In the last lecture

[Priscilla Okaiteye]

In Friday's lecture when the professor was talking about method 2 (using bond enthalpies to calculate the change in the H reaction) I did not quite catch what he was saying when he was going over the example. What did he mean when he was talking about the carbon double bond? What happen to it?

[Albert Duong 4C]

I believe he stated that the carbon double bond breaks in the reaction and then forms a single carbon bond. Since energy is required to break a bond, the energy of a c=c bond (usually taken from a table or given) is a positive value. However, energy is released when a bond forms, so the energy of a c-c bond in this particular reaction is negative. To find the enthalpy of the reaction, we take the sum of these positive and negative bond energy values.

[Cecilia Jardon 1I]

Essentially he was saying that you require more energy to break a double bonded carbon atom (c=c) vs a single bonded atom (c-c). And that energy is required to break a bond and that to form bond energy is released.

[Tuong Nguyen 2I]

Also remember that the second method is the most inaccurate method of calculating the overall enthalpy of a reaction however. The reason being is that the values that we have for the bond enthalpies are only averages taken over a different number of molecules. The actual bond enthalpy for certain bonds is different depending on the molecule it is part of. A C-H bond in one molecule will have a different bond enthalpy than a C-H bond in another molecule.