Phase Changes and internal energy

[gkangelexa]

The internal Energy E is this: $\Delta$ E = q + w = q - P$\Delta$V

where q is heat, w is work, and V is volume

Phase changes occur when you change internal energy of the system, right?

I am assuming that this means when you go from gas to liquid to solid, you must decrease the internal energy (remove heat).

And when you go from a solid to liquid to a gas, you must add internal energy (add heat).

That obviously makes sense, but what happens when you involve the work part of internal energy.
When you do work on the system (volume decreases), you increase its internal energy, yet compression causes a gas to turn to a liquid.
How can this be since you are increasing the internal energy?


Or am i understanding this wrong? Is it the increase in internal energy or the increase in heat that determines phase changes?

 

[Studiot]

Have you ever pumped up a tyre?

What happens?

The tyre becomes hotter.

This is because the work you do in increasing the gas pressure appears as heat according to the gas law

PV/T = constant

Since the volume does not change the temperature must rise.

Also the boiling point rises with increase in pressure. (Really we should call this the condensation point since we are talking about a gas to liquid phase change.)

 

[gkangelexa]

So why when we compress a gas, it turns to a liquid, even though you increase its internal energy?

that seems counter intuitive, since removal of energy is what causes a gas to turn into a liquid not addition of energy

 

[nasu]

So why when we compress a gas, it turns to a liquid, even though you increase its internal energy?

that seems counter intuitive, since removal of energy is what causes a gas to turn into a liquid not addition of energy

The increase in internal energy due to compression happens before the phase transition.
During the actual transition the gas must be able to transfer energy (heat) to the environment.
The increase in pressure brings the gas at the right conditions for molecular interactions to become strong enough for a condensed phase. This may be done by decreasing temperature, increasing pressure or both.
If you cool down the gas rather than compressing it, it looses some energy but the energy lost during cooling is not related to the energy lost during the phase transition.

 

[PJC01]

I would like to clarify a few things about phase changes and internal energy. Firstly, phase changes occur due to a change in internal energy, but it is not the only factor that determines a phase change. Other factors such as pressure and temperature also play a significant role.

In the equation \Delta E = q + w, q represents the heat transferred to or from the system, while w represents the work done on or by the system. This equation is known as the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

When a phase change occurs, there is a change in the internal energy of the system, but it is not solely determined by the heat or work involved. For example, during melting, the internal energy of a solid increases as it absorbs heat, but the temperature remains constant until all the solid has melted. This is because the absorbed heat is used to overcome the intermolecular forces holding the solid together, rather than increasing the temperature. Similarly, during freezing, the internal energy of a liquid decreases as it releases heat, but the temperature remains constant until all the liquid has solidified.

In the case of a gas turning into a liquid, as you mentioned, compression does increase the internal energy of the gas due to the work done on the system, but this increase in internal energy is not the sole factor that causes the phase change. The decrease in volume also increases the pressure, which can overcome the intermolecular forces and cause the gas to turn into a liquid.

To summarize, phase changes are determined by a combination of factors, including heat, work, pressure, and temperature. It is not just the increase or decrease in internal energy that determines a phase change, but rather the overall balance of these factors. I hope this explanation helps to clarify any confusion.