Law for heat exchange and evaporation

In summary, the conversation discusses the laws of heat exchange and how it relates to evaporation. It is explained that evaporation creates a cooling effect even in an isothermal condition, and it is a chemical process. The discussion also delves into the role of average kinetic energy and temperature in this process, and how evaporation results in a temperature gradient within the surroundings. The catch is also mentioned, where non-surface molecules with higher kinetic energy can also contribute to evaporation.
  • #1
rkatcosmos
10
0
I wonder if there is any law for heat exchange. I have learned that heat exchange between systems occurs when the temperature of respective systems differ. But in case of evaporation, the process creates a cooling effect even in an isothermal condition ( I mean by isothermal condition to be a isothermal situation between the evaporating liquid and the surrounding).

I would like to discuss the cooling effects produced by evaporation under isothermal condition.
 
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  • #2
If heating is by radiation, it can only take place at temperature difference. Newtons Law of cooling states that. There are other laws like Kirchoff's Law(search Wiki).

Cooling effects due to radiation is a different process. Exchange of material takes place between System and surrounding. Though i don't know too much details about it. But basically it is a chemical process that happens because change in Entropy(delta S) is positive.
 
  • #3
darkxponent said:
If heating is by radiation, it can only take place at temperature difference. Newtons Law of cooling states that. There are other laws like Kirchoff's Law(search Wiki).

Cooling effects due to radiation is a different process. Exchange of material takes place between System and surrounding. Though i don't know too much details about it. But basically it is a chemical process that happens because change in Entropy(delta S) is positive.

I was asking about evaporation and not radiation.
And in both cases I don't think its a chemical process
 
  • #4
rkatcosmos said:
I was asking about evaporation and not radiation.

Really? See what you wrote in Post1.
rkatcosmos said:
I wonder if there is any law for heat exchange. I have learned that heat exchange between systems occurs when the temperature of respective systems differ.
This was what what i replied to.
rkatcosmos said:
And in both cases I don't think its a chemical process
Case two is definitely a chemical change where,

H20(liquid) → H20(vapour), which in this case happens because spontaneity is positive as liquid changes into gas.
 
  • #5
When a pure liquid system is evaporating under isothermal conditions (at constant pressure), a part of its boundary is receiving heat from the surroundings. But how can this be if the situation is isothermal? For the system to remain isothermal, the rate of heat transfer from the surroundings to the system must equal the rate of evaporation times the latent heat of evaporation. Even if the system is isothermal, the portion of the surroundings through which heat flow is occurring is not isothermal spatially. There is a temperature gradient within the surroundings that is responsible for conductive heat flow to the system. The interface between the system and the surroundings is at the evaporation temperature of the system, but, within the surroundings, the temperature increases with distance from the boundary. The thermal conductivity of the surroundings times the surroundings' temperature gradient at the interface is equal to the heat flux (heat flow per unit area) into the system.
 
  • #6
Initially assuming the liquid and the surroundings are at the same temperature,let us proceed.
First let us consider the evaporating liquid which is the object of interest ,not the surroundings.
When you say a substance is at a constant temperature,the average KE of the molecules is a constant,in this case for the evaporating liquid.Though,we say the evaporating liquid is at a constant temperature,you will always find molecules having KE greater than and lesser than the average KE. Also,we know that Evaporation is a surface phenomenon.Therefore,molecules only at the surface can evaporate,so those molecules which have KE greater than say a "critical KE" will escape into the air.The Surface molecules tend to absorb KE from non-surface molecules which have KE greater than avg. KE and also itself. Once, the surface molecule escapes into the air(or whatever),the remaining liquid is left with a lesser average KE ,which implies a lesser temperature(however small say dT).Therefore ,the evaporating liquid absorbs an elemental energy(dE) from the surroundings for every molecule evaporating.Therefore responsible for cooling effect on the surroundings.

The catch:
If you ask how can there be non-surface molecules which have KE greater than surface molecules,(owing to surface energy...)I would say on an average surface molecules may have higher energy,but still it is an average, in a distribution you will always find "some" non-surface molecules with higher KE than surface molecules
 
  • #7
Hummel said:
Initially assuming the liquid and the surroundings are at the same temperature,let us proceed.
First let us consider the evaporating liquid which is the object of interest ,not the surroundings.
When you say a substance is at a constant temperature,the average KE of the molecules is a constant,in this case for the evaporating liquid.Though,we say the evaporating liquid is at a constant temperature,you will always find molecules having KE greater than and lesser than the average KE. Also,we know that Evaporation is a surface phenomenon.Therefore,molecules only at the surface can evaporate,so those molecules which have KE greater than say a "critical KE" will escape into the air.The Surface molecules tend to absorb KE from non-surface molecules which have KE greater than avg. KE and also itself. Once, the surface molecule escapes into the air(or whatever),the remaining liquid is left with a lesser average KE ,which implies a lesser temperature(however small say dT).Therefore ,the evaporating liquid absorbs an elemental energy(dE) from the surroundings for every molecule evaporating.Therefore responsible for cooling effect on the surroundings.

The catch:
If you ask how can there be non-surface molecules which have KE greater than surface molecules,(owing to surface energy...)I would say on an average surface molecules may have higher energy,but still it is an average, in a distribution you will always find "some" non-surface molecules with higher KE than surface molecules

If particles with KE greater greater than "critical KE" escapes, then how do they absorb energy? Why should they absorb energy? They are already having higher energy...

And can temperature be defined to any defined region? If so what will be the definition of temperature?
If average KE of the system is defined as temperature, then is there flow of heat between different layers of liquid(as you say average KE at the top layer is higher than the bottom layer of liquid)
 

1. What is the law of heat exchange?

The law of heat exchange states that heat flows from a region of higher temperature to a region of lower temperature until an equilibrium is reached.

2. How is heat exchange related to evaporation?

Heat exchange is necessary for the process of evaporation to occur. When a liquid is heated, its molecules gain energy and become more active, resulting in some molecules breaking away from the surface and turning into gas through evaporation.

3. How does the rate of evaporation depend on temperature?

The rate of evaporation is directly proportional to temperature. This means that as the temperature increases, the rate of evaporation also increases. This is because higher temperatures provide more energy for the molecules to break away from the surface and evaporate.

4. What role does humidity play in heat exchange and evaporation?

Humidity affects the rate of heat exchange and evaporation. Higher humidity levels mean that the air is already saturated with water vapor, making it more difficult for the liquid to evaporate. This is because there is less room for the water molecules to escape into the air.

5. How does surface area affect evaporation?

The larger the surface area of a liquid, the faster it will evaporate. This is because a larger surface area allows for more molecules to escape and turn into gas. This is why larger bodies of water, such as oceans, evaporate at a slower rate than a small puddle.

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