What Happens to dU, dQ, and dW During Boiling?

[prasannaworld]

Boiling Internal Energy - URGENT

A quick question:

dU = dQ - dW

What are dU, dQ and dW during Boiling?

I thought:
-the particles do work, hence dW is +
-dU increases as particles gain more energy

so dQ = dU + dW - showing that heat flows in. Which made sense to me.

Or is Boiling Isothermal?

 

[vanesch]

It is difficult to say, because you didn't specify the way things boil. First of all, you shouldn't look at *microscopic* work ("the particles do work"). The whole idea of thermodynamics is that we can take abstraction from any microscopic model. So if you boil water in an open kettle, then yes, the steam does work: it pushes away the air that was there, at 1 atmosphere. You can picture this by considering the boiling water in a cylinder with a piston. The steam will push the piston out, doing work against the pressure of the air.

However, there's another change too. The internal energy of steam is larger than the internal energy of liquid water at the same temperature. So dU is positive (liquid water (lower U) changes into steam (higher U)). dW is positive (the system DOES work on the air), and of course, dQ is positive (heat is given to the system).

If the water is in a closed volume, then the steam doesn't do any work. So now, dW is 0. But dU is still positive (the change in internal energy between water and steam). And dQ remains positive (we're heating the water).

The difference between the U of water and the U of steam is called the "latent heat".

 

[sir-pinski]

Hi there,

Thank you for reaching out with your question about boiling internal energy. Let me try to provide some clarification for you.

First, let's define dU, dQ, and dW. dU represents the change in internal energy of a system, dQ represents the heat added or removed from the system, and dW represents the work done by or on the system.

During boiling, the particles are gaining energy as they are heated, so dU will increase. The heat added to the system, dQ, will also be positive since heat is flowing into the system. However, the work done, dW, can be either positive or negative depending on the specific conditions.

If the system is in a closed container, the work done will be negative as the particles are doing work on the container's walls as they expand. This can also be seen in the equation dU = dQ - dW, where dW is subtracted from dQ.

If the system is in an open container, the work done will be positive as the particles are expanding and pushing against the atmosphere. In this case, the equation would be dU = dQ + dW, where dW is added to dQ.

In terms of whether boiling is isothermal or not, it depends on the specific conditions. In an isothermal process, the temperature of the system remains constant. In boiling, the temperature of the liquid remains constant until all of the liquid has turned into vapor. However, the temperature of the vapor may change depending on the pressure and other factors.

I hope this helps clarify things for you. If you have any further questions, please don't hesitate to ask. Best of luck with your studies.