Click here to go to the NEW College Discussion Forum

since the AP Chemistry is less than two months away, i feel a thread like this will be helpful for me and many others.

feel free to post and past expierences and questions.

i'll start it off:

how can you tell if a reaction is endothermic or exothermic by just looking at the reactions?

example

Which one of the following changes will have a positive value of dH?

A. CO2(s) ---> CO2(g)
B. C(s)+O2(g) ---> CO2(g)
C. 2H2O(l) ---> 2H2(g)+O2(g)
D. NH3(l) ---> 2H2(g)+O2(g)

By Mikus (Mikus) on Tuesday, March 18, 2003 - 10:42 pm: Edit

NH3 doesn't yield that... typo?

By Jason817 (Jason817) on Tuesday, March 18, 2003 - 11:18 pm: Edit

remember:

forming bonds is exothermic (energy is released when bonds are formed)
example: H + H ---> H2

breaking bonds is endothermic (energy required to break bonds)
example: CaCO3 ----> CaO + CO2

as solid becomes a gas, it becomes more endothermic, and as a gas becomes a solid, it becomes more exothermic.

By Eurostar (Eurostar) on Wednesday, March 19, 2003 - 04:13 am: Edit

EDIT.

By Heatwave345 (Heatwave345) on Wednesday, March 19, 2003 - 04:12 pm: Edit

Mikus:
it is not a typo. I guess (d) isn't the correct answer.

Jason:
which do you think is athe correct answer? Based on your explanation it can be either (a), (b), or (c)

By Pat57575 (Pat57575) on Wednesday, March 19, 2003 - 06:14 pm: Edit

A. CO2(s) ---> CO2(g)
B. C(s)+O2(g) ---> CO2(g)
C. 2H2O(l) ---> 2H2(g)+O2(g)
D. NH3(l) ---> 2H2(g)+O2(g)
I'm pretty sure the answer would be B. A would be endothermic b/c you would have to input heat to break the stronger solid bonds to weaker gas bonds, C would be endothermic b/c you have to break stronger liquid bonds to weaker gas bonds (as well as dissociate the compound), and D would be endothermic for the same reason as C. Though you could argue B could be endothermic b/c you have to melt and vaporize the carbon, the energy released in forming the more complex compound CO2 would probably exceed that energy (and therefore by exothermic).

By Jason817 (Jason817) on Wednesday, March 19, 2003 - 06:55 pm: Edit

A and D are endothermic.

By Quarky (Quarky) on Wednesday, March 19, 2003 - 07:29 pm: Edit

I think B is correct.

By Nocalguy (Nocalguy) on Wednesday, March 19, 2003 - 11:36 pm: Edit

what is "dH"??? All of them look endothermic to me. BTW: I got a 5 on the Chemistry AP test last year, but I'm not sure what the question's asking. lol

By Jason817 (Jason817) on Wednesday, March 19, 2003 - 11:44 pm: Edit

C is endothermic too. I dont think B is...

dH = delta H = change in heat

By Nocalguy (Nocalguy) on Thursday, March 20, 2003 - 12:32 am: Edit

On further examination, C looks good to me. How the heck do you get H2 and O2 from NH3? Did I miss something in Chemistry AP? lol

By Jason817 (Jason817) on Thursday, March 20, 2003 - 01:06 am: Edit

Maybe the nitrogen dissociates to a point of nonexistance and is replaced by by oxygen, making it a perfect unbalanced equation.

By Pat57575 (Pat57575) on Thursday, March 20, 2003 - 01:17 am: Edit

The entire mass of nitrogen was converted to an amount of energy E = mc^2... positive about that!

nocalguy- Why does C look good? Try putting a glass of water outside on a cold day and see if it starts evaporating.

By Nocalguy (Nocalguy) on Thursday, March 20, 2003 - 03:53 pm: Edit

For water to turn back into hydrogen and oxygen, you have to add heat. Wouldn't that be endothermic?

By Nocalguy (Nocalguy) on Thursday, March 20, 2003 - 04:27 pm: Edit

I still think it's a typo on D. The Chemistry AP test won't go into specifics like it. More likely, it's NH3---->H2+N2 (not O2)

By Absurdistx (Absurdistx) on Thursday, March 20, 2003 - 05:04 pm: Edit

You're right. Synthesis of ammoniais N2 + 3 H2 --> 2 NH3

By Pat57575 (Pat57575) on Thursday, March 20, 2003 - 06:41 pm: Edit

oh, my bad nocalguy... for some reason I was thinking the question asked for exothermic...

By Nocalguy (Nocalguy) on Thursday, March 20, 2003 - 08:55 pm: Edit

The problem is, wouldn't A and D (assuming they meant N2) be endothermic as well??? That's what I don't get.

By Jason817 (Jason817) on Thursday, March 20, 2003 - 10:04 pm: Edit

Yes. As i said: A, C, and D are endothermic

By Nocalguy (Nocalguy) on Thursday, March 20, 2003 - 10:22 pm: Edit

Just the fact that we're talking so much about this problem shows that it is a rather poor problem.

By Heatwave345 (Heatwave345) on Friday, March 21, 2003 - 08:25 pm: Edit

sorry guys, this question is a dud. i ask my teacher and he scrapped it.

any other questions for this thread?

By Absurdistx (Absurdistx) on Sunday, March 23, 2003 - 10:54 am: Edit

It's not a question, but perhaps it'll help some people. http://www.mhs.k12.oh.us/apchemtests/default.phtml is a site that has all the past Free Response questions *and* the answers. Sometimes just working through those can be a big help.