# Law for heat exchange and evaporation

1. Jun 29, 2013

### rkatcosmos

I wonder if there is any law for heat exchange. I have learnt 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.

2. Jun 29, 2013

### darkxponent

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. Jun 29, 2013

### rkatcosmos

I was asking about evaporation and not radiation.
And in both cases I don't think its a chemical process

4. Jun 29, 2013

### darkxponent

Really? See what you wrote in Post1.
This was what what i replied to.

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. Jun 29, 2013

### Staff: Mentor

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. Jul 9, 2013

### Hummel

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. Jul 13, 2013

### rkatcosmos

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)

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