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### Bond Breaking or forming

Posted: Wed Jan 17, 2018 10:22 pm
When looking at a rxn, is there another way aside from looking at the delta h to know if a bond is forming or breaking?

### Re: Bond Breaking or forming

Posted: Wed Jan 17, 2018 10:46 pm
Bond formation is always exothermic, so a negative H.
Bond breaking is endothermic, so a positive H.
When looking at a reaction, you must draw all the lewis structures of the reactants and products to understand which bonds are broken and which are newly formed.
We do this to calculate the heat of the reaction. If the heat of the reaction is exothermic, we can assume that more bonds are forming than bonds breaking. If the enthalpy of reaction is positive, we can say that more bonds are breaking than there are forming.
Does that make sense? Let me know if I answered your question. :)

### Re: Bond Breaking or forming

Posted: Wed Jan 17, 2018 10:59 pm
yeah that makes sense, so my follow up question would be after drawing the electron configurations how could you tell if a bond is broken or formed like what would it look like?

### Re: Bond Breaking or forming

Posted: Thu Jan 18, 2018 10:16 am
You would have to compare bonds of the reactants with bonds of the products.
Another way to solve for the heat of the reaction, would to break ALL the bonds of the reactants and form all the bonds of the products.
This way you won't miss any bonds, but it is a bit more math!

### Re: Bond Breaking or forming

Posted: Thu Jan 18, 2018 10:20 am
If you're using bond enthalpies, then after you draw the Lewis structures you just need to look at the individual bonds in the reactants/products to see which bonds are broken from the reactants and which bonds need to form to make the products

### Re: Bond Breaking or forming

Posted: Thu Jan 18, 2018 10:20 am
In response to your latest question, you would just notice that a bond in the reactants had been broken and a new bond in the products is formed because there are different combinations. For example, if we have CH4 + HBr --- CH4Br and we drew the lewis structures, we would see that the double carbon-carbon bond in CH4 was broken and instead a single carbon-carbon bond was formed in CH4Br. It's basically just looking at how the bonds between certain atoms changed from reactants to products.