So I've always wondered where molecular energy comes from...

[imported_electron]

Ok that title is probably not the best way to put it but given limited space and my not knowing what to call it....

I've always wondered where the energy comes from to say for example split an engine block when water freezes. I mean it must take quite a bit of energy to break open a hunk of metal like that right?

And what is the limit to that energy? I mean there must be a limit right? How big of a force can you exert with the expansion of freezing water?

And what is it called? I don't think molecular energy is the right term.

 

[iwantanewcomputer]

I think it would be the negative heat of melting. You have to put a lot of heat into ice to turn it into water at the same temperature. That it the energy difference between the two phases, Which I believe is equal to delta(PV). this can give you a much higher pressure at constant volume or a sizable volume increase. Look up a PVT phase diagram of water.

this is due to hydrogen bonding and the unique shape/polarity of H2O forming a regular lattice upon freezing with more empty space between molecules than the liquid phase. THis is one of the very few substances that decreases density as it freezes

 

[brentkiosk]

This is a first law of thermodynamics situation. Writing heat flow, Q, in as +, and work done, W, by the water/ice as +, the first law is Q = W + delta U, where delta U is the change in internal energy. Since heat flows out as the water freezes (so Q is -) it's not the heat flow that does the work. It's the fact that the internal energy of the solid is lower than that of the liquid that does the work.

 

[imported_electron]

Ok thanks that answers my question. So given that then the limit would be dependant on how much water you freeze right? The volume of water? Also the limit is related to the pressure exerted by the walls of the engine block right? I mean like if you have water under pressure that stops it from changing phase, until a higher energy is put into the system. For example a car's radiator is under pressure to prevent boilover by raising the temp at which the water boils.

 

[CycloWizard]

Originally posted by: electron
Ok thanks that answers my question. So given that then the limit would be dependant on how much water you freeze right? The volume of water?

Yes, the amount of work that can be done depends on the amount of water you're freezing. The name for the amount of energy per gram (or per unit) of water freezing is, I believe, the enthalpy of fusion.

Also the limit is related to the pressure exerted by the walls of the engine block right? I mean like if you have water under pressure that stops it from changing phase, until a higher energy is put into the system. For example a car's radiator is under pressure to prevent boilover by raising the temp at which the water boils.

The metal will deform until the pressure it exerts on the water/ice is equal to the pressure that the water/ice is exerting on it (neglecting other forces anyway). Water can form ice at high pressure. The time it takes to do so is longer and it may have to be colder (since this will allow a larger magnitude of heat transfer, therefore increasing the amount of work that can be done). Freeze a can of coke, for example. The force will be great enough to actually break a steel can.

A car's radiator is under pressure to prevent boiling and because the water pump is used to keep the fluid moving. This fluid moves at speed across the heat transfer surfaces, increasing convective heat transfer. I believe (though maybe I'm not remembering correctly) that the addition of antifreeze (ethylene glycol) to the coolant mixture elevates the boiling point significantly.