8.45 question

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Wilson Yeh 1L
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Joined: Fri Sep 29, 2017 7:06 am

8.45 question

Postby Wilson Yeh 1L » Fri Jan 19, 2018 12:27 pm

In question 8.45 it states that carbon disulfide can be prepared from coke and elemental sulfur:

4C(s)+S8(s) --> 4CS2(l)
delta(H) = +358.8 kJ

My question is I thought that whenever something is being formed it would be exothermic, so in this case why is it positive 358.8 kJ not negative?

Deborah Cheng 1F
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Joined: Thu Jul 13, 2017 3:00 am

Re: 8.45 question

Postby Deborah Cheng 1F » Fri Jan 19, 2018 12:32 pm

The problem asks later on, "How much heat is absorbed in the reaction of 1.25 mol S8 at constant pressure?" showing that the reaction needs energy in the form of heat. Not all chemical reactions are exothermic; some such as this one are endothermic and require energy to take place.

Wilson Yeh 1L
Posts: 42
Joined: Fri Sep 29, 2017 7:06 am

Re: 8.45 question

Postby Wilson Yeh 1L » Fri Jan 19, 2018 12:44 pm

Deborah Cheng 1F wrote:The problem asks later on, "How much heat is absorbed in the reaction of 1.25 mol S8 at constant pressure?" showing that the reaction needs energy in the form of heat. Not all chemical reactions are exothermic; some such as this one are endothermic and require energy to take place.

I understand what the question is asking later on and realize that this is an endothermic reaction. I'm just curious because I thought anytime something's being formed it would release energy. I guess my question is... why is this particular reaction endothermic?

Cristina Sarmiento 1E
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Joined: Wed Nov 16, 2016 3:02 am

Re: 8.45 question

Postby Cristina Sarmiento 1E » Fri Jan 19, 2018 2:44 pm

I am also confused because in my notes I have that endothermic reactions absorbs and requires energy so breaking bonds would be endothermic. Exothermic releases and in my notes I put forming bonds would be exothermic but why would you release energy when you form bonds?

Isita Tripathi 2E
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Re: 8.45 question

Postby Isita Tripathi 2E » Sat Jan 20, 2018 3:52 pm

Forming a bond creates a more stable molecule than the individual atoms alone, since they have completed their octets. This is why it takes energy to break bonds (you are forcing something from more stable to more unstable) and releases energy to form bonds (you are forming a more stable complex).


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