## Don't understand bond enthalpy

JaeHoCho1B
Posts: 16
Joined: Sat Jul 09, 2016 3:00 am

### Don't understand bond enthalpy

Hello classmates! I'm very confused on how and when bond enthalpy is used. In addition, I was confused on the example that Dr. Lavelle gave us in our lecture. If anyone can clarify it for me, it will be grateful.

stephanieyang_3F
Posts: 62
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Don't understand bond enthalpy

During lecture, Prof. Lavelle gave us three methods to calculate the ΔH(rxn). One of these methods is by using the bond enthalpies. So keep in mind in a chemical reactions, when bonds break, energy is absorbed or required and when bonds form, energy is released. It's important to know the structure of the compounds being discussed in the reaction so you know what bonds are actually being formed or broken. In that particular example, the double bond in C2H4 was broken along with the bond in HBr, but a C-H bond, C-C bond and C-Br bond was formed. That being said, those bonds that were formed would have a negative value and the ones broken would have a positive value. The overall enthalpy change is the sum of all those bond enthalpies, giving you a net change in enthalpy of -58 kJ. Change in enthalpy is really just the amount of heat released or absorbed at a constant pressure, so calculating sum of these bond enthalpies will give you a good idea of the amount of energy exchanged throughout the whole reaction. In general, you'd use each method based on what information is given in the problem. Hope I helped!

Posts: 2
Joined: Wed Nov 18, 2015 3:00 am

### Re: Don't understand bond enthalpy

Hello! As reviewed in class, bond enthalpy can be used as a method of finding the change in enthalpy of a reaction, (delta H). We look at the reactants and the products of a given reaction and figure out which bonds are being broken and which bonds are being newly formed. Then we add up the energies of all the bonds being formed together, and do the same for the bonds being broken. Because broken bonds require energy to break, they are represented positively, while bonds that are being formed are represented negatively.

For example, if it takes 600 kj.mol to form a bond, then it becomes -600 kj.mol. After we find the energies, we just add them together to find the overall change in enthalpy (delta H).

In the example given in class, we had [ CH2 = Ch2 + H - Br ] -> [CH3 - CH2BR]. In order to figure out which bonds are being broken and which ones are being formed, it would be easier to draw the reaction in their chemical compounds. Essentially draw out the bonds, and going from left to right, find which elements are newly bonded, broken, etc.

For example, in the reactants side we see a double bond between CH2 and CH2. However on the products side, there is not a double bond anymore, meaning that the double bond was broken. Hope this helps!

604746385
Posts: 26
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Don't understand bond enthalpy

Eric_pierce_3E
Posts: 13
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Don't understand bond enthalpy

Kudos on the explanations! Very clear and helpful

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

### Who is online

Users browsing this forum: No registered users and 1 guest