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Bond enthalpies vs enthalpy of rxn - conceptual

Posted: Thu Jan 25, 2018 12:57 pm
by Lucia H 2L
I know that breaking bonds is endothermic and making bonds is exothermic, so total avg enthalpy is bonds broken - bonds formed, aka

(bonds in reactants) - (bonds in products).

However, in general, total change in enthalpy is (enthalpy of products) - (enthalpy of reactants). This seems opposite of the expression above. These both make sense separately but I'd like to see how they go together.

How does this work?

Re: Bond enthalpies vs enthalpy of rxn - conceptual  [ENDORSED]

Posted: Thu Jan 25, 2018 2:12 pm
by Clara Rehmann 1K
No matter how we're calculating enthalpy, we want the total "additive" energy in the reaction.

When calculating enthalpy using bond enthalpies, I look it like this: breaking bonds requires energy, contributing positive values to my total (energy going in), while making bonds releases energy, contributing negative values to my total (energy coming out). Adding these up gives the enthalpy, the total amount of energy either put out (if more energy is released by bonds forming) or taken in (if more energy is used up by bonds breaking).

When calculating enthalpy using enthalpy of formation, I look at it like this: we have products, which release a certain amount of energy when formed, and reactants, that take up a certain amount of energy when broken up. Thus, the enthalpy values contributed by the products are usually negative, and the enthalpy values contributed by the reactants are usually positive. Adding these up gives us the enthalpy of the reaction - if the number is positive, then the reactants require more energy than the products produce, and if the number is negative, then vice-versa.

I hope this helps (and that I am correct in my thinking!)

Re: Bond enthalpies vs enthalpy of rxn - conceptual

Posted: Thu Jan 25, 2018 5:44 pm
by Helen Shi 1J
However, when bonds are broken isn't energy released as well? So why is it not exothermic?

Re: Bond enthalpies vs enthalpy of rxn - conceptual

Posted: Thu Jan 25, 2018 7:04 pm
by Sarkis Sislyan 1D
No, breaking bonds does not release energy, it requires energy to break. As a result, breaking a bond is an endothermic process.