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8.45 question

Posted: Fri Jan 19, 2018 12:27 pm
by Wilson Yeh 1L
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?

Re: 8.45 question

Posted: Fri Jan 19, 2018 12:32 pm
by Deborah Cheng 1F
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.

Re: 8.45 question

Posted: Fri Jan 19, 2018 12:44 pm
by Wilson Yeh 1L
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?

Re: 8.45 question

Posted: Fri Jan 19, 2018 2:44 pm
by Cristina Sarmiento 1E
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?

Re: 8.45 question

Posted: Sat Jan 20, 2018 3:52 pm
by Isita Tripathi 2E
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).