Method 2

Vuong_2F
Posts: 90
Joined: Sat Sep 14, 2019 12:17 am

Method 2

Hi can someone please summarize how to use Method 2? Also, will there ever really be a case where only Method 2 is applicable (would Lavelle test us on this method specifically)?

405268063
Posts: 102
Joined: Thu Jul 25, 2019 12:17 am

Re: Method 2

I think the method you are referring to is using bond enthalpies to calculate delta H. In class, Dr. Lavelle said that this was a less accurate way of finding delta H than using Hess' Law. I'm not quite sure if that means we would be tested on it, but it is possible he could give us bond enthalpies and ask us to find delta H using those numbers.

Christine Honda 2I
Posts: 116
Joined: Sat Sep 14, 2019 12:17 am

Re: Method 2

In Method 2 you use each individual bond enthalpies to calculate delta H. Using the Lewis structures you look at which bonds are broken in the reactants and which bonds are formed in the products. The bonds broken are positive enthalpies and bonds formed are negative. Then you add the enthalpies of reactants and subtract the enthalpies of the products!

Jasmine 2C
Posts: 184
Joined: Wed Sep 18, 2019 12:18 am

Re: Method 2

I don't have my notebook with me currently but if I remember correctly, Method 2 was adding the bond energies of every bond in the molecule before and after the reaction and subtracted them from each other. And whether the result is positive or negative, it would indicate that the reaction is endothermic or exothermic. I think this method is only applicable when all of the bond energies for EVERY bond is provided for us to calculate.

Jeremy_Guiman2E
Posts: 82
Joined: Fri Sep 28, 2018 12:29 am

Re: Method 2

Method 2 involves using specific bond enthalpies to calculate delta H of the reaction. Basically, for the reactants, bonds would need to be broken and therefore energy is required (positive delta H). Conversely, for the products, new bonds are formed, meaning that energy is released (negative delta H). It'd be most helpful to draw out the structures of the reactants and products to see which bonds would need to be broken and which bonds are newly formed. From this, you can add up each of the individual delta H's from the reactants' and products' bonds to determine delta H for the overall reaction.

However, this method is the least accurate because these measurements are average except for the bond enthalpies of diatomic molecules (as they are measured for diatomic molecules).

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