CHEM 1050 Lecture 3: Note 3 Thermochemistry Part 3.pdf
11 Feb 2015
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Bond energies in covalent molecules (refer to section 9. 11 in text) Covalent bonds bonds resulting from the sharing of electrons between atoms. e. g. , (cid:60) (cid:60) (cid:60) (cid:60) The distance between the atoms is the bond length (depends on the atoms). Not all covalent bonds are equal they involve different nuclei and different electron distributions, (e. g. , c c, c=c, c c, etc. ). Hence the energy to break bonds can differ. Energy required to break a mole of bonds in the gas phase (does not hold for liquid or solid state). Note: products are neutral species not ions! is always positive, since enthalpy change is associated with breaking a covalent bond (energy must be supplied). is a measure of the strength of a covalent bond. We can then say that the bond enthalpy of h2 is 436 kj. Note: hbd = be(h h) = +436 kj 2 hf [h, g] Hf [h, g] = +436 kj/mol 2 = 218 kj/mol.
