Water and Specific Heat Capacity

LLMMML

As part of a Biology course, there was a minor piece on the specific heat capacity of water.

I understand how some of the heat energy you add to water is used to break hydrogen bonds, thereby having a smaller impact on the average kinetic energy of the molecules and therefore a smaller rise in temperature than you would see in other liquids.

I don't really understand the opposite process though.

Say you remove a lot of heat from water by whatever process. The molecules lose kinetic energy, more hydrogen bonds form. Why does this lead to a smaller fall in temperature than in other liquids?

Oregano_State

I'm not sure what level you're studying this at but this gives a good, concise explanation. http://www.lsbu.ac.uk/water/explan4.html

The general idea is that since water has a relatively low molecular mass and size compared to many other liquids, heat (energy) loss is spread out across many more molecules, thereby reducing the average energy loss per molecule, and keeping the average RMS velocity higher than in other liquids.

krd

Oregano_State wrote: »

The general idea is that since water has a relatively low molecular mass and size compared to many other liquids, heat (energy) loss is spread out across many more molecules, thereby reducing the average energy loss per molecule, and keeping the average RMS velocity higher than in other liquids.

But as far as temperature goes. Wouldn't molecular momentum be more important than velocity?

I'm not sure of this, but imagine, that heavier molecules, as they form Van Der Waal bonds, would take more momentum from the fluid than smaller molecules. Which would mean, water would cool slower.

Oregano_State

According to wikipedia, the temperature of an ideal gas is directly proportional to the mean kinetic energy of the molecules. Ek=(1/2)M*V^2

This means that V, the velocity is the most important term in determining the temperature.

Obviously for a real gas, and even more so for a liquid, inter-molecular forces beome much more important, but I'm pretty sure they're not of the same order as the Kinetic Energy, so velocity is still the most important term.

krd

Oregano_State wrote: »

According to wikipedia, the temperature of an ideal gas is directly proportional to the mean kinetic energy of the molecules. Ek=(1/2)M*V^2

That formula contains the momentum.

This means that V, the velocity is the most important term in determining the temperature.

Since the mass is constant, it's fair to say that.

Obviously for a real gas, and even more so for a liquid, inter-molecular forces beome much more important, but I'm pretty sure they're not of the same order as the Kinetic Energy, so velocity is still the most important term.

No, they are very important. For two important reasons. One, the intermolecular collisions, distribute the velocity of the molecules - they disperse the momentum, eventually reaching thermal equilibrium, where there is an average momentum that's the same at every point in say a volume of gas - why I say momentum instead of velocity, is in a mixed gas at thermal equilibrium, all the molecules will have the same average momentum. But, the light molecules will have a higher average velocity.

Okay, that bit was obvious. The collisions and recoil, of the molecules, is an interaction of the electron field - and the protons too. It's the Pauli exclusion principle in action. The electric potential field around an atom or a molecule is not even. In gas form, the field seems even - but that's an illusion. In liquid form, you can see the stickiness appear - and then in solid, you can see the unevenness form into ordered crystalline shapes.

The second reason intermolecular forces are important. Is black body radiation. The molecules brush against each other. This creates photons - there is quite a bit of this going on. And if you like - you can consider the photon momentum to be the included in the average momentum. You'd be tricking yourself if you thought it was just molecular momentum/velocity.

And there is a direct relationship, between the momentum of the photons (peak wavelength not average) and the average molecular momentum. And of course temperature.


It sounds like a more convoluted way of thinking about it - but it encapsulates a lot at once. And with a little push, it explains, the Archimedes principle, why bubbles rise in fizzy drinks, it even explains why the wind pushes things around.

It tells you why chemical reactions, like cooking, need heating. When you put sugar in direct sunlight, it doesn't caramelise in front of your eyes. Even though it's being hit by high energy photons. It's because when the electrons in the sugar molecules are energised and jump - I think it takes pico seconds - the average momentum in the sugar is so low - there's nothing to push the nuclei apart. In a hot sauce pan, the average momentum is higher - so the chance of nuclei being pushed away from each other is higher.

And for the same reason stirring, like stirring custard, helps the chemical reactions you're looking for to happen, happen.

The same reason a chemist heats a mixture with a Bunsen burner, and maybe uses a stirrer. The greater the momentum the greater the chance molecules will slam into each other and react.