Bond Enthalpies with Molecules

[Gabriel Ordonez 2K]

How do we calculate bond enthalpies if we are given a chemical equation showing only the molecules and their number of bonds/atoms?

[Pegah Nasseri 1K]

You can draw the Lewis structures of each molecule to look at all the bonds present between the atoms of each molecule. From this you can deduce which bonds are broken and which ones are formed to produce the products of the reaction. Use the values of the bond enthalpies given to you for each bond but remember to assign negative signs for any bonds formed and positive signs for any bonds broken. Bonds formed will have negative bond enthalpies because energy is released in the formation of the bond, while breaking a bond requires energy. Add up all the bond enthalpies to obtain the total enthalpy of the reaction.

Alternatively, if it is hard for you to distinguish which bonds are broken and which ones are formed, you can assume all the bonds on the reactants side are broken and add up all these bond enthalpies with the negative bond enthalpies of all the products side (assuming that all bonds on the products side are formed). The bonds that were left unchanged in the reaction will have positive and negative bond enthalpies that will cancel each other out.

[Amy Kumar 1I]

Use the given positive values for all the bond enthalpies and use the equation (bonds broken)-(bonds formed) to calculate the overall delta h.

[BCaballero_4F]

To calculate this you would add together the bonds broken, then subtract that number by bonds formed.

[TimVintsDis4L]

The type of bond has a constant enthalpy which will always be provided were we to have to solve for it. My advice for calculating these problems is to draw it out first and write out how many of what type of bond you have. Once you have that, you can cancel out duplicates as this will make the calculations much simpler. Once you're at this point, just plug in the numbers given.