Chem: Why more heat released from steam than liquid water?

[Girlygeek]

I was sick so I actually missed this lesson in class and this wasn't mentioned in the notes I got from a fellow student, nor is it in my textbook. I can't even find a definition for what it means that the heat of condensation is "evolved," so if anyone can fill me in on that I'd greatly appreciate it. Any help/perspective would be great. Thanks!

Homework Statement

More heat is derived from cooling one gram of steam at 100 degrees C to water at 50 degrees C than from cooling one gram of liquid water at 100 degrees C to 50 degrees C because:

a. The steam is hotter than the water
b. The steam occupies a greater volume than the water
c. The density of the water is greater than that of the steam
d. The heat of condensation is evolved.

The Attempt at a Solution

I know it isn't "A" (lol, which weighs more, a pound of feathers or a pound of lead?). As for the other three, I'm totally stumped. If I had more of an idea what the heat of condensation being evolved meant, that would help. As it is, I'm leaning a bit more towards that as the answer. My second choice would be the density, then the volume. I would choose density because the closeness of the individual molecules might actually hamper the energy releasing process because they are so close, maybe the nearby atoms would re-absorb some of the energy, or act a bit like insulation. Just my guess.

Mostly I'd just really like to understand the concept behind this problem. Thanks!

 

[mgb_phys]

Heat of condensation is the heat given out when something condenses (ie gas->water)
It's more commonly called the heat of vaporisation = the heat you have top put into boil something.

You obviously need to put energy into boil water, so you must get heat out when it condenses!

ps. The phrase 'heat of' is an old fashioned word for 'energy'

 

[Girlygeek]

Thank you for the quick response. I actually understand the heat part, I'm just not sure what they really mean when they state that the heat of condensation is evolved. Does that imply that it is on a different energy level or something? So it would have more heat/energy stored up to release when it cools back down, than liquid water than never attains steam status?

 

[mgb_phys]

the heat of condensation is evolved.

= energy given out. For some reason all thermal questions seem to be written in the 17th century!

So it would have more heat/energy stored up to release when it cools back down, than liquid water than never attains steam status?

Yes - the extra energy you put into turn the water into steam is given off when it condenses.

 

[Girlygeek]

Great! So, that is the correct answer, then, right?

I just want to make sure I understand. The energy (heat) released varies in different states based on how much energy was required to put it in that state in the first place. So, liquid water would release more energy than ice, just like steam releases more energy than liquid water, because liquid water actually has more energy in it. The density or volume has absolutely no part to play when we are talking about cooling the same substance in different phases from one temperature to another, right? Do I understand it properly? I think it makes sense...

Thank you!

 

[mgb_phys]

Phase changes take in energy to go to a more disordered state so solid->liquid (ice->water) or liquid->gas (water->steam) and give off energy going the other way.
Generally the liquid->gas takes more energy (because it's more disordered) than solid->liquid, but it depends on the chemical bonds in the material.

eg for water:
Heat fusion (solid->liquid) = 6.01 kJ/mol,
Heat of vaporization (liquid->gas) = 40.68 kJ/mol

The temperature change is a little more complex.
There is no temperature change when freezing/melting or boiling/condensing.
The heat capacity (the energy needed to heat a fixed mass of stuff through 1 degree) also depends on the state, so the energy needed to heat 1kg of ice 1 deg (if you don't melt it) is different form that needed to heat water. This is because the bonds in the water or ice are different.


pps. the heat of vapourisation/fusion is also sometimes called the "latent heat" - latent means hidden.
In a way the extra heat is 'hidden' because there is no temperature change.


ps. yes the answer is d

 

[Girlygeek]

Thank you so much! That really helps. It is fascinating to me how the bonds change. It must be very complicated to all those little atoms being twisted and pulled, charged with energy and deflated from all the different forces that act on them and that they act on each other.