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Topic: How do you actually know if enthalpy change is positive or negative?  (Read 1617 times)

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Offline Ilikecats12344

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How do you actually know if enthalpy change is positive or negative?
« on: October 07, 2015, 05:02:07 PM »
Is it if the sum is just a positive or negative number? Because for my displacement of copper from copper (II) sulphate by zinc the initial temperature dropped from 57 degrees to 26 degrees. Would that mean that the enthalpy is negative because it gave off heat?
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Offline Enthalpy

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Re: How do you actually know if enthalpy change is positive or negative?
« Reply #1 on: October 08, 2015, 02:12:20 PM »
The reaction from CuSO4 (Hf about -180kcal/mol aq) to ZnSO4 (Hf = -252kcal/mol aq) is clearly exothermal, hence is conventionally written with a negative enthalpy change.

Your temperature dropped for other reasons, for instance through cooling by air. Possibly, adding cold zinc cooled the solution too.
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