Enthalpy [ENDORSED]

[Anjali_U_3L]

How does enthalpy change when a single bond in a compound is converted to a double bond?

[Tara_Shooshani_3N]

It depends on the elements being used, but in general, you subtract the mean bond enthalpy of the double bond you are forming from the single bond you are breaking.

For example, if we take a single bonded C-C and convert it to a C=C bond, we first have to add energy to break the single bond, and then energy is released when the double bond forms. The equation (using mean bond enthalpies from Table 8.7) looks like the following: 348 - 612 = -264 kJ. This means that 264 kJ are released when a single bonded C-C converts into a double bond.

[Eunnie_Lee_3H]

Enthalpy is the amount of heat released or absorbed at a constant pressure. For a single bond in a compound to become a double bond would require the compound's single bond to be broken, and then a double bond to be formed.
Breaking bonds require energy, while forming bonds would release energy.
Breaking or forming single bonds also requires less energy than breaking or forming double bonds since single bonds are weaker and easier to break, while double bonds are stronger.
Assuming that this system is at a constant pressure, etc, this would mean that you would need to put in energy when breaking the weaker single bond, and then have energy released in forming the double bond. Thus overall, energy was released in the system. This means enthalpy would be (become?) negative, as the reaction was most likely an exothermic reaction.

Please correct me if I am wrong! I hope this helps.

Oh, whoops, I was just about to post when I saw that the above post made. I hope this also helps.

[Patrick Ricaflanca 2H]

Since a bond is being formed, energy is being released from the system and the enthalpy of that part of the system would be negative.
In the case where a bond is being broken, energy is being taken up by the system to break a bond and the enthalpy of that part of the system would be positive.