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The second way Dr. Lavelle suggested to measure the enthalpy of a reaction was to use bond enthalpies. Essentially, you would find how much energy is required to break every bond in the reactants and add how much energy is released when every bond in the products is formed. However, a shortcut is to only pay attention to the bonds that actually break and form, which saves you from having to examine every bond.
Sarah Zhari 1D wrote:To add on, bond breaking is an endothermic process and has a positive value, and bond forming is an exothermic process and has a negative value.
Also note that the table of bond enthalpies that they give you are going to have all values be positive. You need to change the value to negative for bonds that are being formed, because forming bonds releases heat (as you said).
EvaKaganovsky_1G wrote:Can someone explain Lavelle's second method to measure enthalpy changes by using bond enthalpy. I got a little lost. Thanks!
With the second method, it is best to draw out the Lewis structure to fully understand what is going on. So, for the second method, you are looking at how the two molecules become the one for the addition reaction. You will look at the Lewis structure, and think where the bonds have to be broken and where the bonds have to be made in order for the product to be a result. So for the example given in class the double bond is broken (+ kj/mol) and then "replaced" buy a single bond (-348 kj/mol). This idea is repeated for every new bond formed (- energy) and every bond that needs to be broken (+ energy).
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