Chemical Forums
Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: ysk1 on September 27, 2007, 11:18:57 PM
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In general chemistry and organic chemistry, I learned that ionic bond is the strongest type of bond out there.
I'm taking biochemistry right now, and its biochemistry textbook says the opposite. I'm really confused. I told this to my prof. He said he, too, initially thought as I did, but, because the textbook says otherwise, he'll have to teach the class that way.
The biochemistry textbook is Molecular Cell Biology, Sixth Edition by W.H. Freeman and Co.
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There are a plenty of ionic and covalent bonds of different strength.
General chemistry books refer to simple inorganic compounds, eg NaCl, organic and biochemistry textbooks take into account mainly organic compounds in which C-C bond is rather strong. But a simple test - heating eg NaCl and organic compound without oxygen in atmosphere to, say 800 C, shows organic compound will decompose.
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I don't think its fair to compare the decomposition point of organic compounds to the melting of NaCl. In organic compounds, you generally have one bond holding things together. In ionic compounds, however, you have a crystal lattice that holds atoms in place. The stability of the lattice probably contributes a great deal to the thermal stability of ionic compounds.
Instead, compare the melting points of diamond and sodium chloride. Both are crystaline solids and one is held together by covalent bonds while the other is held apart by ionic bonds. Wikipedia quotes the melting point of diamond as 3547 °C (since this is wikipedia, take this with a grain of salt. I thought diamond decomposed before melting, so can anyone confirm this number?) whereas the melting point of sodium chloride is 801oC.
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Wikipedia quotes the melting point of diamond as 3547 °C (since this is wikipedia, take this with a grain of salt. I thought diamond decomposed before melting, so can anyone confirm this number?) whereas the melting point of sodium chloride is 801oC.
In fact, this is melting point of graphite. Diamond is thermodynamicaly unstable at temperate pressures.
See phase diagram of carbo:
http://phycomp.technion.ac.il/~anastasy/teza/teza/node5.html
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it may be the idea that covalent bond in organic compounds is stronger than ionic bond in organic compounds while the ionic bond in inorganic compounds is stronger than covalent bond in organic compounds since organic compounds are not very polar