chemistry ponderable (not homework)

[rajan]

last week the students in the lab section i TA worked on colligative properties, particularly freezing pt depression. i was thinking about a particular part of the lab, and i would like to pose a question:

if you dissolve salt (any salt) in water, will you lower the heat of fusion? i would think that it would be lowered slightly so that it is less positive from solid --> liquid. My thinking is that because you are lowering the energy of the liquid form (evidence of this is that the melting point is lower), your transition will not only occur at lower T, but also have less of an energy difference.

any thoughts?









another reason i bring this up is that here in minnesota, we have cold winters. we also have refrigerators and freezers that run during these cold winters. it doesn't make sense to me to heat up our house and cool down our fridges at the same time. i was thinking that if you could construct a solution of salt water that would freeze about 10 degrees F, you could put it in your car overnight and then take it into your fridge during the day and it would basically power the fridge... the phase transition from solid->liquid could absorb a lot of heat energy, but would the salt-water mix have a lower heat of fusion than pure water (which is very high)?

[kaspar_silas]

What your saying will work I think. From what I understand:

Take a easier example, a pure element say Bismuth. Nearly any other element added to it (in small amounts) that forms an alloy will lower the melting point. It does this as the phonons in the heated alloy are scattered more due to the disruption of the Bismuth structure. Thus it melts at a lower temperature soaking up less energy. Hence when it solidifies it also releases less energy.

I actually heard (I think in new scientist) about a housing estate in germany doing a similar scheme to what you suggest. Well the opposite but you get the idea. The houses were made with walls that had a form of low temp melting wax encapsulated. During the day the wax melted and then on cold nights it released the energy. I vaguely remember a claim of 60% reduction in heating.

[rajan]

It does this as the phonons in the heated alloy are scattered more due to the disruption of the Bismuth structure.

so it's actually the instability of the solid instead of the increased stability of the liquid that actually causes mp lowering? very interesting, it's quite different from what i thought initially. maybe there's some of both.

I actually heard (I think in new scientist) about a housing estate in germany doing a similar scheme to what you suggest. Well the opposite but you get the idea. The houses were made with walls that had a form of low temp melting wax encapsulated. During the day the wax melted and then on cold nights it released the energy. I vaguely remember a claim of 60% reduction in heating.

that's a really good educational lesson (my students may hear about it)

the link is http://www.worldchanging.com/archives/002404.html
--- i'm always amazed at the ease in which Germans are able to bring pure science to an applied state. thanks for the information.

[bruceschaller]

Well, you could do that. (Bring frozen salt water and put it in the freezer to 'power' it.) But, In winter the waste heat from the refrigerator heats the house, which is actually good use of 'waste' heat.

The calculations that you would need to do to see if it is feasible:

Convective, conductive, and radiative heat transfer from the ice block, inside of the refrigerator. (Radiative heat is small) Also, it would be good to know how fast the refrigerator absorbed heat from the outside (lost cold/gained heat).

Convective heat transfer is actually the most important. Usually refrigerators have air blowing by the cold coils to maintain the temperature evenly throughout the refrigerator. So you may need a small fan inside the refrigerator to replace that effect. Otherwise you will get a layering of air in the refrigerator.

The required variables are:
1) Area of the ice block, or pan in which you are keeping the ice
2) the convective and conductive coefficients between the ice and the air
(The convective coefficient is a function of how quickly air is moving over the block of ice, so you would need to figure that too.)
3) The temperature difference between the ice and the air
4) how quickly the refrigerator heats up under the influence of the house.

A textbook that can help you make this calculation is Geankoplis' Transport and seperation proceeses.

You might find it easier to move the refrigerator outside, and place distilled water in the fridge, with a small fan. That way the temperature won't drop below 32 before the water freezes.

Bruce Schaller
Chemical engineering student

[kaspar_silas]

Well, you could do that. (Bring frozen salt water and put it in the freezer to 'power' it.) But, In winter the waste heat from the refrigerator heats the house, which is actually good use of 'waste' heat.

The calculations that you would need to do to see if it is feasible:

Surely the proof that's it feasible is it used to be done. Old houses often had ice houses that worked on pretty much this principal. Here is one one I have seen myself.
http://www.antrimtown.co.uk/The%20Ice%20House.htm
Okay, it's with fresh water but the principal is the same. Plus this was in Antrim (Ireland) where it very very rarely goes below -5C or (23F).

O, I just thought maybe you mean feasible on the small scale in a house? Well then fair enough. Actually I suppose if you really wanted to model the heat losses you could make a finite element model of the whole system. More importantly this would also look really nice.

I did actualy look on one FE product gallery, just in case,
http://www.comsol.com/showroom/apparea/644/
but alas no.

[rajan]

Well, you could do that. (Bring frozen salt water and put it in the freezer to 'power' it.) But, In winter the waste heat from the refrigerator heats the house, which is actually good use of 'waste' heat.

yeah i always find it a tradeoff whether i should switch back to incandescent lights in the winter when i would really like some radiative heating. the refrigerator problem is that the heat is just heating up the wall and probably exchanging with the outside before it actually does something useful. on a related note, i laugh when a space heater advertises to be 100% efficient, as if it is something special to that brand.

kudos to kaspar_silas for yet another interesting internet reference.