## How to calculate enthalpy of reaction

Anna O 2C
Posts: 98
Joined: Fri Sep 28, 2018 12:19 am

### How to calculate enthalpy of reaction

When we are adding up bond enthalpies or enthalpies of formation for specific reactions to get the overall reaction enthalpy, do we need to change the signs of the values we will be given in a table for bond enthalpies or enthalpies of formation? Or can we just have all the bonds in absolute value and just use "(+)broken - (+) formed" (using all positive values) and "(+)products - (+)reactants" ? Can we use just positive values for both and then subtract the ones being formed in both cases?

Joonsoo Kim 4L
Posts: 61
Joined: Fri Sep 28, 2018 12:29 am

### Re: How to calculate enthalpy of reaction

I believe you use the formula using the value you find in the table, and not the absolute value. I don't believe you'll find any negative values for bond energy, since it's the energy required to break a bond, and if that value was negative, then the bond probably wouldn't exist.