Lecture Note - Covalent Bond Strengths

Lecture Note - Covalent Bond Strengths - Bond H (kJ/mole)...

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Covalent Bond Strengths For a given chemical reaction we'll have bonds being broken, bonds being formed, and energy either being absorbed or emitted by the reaction. Let's look a little closer at the change in enthalpy associated with each individual bond being broken or formed. Process ΔH (kJ/mole) CH 4(g) → CH 3(g) + H (g) 435 CH 3(g) → CH 2(g) + H (g) 453 CH 2(g) → CH (g) + H (g) 425 CH (g) → C (g) + H (g) 339 total = 1652 If we wanted to know what is the enthalpy change associated with breaking a C-H bond we find that it is slightly dependent on what molecule it is in. Thus, we take an average change in enthalpy when a C-H bond breaks as D C-H = 1652/4 kJ/mole = 413 kJ/mole. Other average bond enthalpy changes in kJ/mole are. .. Bond ΔH (kJ/mole) Bond ΔH (kJ/mole) H-H 432 C-H 413 H-F 565 C-C 347 H-Cl 427 C-N 305 C-O 358 Notice how multiple bonds are shorter and require more energy to break than single bonds.
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Unformatted text preview: Bond H (kJ/mole) Bond Length C=C 614 1.37 CC 839 1.20 C-N 305 1.43 C=N 615 1.38 CN 891 1.16 We can use these average bond enthalpy changes to calculate the approximate enthalpy change for reactions. For example, to calculate the change in enthalpy for the following reaction: H 2(g) + F 2(g) 2 HF (g) we identify and count all the bonds that are broken (shown in red) and formed (shown in blue). H H + F F 2 H F Substituting the average bond enthalpies: D H-H = 432 kJ/mole, D F-F = 154 kJ/mole, D H-F = 565 kJ/mole in the expression H = (H of bonds broken) - (H of bonds formed) we obtain H = [ (1 mole) (432 kJ/mole) + (1 mole) (154 kJ/mole) ]- [ (2 moles) (565 kJ/mole) ] = -544 kJ...
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This note was uploaded on 07/31/2008 for the course CHY 152 taught by Professor Foucher during the Winter '08 term at Ryerson.

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Lecture Note - Covalent Bond Strengths - Bond H (kJ/mole)...

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