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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: asheys666 on March 31, 2020, 03:23:31 PM

Title: Question regarding the potential and kinetic energy of two substances
Post by: asheys666 on March 31, 2020, 03:23:31 PM
Let's say we have a substance Y and a substance Z (Both are liquids), Y starts out at a higher temperature than Z. These two substances come into contact and exchange heat. Assume a closed system and that heat exchange can only occur between Y and Z

We are asked to describe the PE and KE changes for both substances at the moment substance Y is freezing (at freezing point).

I stated that substance Y's potential energy is increasing in the negative direction as new intermolecular attractions are being formed between the molecules as it goes from liquid to solid.

The KE of substance Y is not changing because at the moment substance Y is freezing the temperature is not chaning.

The KE of substance Z will be increasing slightly because of the heat transferred from Y to Z.

However, I am confused as to whether the potential energy of Z would be unchanging or not because it continues to remain a liquid when Y is freezing meaning that no intermolecular attractions have been broken. I am just very confused if my statement is correct or not?
Title: Re: Question regarding the potential and kinetic energy of two substances
Post by: Enthalpy on April 01, 2020, 07:05:10 AM
Let's admit that the vibration energy is not counted by half as kinetic energy, but separately.

"The temperature is not changing" is no good reason. Molecules have an individual speed and kinetic energy in the liquid, they don't in the solid. Here the kinetic energy changes at constant temperature.

But if we recognized that vibration is just half kinetic energy and half deformation energy, the situation would be much more complicated and about impossible to answer. In stiff small liquid or gaseous molecules at moderate temperature, some vibration modes are not excited and store no energy, while if building a solid, they have collectively more vibration modes at lower energies that are excited and store energy. That's why nearly all metals have 3R/mol heat capacity. Then freezing may increase the energy stored in vibrations, and in some molecules, this increase could exceed the loss of energy in individual translations.

About potential energy: intermolecular forces and energies change in a liquid when, for instance, the liquid expands. No melting needed. You could compare the heat capacity of the same substance at the same temperature, as a liquid and as a gas. Big difference. Tables for water and steam are easily found.