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Q & A: endothermic temperature changes

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Most recent answer: 11/15/2011
Q:
Is an endothermic reaction always accompanied by an increase in temperature? For example, when water is heated and evaporates, the molecules move faster which normally indicates an increase in temperature but in the case of evaporation, we consider the process endothermic no?
- Manon (age 45)
Qu├ębec, Canada
A:
"Accompanied by" is a tricky phrase. In the case you describe, something (maybe a stove) is pouring heat into the water. Endothermic evaporation is occurring. The water is getting hotter. However, the endothermic process is cooling the water. Think of how you feel when sweat evaporates from your skin when there's a breeze. The evaporation definitely cools you down. Now if at the same time something else, like the stove, is putting even more heat in, then it's true that the evaporation is "accompanied by" a net temperature increase. However, by itself, the evaporation causes the temperature to decrease.

Mike W.

(published on 11/13/2011)

Follow-Up #1: liquid-gas equilibrium

Q:
O.K. but then even if we don't take into account the heat coming from the stove, the molecules are moving faster and should be at a higher temperature no? I thought that temperature was an indication of the level of agitation of the molecules.
- Manon (age 45)
quebec, canada
A:
In equilibrium, the liquid and the gas have the same temperature. Since at that temperature it's a good approximation to say that the molecules' kinetic energy is proportional to the absolute temperature T (via the equipartition theorem),  the kinetic energy per molecule in the liquid and the gas will be the same.

In practice, as you boil some water it will superheat a bit, i.e. get slightly above the boiling point. In that case the liquid will be a little hotter than the vapor. Under other circumstances (say a cold night, where things are cooling via infrared radiation) the vapor can also be a bit cooler than the liquid for a while. In other circumstances (a sunny day, with things warming from solar radiation) the vapor may be a bit warmer than the liquid for a while.

Mike W.

(published on 11/15/2011)

Follow-up on this answer.