[Tekker]

Hi all,

I’ve got a chem extra credit assignment that my friend and I cannot figure out.

Here’s what it says on the handout.

Consider the process:

NaCl(s) --> Na+(aq) + Cl-(aq)

Break the solution process down into three steps:

?Hsolution = enthalpy change for solution formation
?H1 = energy required to overcome solute-solute interaction
?H2 = energy required to overcome solvent-solvent interaction
?H3 = solvent-solvent interaction energy

Calculate ?H2 for the dissolution of 1 mol of NaCl(s) in water at 298.15 K (25°C). You will need thermodynamic data from appendix C of your text and some very helpful hints and data in the “Closer Look” box on page 280 of your text.

Draw and energy diagram for the solution process and label it.

Here is what our energy diagram has to took like:
http://www.artistcollaboration.com/users/tekker/Physics%20&%20Chemistry/Enthalpy%20Diagram%20-%20NaCl%20&%20H2O.JPG

Ok, so basically the “Closer Look” box in our book gave us the energy required to separate NaCl into Na+(g) + Cl-(g). So ?H1 is equal to 788 kJ/mol.. But unfortunately that is about as far as we can get. lol

In order to be able to calculate the energy for ?2, it seems to me like we would have to know the energies for ?H(solution) as well as for ?H3 and I just can’t figure out how to get all of that from the stuff in the book.

We asked our teacher if it had to do with hydration and he said yes. But I don’t see anything like that in the back of the book. There are only three columns for the thermodynamic quantities in Appendix C, one of them is the standard enthalpies of formation (?Hf°) and then there are two other symbols that I don’t know, ?Gf° and S°. I don’t know how he expected us to get this from the info in the book, because it doesn’t seem to be there (unless we are overlooking something obvious). Even searching online hasn’t been any help at all.

My friend and I are completely stuck on this after wasting a lot of time on it yesterday trying to get numbers to work from the standard enthalpies of formation, but I’m not sure it has anything to do with that.... So if anyone can help out with this we’d really appreciate it.

Thanks
-tkr

[Mitch]

Enthalpy of Hydratyion

[Tekker]

Yeah, my friend and I have tried searching for stuff for the last few days. We've litterally wasted hours and hours trying to figure it out with nothing to show for it. We compiled a word document from several websites but we haven't found anything that puts all the pieces together yet. The closest one we found gave values for ?Hsolution, ?H1, and ?H3, which is exactly what we were looking for, however.... When the values were added up ?H2 would have essentially equaled "zero". In other words, ?H1 + ?H3 = ?Hsolution. There was nothing left for ?H2. Which makes absolutely no sense, because ?H2 is the separation of water molecules and to overcome the hydrogen bonding in water there HAS to be some energy required to separate them. So I think most of the sites we've found has only served to confuse us even more. lol

I think we'll just have to talk with our teacher tomorrow and see if we can get some help on it. This is a big point extra credit assignment, so I really (really) want to turn this one in.  

-tkr

[Tekker]

Ok, we managed to put another piece of the puzzle together today and I thought I had it for a split second until I realized that I didn't.. Grrrr..

Ok, here's an updated version of our enthalpy diagram with the values that we "think" we know.... lol
http://www.artistcollaboration.com/users/tekker/Physics%20&%20Chemistry/EnthalpyDiagram2_NaCl_H2O.JPG

The problems we still have are:

1) We can't figure out what ?H(solution) is supposed to be:
We have looked up the standard enthalpies of formation for Na+(aq) (= -240.1kJ) and Cl-(aq) (= -167.2kJ), which gives a total of -407.3kJ. But the problems with this are these numbers indicate an exothermic reaction when according to how our teacher drew the diagram, it appears that it's supposed to be endothermic. So this number should be positive. The other problem is that the NaCl(ag) was formed from Na(s) and Cl2(g) elements, but our diagram starts at the NaCl compound. So that distance can't be correct.

So another thing I tried was using the standard ?H of formation of NaCl and NaCl(aq) to find ?H(solution) which gave a total of about 3.6kJ. This matches the other websites we have found online that has ?H(solution) in the range of 3kJ to 5kJ. However, putting 3.6kJ int ?H(solution) means ?H3 has to be wrong because ?H1 alone is a lot bigger than ?H3 and ?H(solution). Otherwise ?H2 would have to be negative and that certainly can't be right. So that leads us to our next problem....

2) The stuff I have seen online had ?H3 in the range of 780 or close to it.
But we cannot figure out how to get anything close to that number using the info in our text.

However (and this is a big however). ALL of the info we have found online has had absolutely no mentioned of the energy required for ?H2. That certainly hasn't helped any because those numbers may actually be completely different from what we need to get.


We could really use some help here, does anyone know how to do something like this? This seems like an awefully hard question for a gen chem class....      


Also, Would it help if I scaned the pages in our text that we were told we "should" be able to find the info? Like the appendix info and that "closer look" section on page 280? It turns out that the closer look section was also needed to find ?H3 because they don't give an enthalpy value for Cl-(g) in the appendix, so we had to figure it out from the enthalpies from the inonization of Na and Cl2.. Like I said, this is a HARD problem.

Thanks,
-tkr

[Hunt]

dH_soln is the sum of energies used in expanding both the solvent and the solute in addition to to the energy of solvent-solute interaction.

That is, dH_soln = dH₁ + dH₂ + dH₃

dH_soln is either exothermic or endothermic depending on the magnitude of dH's. dH₁ & dH₂ are always endothermic since energy is absorbed to overcome the forces while dH₃ is usually exothermic.

dH_soln can be determined from dG_soln and dS_soln at 298.15 K

dG_soln = dH_soln - TdS_soln

Claculate dH_soln.You know dH₁ and you'll need dH₃ inorder to compute dH₂. That's what I think dude ...