Sapling week 5/6 #3

[luludaly2B]

I am having a little trouble understanding this concept. Could someone explain this question to me? It reads:
Classify the phase changes by the signs of the system's ΔH and ΔS.

I'm pretty sure when liquid goes to gas and solid goes to liquid, both ΔH and ΔS are positive. However, I am not completely sure.

[Leo Naylor 2F]

Yes, you are correct that for fusion and vaporization, both the change in enthalpy and the change in entropy are positive. The same is true for sublimation (solid to gas) since it is simply the sum of fusion and vaporization. For the opposite processes, both the change in enthalpy and the change in entropy are negative.

[Edward Tang 1k]

You are right! liquid to gas and solid to liquid have positive values for both enthalpy and entropy because 1) you need energy to break the existing bonds, hence the positive delta H, and 2) the phase change moves to a state with higher entropy because particles in gas has more possible states than in liquid, which has more possible states than those in the solid state, hence the positive delta S

[Lea Chamoun 2J]

For each of the phase changes given, first determine if heat is absorbed or released. For example, for liquid to gas, heat needs to be put into the system in order for this phase change to occur, so ∆H is positive. Because the entropy of a gas > entropy of a liquid > entropy of a solid, the gas is going to have a higher entropy than the liquid, meaning ∆S will also be positive. You are also right that both ∆H and ∆S are positive for the phase change between solid and liquid because heat is being absorbed by the system and the liquid has greater possible positions than the solid, and a greater entropy.

[Kayla Law 2D]

Hi! The heats of vaporization (liquid to gas), fusion/melting (solid to liquid), and sublimation (solid to gas) are all positive since heat must go into the system to break apart hydrogen bonds in order for the phase change to occur. The entropy also increases as you go from solid to liquid to gas since a gas has the highest degeneracy (can occupy the most positions/states) and a solid has the lowest degeneracy (can occupy the least positions/states). So as you go from a solid to a liquid, a liquid to a gas, or a solid to a gas, the entropy is also positive. On the other hand, going the opposite direction will release heat and decrease entropy. So going from a gas to a solid, a liquid to a solid, or a gas to a liquid will release heat and decrease entropy. Hope this helps! :)

[SainehaMaddineni_3I]

Yes, you are right! The entropy of a material increases with an increase in the number of microstates. Entropy decreases when going from a freely moving state to one of limited movement where there are fewer possible arrangements of molecules, such as gas to solid. Enthalpy increases as a substance goes from a limited state to a more freely moving state since it requires energy to break the intermolecular forces (solid to liquid and liquid to gas). Enthalpy decreases when going from a state with fewer intermolecular forces to a state with more intermolecular forces (gas to solid).

In a solid, there is limited movement of molecules, so the number of possible arrangements in a solid is very small compared to a liquid or gas. When you go from a solid to a liquid, there is more molecule movement since there would be fewer intermolecular forces. As a result of an increase in possible microstates, solid to a liquid means ΔS is positive. Going from a solid to a liquid also requires energy to break the intermolecular forces, making ΔH positive.

The same reasoning could work for going from a liquid to a gas. You would be going from a more orderly state to a more freely moving state that has less intermolecular forces. A gas has more possible arrangements than a liquid. This means that entropy is increasing and ΔS is positive. Going from a liquid to a gas involves breaking intermolecular forces, which requires energy, and makes ΔH positive.

On the other hand, going from a gas to a liquid means that you would be moving from a freely moving state to one of limited movement. By going from more microstates to fewer microstates(less possible arrangements), the ΔS is negative (entropy decreases). A liquid has more intermolecular forces, so going from a gas to a liquid involves forming bonds, which indicates that enthalpy decreases and ΔH is negative.

[Juliet Cushing_2H]

To determine the ΔH value I tend to visualize a phase change diagram where you see how the phases change mapped against q, heat added to the system. In the process of phases changing from solid-->liquid and liquid-->gas, q increases which means that heat is being added to the system (endothermic). The opposite phase changes would be exothermic, ie gas-->liquid and liquid-->solid given that along the phase change line q decreases.

To determine ΔS think about the disorder in the system and possible movement. Solids are rigid and unable to move. Liquids have more mobility and freedom (more disorder) so changing from solid-->liquid would have a +ΔS value given that disorder increases. Likewise for liquid-->gas.

[Gwirnowski 3B]

For figuring out Delta S changes, I think of how organized the particles are. The more organized, the lower the entropy, so Solids (very organized) are a lot lower in entropy than liquids (somewhat organized) which are lower in entropy than gases (not very organized). So any shift from organized to less organized will be a positive Delta S, and any shift to being more organized will be a negative Delta S.

For Delta H, you can just think endothermic or exothermic. Exothermic processes will have a negative Delta H and endothermic will have a positive Delta H.

And finally for this problem, there will not be a phase change where it will require heat yet the entropy will decrease, or it is giving off heat and the entropy is increasing. For this problem you will only put the phase changes in areas where the Delta H and Delta S have the same sign.

[luludaly2B]

Yes, you are correct that for fusion and vaporization, both the change in enthalpy and the change in entropy are positive. The same is true for sublimation (solid to gas) since it is simply the sum of fusion and vaporization. For the opposite processes, both the change in enthalpy and the change in entropy are negative.

Thanks so much, this was super helpful! Is there ever an instance in which delta H and delta S are opposite signs? If so, when? Thanks again!

[Seraphina Joseph 1C]

These explanations really helped me! Thanks

[Mingzi Yang 1E]

Thank you for all the explanations! They are helpful!

[Mingzi Yang 1E]

Yes, you are correct that for fusion and vaporization, both the change in enthalpy and the change in entropy are positive. The same is true for sublimation (solid to gas) since it is simply the sum of fusion and vaporization. For the opposite processes, both the change in enthalpy and the change in entropy are negative.

Thanks so much, this was super helpful! Is there ever an instance in which delta H and delta S are opposite signs? If so, when? Thanks again!

I have the same question too! It will be great if anyone can clarify this question.

[Sara Dolmo 2J]

These explanations are really helpful thats so much!