Lecture19sf

# Lecture19sf - Bond Energy The bond energy of bond X-Y is...

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Bond Energy The bond energy of bond X-Y is the energy needed to break the bond into separate atoms. It can be accurately measured for diatomic molecules. H (kJ/mol) H 2 (g) → 2H(g) 436 Cl 2 (g) → 2Cl(g) 243 HCl(g) → H(g) + Cl(g) 431 All of the above numbers would be called the bond energy of H 2 , Cl 2 , and HCl respectively. For bonds between atoms which are not from individual diatomic molecules, we can measure average bond energies based on the energy needed to break these bonds within different molecules. Table 9.1 in your text shows a number of bond energies, some from diatomic molecules, most of them averaged among different molecules. Example: bond energy (kJ/mol) C-H 414 C-C 347 C-O 360 Bond energies are different for double and triple bonds. Example: bond energy (kJ/mol) C-C 347 C=C 611 C C 837 N-N 163 N=N 418 N N 946 C-O 360 C=O 736 C O 1072 Multiple bonds are stronger and also shorter than single bonds. Comparing C-C, C=C, and C C, the bond lengths are 154,134,120 pm respectively. (also listed in table 9.1).

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We can use the table of bond energies to calculate H for reactions. Calculate H for the reaction: H 2 (g) + Cl 2 (g) → 2HCl(g) Using the bond energy chart, imagine a process where we first break every bond of the reactants into separate atoms, and then join these atoms together to form the product. H H 2 → 2H 436 (bond energy of H 2 ) Cl 2 → 2Cl 243 (bond energy of Cl 2 ) 2H + 2Cl →2HCl -2(431) (-2 times the bond energy of HCl) Adding: H 2 + Cl 2 → 2HCl -183 kJ This compares favorably with the H calculated using the H f chart: From the H f chart, H f for HCl = -92.3 kJ/mol. Thus, H for this reaction should be 2(-92.3) = -184.6 kJ. Bond energies are always positive, since it requires energy to break bonds. When we form bonds, H is negative and we use the negative of the bond energy, as shown in the above example. Generalizing from this example: H rxn = ∆♦ BE reactants) - ∆Γ BE products) where BE = bond energy. Note that in this formula it’s reactants minus products. H for the overall reaction would equal the energy needed to break all reactant bonds (sum of bond energy of reactants ) minus the energy released when all bonds of products are formed (minus the sum of bond energy of products). Calculate H for the reaction: 2H 2 (g) + O 2 (g) → 2H 2 O(g) Diatomic oxygen is a double bond with a bond energy of 498 kJ/mol. H 2 has a bond energy of 436 kJ/mol Bond energy of reactants: 2(436) + 498 = 1370 kJ/mol This would be the energy required to break 2H 2 + O 2 into separate atoms.
Water, H 2 O has an H-O-H structure, and thus has 2 O-H bonds. The O-H bond energy is 464 kJ/mol. Since there are 2H 2 O, there are 4 O-H bonds. Bond energy of products:

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Lecture19sf - Bond Energy The bond energy of bond X-Y is...

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