Can Delta U for Water Systems in Contact Be Equal and Opposite?

[Henrique Silva]

If we have 2 systems like water(A) and cold water(B) in contact, if we ignore the energy exchanges between air and those systems, could we consider that the deltaUB=-deltaUA??

 

[Chestermiller]

Are A and B kept separate, and just allowed to exchange heat, or are they mixed with one another?

Chet

 

[Henrique Silva]

They are mixed

 

[Chestermiller]

They are mixed

If they are mixed, then it is not possible to determine the separate identities of A and B in the final mixture. So you you can't determine ΔU_A and ΔU_B separately. However, you can determine the difference in internal energy between the final mixture and the initial separate internal energies of A and B, and set this difference equal to zero.

Chet

 

[Henrique Silva]

If deltaUA and deltaUB=weight . mass thermal capacity . delta temperature, I can determine the deltaU of both A and B even if they are mixed together

 

[Henrique Silva]

Now I understand what you were saying, ok but isn't it the same measuring the deltaU with the final mixture?

 

[Chestermiller]

If deltaUA and deltaUB=weight . mass thermal capacity . delta temperature, I can determine the deltaU of both A and B even if they are mixed together

You can do that, and you would obtain the same result as if you used the method I described in post #4. However, conceptually, if the fluids are mixed, it is not really appropriate to identify separate changes for the internal energies of A and B. After all, at the molecular level, the fluids would be intimately mixed, and you could no longer identify either liquid.

Chet

 

[Henrique Silva]

So This isn't right deltaUb=-deltaUa?

 

[Chestermiller]

Also, if you were mixing different fluids to form a solution, there might be a heat of mixing, and it would then be problematic to identify the separate contributions of each of the two fluids.

 

[Henrique Silva]

Thanks

 

[Chestermiller]

So This isn't right deltaUb=-deltaUa?

That would be OK for mixing two bodies of the same liquid, but what would you do if they were two different liquids, and there was a heat of mixing?

Chet

 

[Henrique Silva]

So if they were two different substances that expression wasn't apropriated. Thanks about your help