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Calculating temperature change when dissolving Urea

by Neptulin
Tags: dissolving urea
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Aug30-11, 06:07 AM
P: 10
If 5g grams Urea is dissolved in 10mL water, what is the temperature change?

We are not given any constants as we are meant to research.

Here is what I think I have to do:
Calculate enthalpy of solution of urea. That is, enthalpy of hydration - lattice energy of urea.
Then I multiply this by the number of moles of Urea (mass divided by RMM) to get the total enthalpy change.
After this step I have a problem. With just water I could isolate ΔΤ in Q=mCΔΤ, although here I am dealing with a mixture of Urea and water, so the heat capacity value is inaccurate. If I had equal quantities (in moles) of Urea and water could I average the heat capacity of both and use that (and thus use the ratio between the moles of urea and water to work something out in this scenario)? Or have I done something completely wrong to begin with?
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Aug30-11, 03:23 PM
Borek's Avatar
P: 23,533
If I understand you correctly, you are on the right track.

If you are not able to find specific heat of the solution (it can be difficult to find), assume that of pure water. It won't be precise, but better than nothing, and most likely better than average value you were thinking about.
Sep3-11, 06:11 AM
P: 10
Thanks. I found it was much easer to calculate the enthalpy of solution by using enthalpy of formation, and then I followed your advice.

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