S117Lecture7B2011

S117Lecture7B2011 - Bond strength and bond length As the...

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Bond Bond Energy (kJ/mol) Bond Length (Å) C-C 348 1.54 C=C 614 1.34 C=C 839 1.20 As the number of bonds between two atoms increases, the bond grows shorter and stronger Carbon is remarkable in its ability to catenate , that is bond to itself so as to form long chains. Ofcourse life as we know it depends on this abilility. Why does this happen? Compare carbon to the other elements in its group. C—C 356 kJ/mol C==C 614 kJ/mol C=C 839 kJ/mol Si—Si 256 kJ/mol Si==Si 230 kJ/mol Pb—Pb even weaker In 2004 a Si=Si was synthesized for the first time. Bond strength and bond length
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-----An interesting aside----- Why is carbon the ONLY element to link in long chains? Why, for example, does nitrogen not do this, forming, say a polymer (or smaller molecule) of formula (NH)n, ditto phosphorus? Isaac Asimov wrote a very good article that gives a comprehensive answer to your question and several that are closely related. It is called "The One and Only" and appears in his essay collection The Tragedy of the Moon (now out of print). This answer is largely based on Asimov's; he seems to have anticipated every point I wanted to use! It all comes down to thermodynamics. Specifically, for an element to link to itself in long chains, you need three things: 1.The ability to form more than one bond per atom. This excludes hydrogen and the halogens, for example. We will also exclude the metals, since they don't form localized covalent bonds to themselves and so cannot form chains of atoms. 2. A preference for single bonds over multiple bonds . That is, two single bonds should be more energetically stable than one double bond, and three single bonds than one triple bond. 3. The oxide must not be too much more stable than the pure element . Oxygen is the third most common atom in the universe, and an overwhelming tendency to bond to oxygen will mean that an element will never be found except as the oxide. 4. Finally, the bond to hydrogen must be reasonably strong , compared to the hydrogen- oxygen bond. Hydrogen is the most abundant atom in the universe and (as you indicated in your question) is commonly used to "cap off" chains of atoms. But many compounds of hydrogen are spontaneously flammable in the presence of oxygen!
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Bond type Strength Bond type Strength Bond type Strength C-C 83 kcal/mol N-N 39 kcal/mol O-O 35 kcal/mol C=C 146 kcal/mol N=N 100 kcal/mol O=O 119 kcal/mol C=C 200 kcal/mol N=N 226 kcal/mol Single vs. Multiple Bonds If we confine ourselves to the first-row elements carbon, nitrogen and oxygen, we find the following bond strengths: Information in this table is taken from M.A. Fox and J.K. Whitesell, Organic Chemistry, 2nd Ed. , Jones & Bartlett, 1997. Values are averages and will differ somewhat depending
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This note was uploaded on 01/17/2012 for the course S 117 taught by Professor Stephenjacobson during the Fall '11 term at Indiana.

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S117Lecture7B2011 - Bond strength and bond length As the...

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