# Water and copper heat capacity difference due to potential energy?

1. Apr 29, 2013

### pa5tabear

I was looking at this page and it explains the heat capacity difference between copper and water in terms of a difference in potential energy change as the molecules are heated.

What does that mean? I know water has polar bonding so is it due to the water molecules having much stronger attractions which must be weakened by the heating?

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/inteng.html#c4

2. May 6, 2013

### jartsa

It's quite obviously wrong, so it doesn't really matter what they mean at that page.

One thing that matters regarding specific heat capacities of copper and water is that one gram of copper has different number of molecules compared to one gram of water.

Last edited: May 6, 2013
3. May 6, 2013

### sophiecentaur

I think you may be referring to the Dulong Petit law, which was an early law about the thermal behaviour of crystals.

4. May 7, 2013

### Staff: Mentor

What is wrong in particular?

5. May 7, 2013

### jartsa

Somehow I just guess that if we want two materials with very different specific heats, then hydrogen and uranium are good choices.

Wikipedia tells us the specific heat of hydrogen gas is 123 times the specific heat of solid uranium.

There's no huge amounts of potential energy in hydrogen gas that could explain the huge specific heat. (that was the wrong explanation at the page)

6. May 7, 2013

### jartsa

Wrong type of heat capacity explained with an explanation that is wrong.

Text books explain that the heat capacity per mole is different in different materials, because molecules have different number of degrees of freedom ... or something like that.

7. May 7, 2013

### Staff: Mentor

IMHO hyperphysics page explanation is perfectly correct, what is incorrect is overgeneralization and trying to apply the same logic to every other case.

8. May 7, 2013

### OmCheeto

Interesting question, which I interpret simply as; "What does 'potential energy' mean in this context?"

Trying to find the answer myself, I watched a video, where a gentleman presented a graph showing "intermolecular potential energy".

Something similar to this:

Which still left me a bit bewildered, so I googled some more and ran across a paper by some Iranian gentlemen studying something called asphaltene. They had many similar graphs, including this one:

Which kind of told me why the first graph, from right to left, curves down, and then up.
The x-axis in both graphs represents the distance between the molecules.

But neither graph told me where these forces came from, so I googled some more and ran across a video where a nice young lady described intermolecular forces:
Hydrogen bond
Dipole Dipole
Van Der Waals​
She stated that the Hydrogen bond was the strongest of the three, so I was leaning towards that as an explanation.

But it was just a hunch, so I googled some more and ran across a page, where they kind of imply that water is very weird, and made the following comment:

So putting this all together, if you look back at the first graph, I would imagine that you would start out on the far left hand of the graph, where the energy is positive, and as you add heat to the system, the hydrogen bonds breaking consume energy, which pushes you into the negative region.

My final guess is that the broken hydrogen bonds are responsible for the "potential energy" portion of the graph that you originally questioned:

Oh, and another thing, if you look at the molar heat capacities of water vs copper, water has a value only 3 times as high. And metals are all nearly identical!

Please don't take any of this as a true or accurate picture of what is really going on. As my name implies, I know nothing, and I look forward to a clearer explanation.