How much heat is evolved Thermochemistry Question

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"How much heat is evolved" Thermochemistry Question

This is a question from a general chemistry I quiz i had a few days ago. I got this question, and all other questions like it correct, but here's the thing.

Calcium oxide and water react in an exothermic reaction:
CaO(s) + H2O(l) → Ca(OH)2(s) DHrxn° = −64.8 kJ/mol

How much heat would be liberated when 7.15 g CaO(s) is dropped into a beaker containing 152 g H2O?

We have an exam tomorrow and I've been studying the majority of the weekend and I just started working on a thermochemistry worksheet that our instructor was kind enough to provide for us, and for every single "how much heat is evolved" or "how much heat would be released" question has an answer of negative _____.


The answer to this question would be positive, right? Because the negative enthalpy says that heat is released, and heat released = heat evolved = heat produced.

I think he's just trying to stress the concept of enthalpy but if this is the case i think he is definitely going about it in the wrong way > <


(PS: If i was allowed to see the answer that i recorded, this post would not be necessary, sorry)

Your Friend,
Anonymous
 

Redbelly98

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Re: "How much heat is evolved" Thermochemistry Question

The answer to this question would be positive, right? Because the negative enthalpy says that heat is released, and heat released = heat evolved = heat produced.
Yes.

Here is how I think of it. The products of the reaction are actually:
Ca(OH)2(s) + 64.8 kJ of heat​
If you think of it that way, then:
1. The reaction is now balanced in terms of enthalpy. Taking the +64.8kJ into account gives zero for ΔH
2. A product of the reaction is 64.8 kJ of heat (per mole), meaning that heat is given off.​

Hope that helps.
 
Re: "How much heat is evolved" Thermochemistry Question

yeah that helps. That's what i thought the whole time but my professor did it wrong on our homework answer sheet so i just had to make sure before the exam.

Thanks alot,

anonymous

edit:

felt that i needed to tell you that you devised a very clever way of thinking about that. I more-or-less just drilled "NEGATIVE ENTHALPY IS HEAT GENERATED" into my brain, but you created a working system. -.- cool.
 

Redbelly98

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Re: "How much heat is evolved" Thermochemistry Question

I have found it's easier to remember things when they make sense :smile:
 

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