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Prepared by Lawrence Kok

Video Tutorial on Energetics, Bond Enthalpies, Enthalpy of Formation and Combustion.

Video on Energetics and Bond EnthalpiesVideo on using average BEVideo on using average BE

Video on average BE Detail lecture on bond enthalpies

Enthalpy – Heat content of a system stored as chemical energy in chemical bonds

Bond Making – Heat energy given off/released

Bond Breaking – Heat energy taken in /absorbed

During bond making + bond breaking – certain amount energy is either ABSORBED or RELEASED

Energetics ( Enthalpy Change )

Bond Enthalpy

Bond Enthalpies

• Measure of strength of a covalent bond

• Stronger the bond ↑ → Higher bond enthalpy ↑ → Greater ↑ energy needed to break bond

How Average Bond Enthalpies are calculated (Average BE for C-H bond)

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2

Data for enthalpy of formation /atomization

How Average Bond Enthalpies are calculated (Average BE for C-CI bond)

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Data for enthalpy of formation /atomization

Calculation of Enthalpy changes, ΔH using Average BE data

Example 1: Hydrogenation of EtheneCalculate ΔH rxn for C2H4 (g) + H2 → C2H6

Example 2: Combustion of N2H4

Calculate ΔH rxn for N2H4 (g) + O2 → N2 + 2H2O

Calculation of Enthalpy changes, ΔH using Average BE data

Example 1: Hydrogenation of EtheneCalculate ΔH rxn for C2H4 (g) + H2 → C2H6

Example 2: Combustion of N2H4

Calculate ΔH rxn for N2H4 (g) + O2 → N2 + 2H2O

Calculation of Enthalpy changes, ΔH using Average BE data

Example 3: Calculate ΔH rxn for N2 (g) + 3H2 → 2NH3

Example 4: Calculate ΔH rxn for 6 H (g) + 3 CI (g) → C3H6

Calculation of Enthalpy changes, ΔH using Average BE data

Example 3: Calculate ΔH rxn for N2 (g) + 3H2 → 2NH3

Example 4: Calculate ΔH rxn for 6 H (g) + 3 CI (g) → C3H6

Calculation of Enthalpy changes, ΔH using Average BE dataExample 5: Calculate ΔH rxn for CH4 (g) + CI2 → CH3CI + HCI

Method 1 Breaking ALL bonds in reactants and products

Calculation of Enthalpy changes, ΔH using Average BE dataExample 5: Calculate ΔH rxn for CH4 (g) + CI2 → CH3CI + HCI

Method 1

Method 2

Breaking ALL bonds in reactants and products

Breaking only CERTAIN bonds in reactants and products

Limitation using Average Bond Enthalpy• Calculation of Enthalpy change ΔH using average BE is only an approximation and is NOT accurate• Average BE is used for similar bonds within an identical molecule

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Limitation using Average Bond Enthalpy• Calculation of Enthalpy change ΔH using average BE is only an approximation and is NOT accurate• Average BE is used for similar bonds within an identical molecule

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Calculation of Enthalpy change ΔH for C2H4 (g) + H2 → C2H6 using 3 methods

1. Average BE ( only approximation and not accurate )

2. Enthalpy change of combustion ΔHc ( accurate )

3. Enthalpy change of formation ΔHf ( accurate )

Calculation of Enthalpy change ΔH for C2H4 (g) + H2 → C2H6 1. Using Average BE Method ( only approximation and not accurate )

Energy Cycle

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Calculation of Enthalpy change ΔH for C2H4 (g) + H2 → C2H6 1. Using Average BE Method ( only approximation and not accurate )

Energy Cycle or Energy Level Diagram

Answer = -124 kJmol-1

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Calculation of Enthalpy change ΔH for C2H4 (g) + H2 → C2H6 2. Using Enthalpy change of combustion ΔHc

Energy Cycle

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Calculation of Enthalpy change ΔH for C2H4 (g) + H2 → C2H6 2. Using Enthalpy change of combustion ΔHc

Energy Cycle or Energy Level Diagram

Answer = -137 kJmol-1

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Calculation of Enthalpy change ΔH for C2H4 (g) + H2 → C2H6 3. Using Enthalpy change of formation ΔHf

Energy Cycle

3

Calculation of Enthalpy change ΔH for C2H4 (g) + H2 → C2H6 3. Using Enthalpy change of formation ΔHf

Energy Cycle or Energy Level Diagram

Answer = -137 kJmol-1

3

Video on Energetics and Bond EnthalpiesVideo on using average BEVideo on using average BE

Video on average BE Detail lecture on bond enthalpies

Conclusion

Calculation of Enthalpy change ΔH of reaction for C2H4 (g) + H2 → C2H6

THREE methods can be used

1. Average BE = - 124kJmol-1

(Approximation )2. Enthalpy change of combustion ΔHc = - 137kJmol-1 (Accurate )3. Enthalpy change of formation ΔHf = - 137kJmol-1 (Accurate )

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

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