Refrigerators

[pm1]

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I'm trying to understand how refrigerators work and I found a pretty useful thread (I must forewarn that it is pretty lengthy though).
http://forums.studentdoctor.net/showpost.php?p=3852353&postcount=13

If anyone is willing to learn about refrigerators and read the thread, I have a question 🙂

On the third paragraph it says that the refrigeration "occurs in a closed process, there is no change in enthalpy and heat is lost--that is why it usually is warm behind your refrigerator."

I thought closed process only meant that there is no mass exchange with the surroundings. How come do we know that there is no change in enthalpy? By loosing heat isn't the enthalpy changing?

Thanks

 

[SaintJude]

I'm not sure myself, but I'll summarize what he said.

The post said: expanding gases will cool, and compressed gases will warm. (wasn't backed up)

Basically:
1.) The "compressor" compresses refrigerant gas, which will warm the surroundings in a constant volume system.
So P ↑ and temp ↑ . Then he says B/c Δ V = 0 and W=P Δ V, the 1st law is reduced to U = Q.

That's when he says:

But, since this occurs in a closed process, there is no change in enthalpy and heat is lost--that is why it usually is warm behind your refrigerator.

Now that everyone's up-to-speed, can someone explain?

 

[milski]

It's closed in a sense that the gas inside the system is isolated from the outside and can exchange only heat. It's still the same gas, it did not participate in any chemical reactions, etc, so when it returns to the same initial state, its enthalpy would not have changed either. As such, all the generated heat Q will be really transferred as heat outside of the system.

It's probably a bit obvious point but it's brought up to support the claim that all Q is transferred outside. If some part of the gas was reacting with itself, making it diatomic from monoatomic, you'll have some change of enthalpy at the end of the cycle and some of that Q will actually go towards that change and not as a transfer to the outside. Fortunately, that's not something we need to worry about.

 

[pm1]

Thank you guys!! I think I'm finally coming in good terms with thermo 🙂

SaintJude, fyi, if you want some practice about refrigerator from TPRH passage 74 in gen chem is a pretty good one! - in case if you haven't done it yet