bond enthalpies

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becky a 3D
Posts: 7
Joined: Sat Jul 09, 2016 3:00 am

bond enthalpies

Postby becky a 3D » Tue Jan 24, 2017 7:56 pm

when identifying which bonds are broken and formed in order to calculate reaction enthalpy does anyone have and tips on how to easily identify whether bonds (mainly c-c) are single double or triple?? for many im actually just visualizing the lewis structure, but for some such as CH3CHCH2, the lewis structure is not so obvious (to me).

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

Re: bond enthalpies

Postby stephanieyang_3F » Tue Jan 24, 2017 8:22 pm

For these bond enthalpy questions, it's really important to be able to visualize the lewis structure so you know which bonds are formed and broken. When it comes to drawing lewis structures, you should focus on satisfying the octet rule above all things. Making sure each atom in the structure has 8 valence electrons (there are some exceptions... as you have probably learned last quarter in Chem 14A) will help you determine if there is a double/triple bond in the actual structure. I'll try to explain with CH3CHCH2.

If you assign and count up the total number of valence electrons that every atom should have, (like C should have 4, H should have 1) you'd find a total of 18 valence electrons. Then divide this number by two since every "bond" you draw will essentially indicate that a bond is a pair of electrons being shared. So you'd have 9. Try to assign bonds between every atom included in the structure. In this case the way they wrote it sort of helps you how the structure should look like. You'll notice that you used 8 bonds and have 1 more left over. Check to see which atoms don't have a full (8) valence electrons around it. You'll notice that 2 carbons' valence shell is not complete, since both of these C only have 3 bonds around it. You assign the last bond between C-C as a double bond to satisfy the octet rule.

https://wn.com/ch3chch2 This video also helps explain how CH3CHCH2 is drawn out. I hope this helps!

becky a 3D
Posts: 7
Joined: Sat Jul 09, 2016 3:00 am

Re: bond enthalpies

Postby becky a 3D » Tue Jan 24, 2017 9:14 pm

yes this was very helpful! thank you! so to my understanding, there really is no "shortcut" (i guess you could say) in identifying these bonds, as in, one would have to visualize (or physically draw out) the lewis structure of the molecule each time. which im guessing with practice and familiarity, this visual image comes a lot more naturally (??)

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

Re: bond enthalpies

Postby stephanieyang_3F » Tue Jan 24, 2017 9:22 pm

Yeah that's exactly right! I like to draw them out cause I tend to mess up when I just visualize it in my head... Eventually you won't even have to go through each step cause you'll be so used to it. For even harder structures it's really useful to know how to approach the problem especially if you haven't seen it before. I'm glad I helped! :)


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