In my Calculus class today we were discussing thermodynamics, and one person claimed that he has thought of a way to violate the laws of thermodynamics. His proposal goes like this.

 

Water expands when it freezes, so as energy is released from water, and it freezes, the expansion could be harnessed to give more energy then you started with, so you get the energy that is released from the water, in addition to the energy generated by the movement of the water as it freezes.

 

I don't have enough of a background in physics to counter this, could someone else help me to understand how this fits in with thermodynamics?

In order to harness the energy of expansion, you would have to exert some pressure on it (for it to work against, in a way) -- in fact, just enough so that the freezing point depresses below the temperature you are using. If you try to put even more pressure on it, the temperature will increase (due to PV=nRT) thereby further preventing it from ever reaching that freezing point.

nope, the work done by expansion is already considered. its in the enthalpy of the phase change. thermodynamics considers volume changes as well as temperature changes.

q = wK + du/dt (1st law of thermodynamics)

where Q is the heat transfer rate and Wk is the work transfer rate. They

may be expressed in joules per second (J/s) or watts (W). The derivative

dU/dt is the rate of change of internal thermal energy, U, with time, t.