see links. ].net> wrote:
I agree with most of that. But cellulose is not spaghetti, rather an
ordered array of strands. And the more crystalline -- the term the
cellulose folks use -- as in cotton or bacterial cellulose, the more
ordered it is.
I think the key point in context, tho, is whether it is largely held
together by H-bonds. As I understand it, that is so -- and made
possible by the specific geometry that allows extensive inter-strand
H-bonding for this particular stereoisomer.
Melting polysaccharides is not a common sport. Trying to dissolve them
is. Starch is soluble (even if not easily at times), cellulose is not.
This is due to the H-bonding difference, not to the spaghetti factor.