The Laws of Thermodynamics From Heat to Entropy

The Zeroth Law Came after the others; that is why it is zero. Statement of thermal equilibrium. If A is in thermal equilibrium with B and B is in thermal equilibrium with C; then A and C are in thermal equilibrium.

The First Law A Statement of Conservation of Energy. ΔU = q + w U is the internal energy q is the heat put INTO the system w is the work done ON the system. Any change in energy of a system must come in one of two forms: work and heat.

Sign Conventions of the First Law Always from the point of view of the system. Any energy INTO the system is + and any energy out of the system is -. q > 0  heat put INTO the system q < 0  heat leaving the system w > 0  work ON the system w < 0  work done BY the system

Thanks for the confusion… Chemists and physicists rarely see eye to eye! ΔU = q + w (Chemists) ΔU = q – w (Physicists) They are the same equation!! The physicists just put the negative sign in for you already!

Units of Energy U is measured in Joules q is also measured in Joules. w is also measured in Joules, but remember joules is a complex unit. Work is also F.d. So w is N.m. Pressure is N/m2 and volume is m3; when you multiply PV, you get Joules.

Units of Energy (cont’d) So that means that work done in thermodynamics is often called PV work. It can be calculated as PΔV. For this reason, we worry a lot about P and V and a graph of P and V is very useful!

PV Diagrams There are four basic ways to transfer heat and work: Isothermal Isobaric Adiabatic Isochoric These processes can be represented on a graph called a PV diagram.

Isobaric – Constant Pressure A process by which the pressure is not changed. To calculate work in an isobaric process, take the constant pressure and multiply by the ΔV. W = PΔV What if we graph an isobaric process on a PV diagram?

Isochoric – Constant Volume A process by which the volume is not changed. To calculate work in an isochoric process, take the changing pressure and multiply by the ΔV. W = PΔV; but ΔV is ZERO! Work done in an Isochoric process is zero. If there is a change in U, it is all heat!

Isothermal – Constant Temperature A process by which the temperature is not changed. To calculate work in an isothermal process is much more difficult. It is still the area under the curve, but the shape is more complex. Temperature is directly related to the energy of a system, so if ΔT = 0, then ΔU = 0. |q| = |w|

Adiabatic – No heat transfer When no heat is transferred in a process. Implies that q = 0! So ΔU = w!

Summing it All Up Isobaric: W = PΔV = area under the curve Isochoric: W = 0; ∆U = q Isothermal: ∆U = 0; |q| = |w| Adiabatic: q = 0; ΔU = w

Second Law of Thermodynamics There are a couple of different statements of the second law. Systems move spontaneously from states of low entropy to high entropy.

Entropy Entropy, S, is the disorder of a system. ΔSuniv > 0 for any process. This is based on statistics. There are more states available for a better chance at being disordered.

Second Law and Physics In physics, the second law usually is related to heat engines. Heat engine – a device that changes heat energy into mechanical energy. In this situation, the second law says that heat flows from hot to cold and not the other way around. Why is that a statement of the second law?

Heat Engines A common pictorial representation of a heat engine is shown. Since heat flows spontaneously from hot to cold, there is a drive for the heat to move. Work can be done.

Properties of Heat Engines There must be a difference in temperature reservoirs. Conservation of energy: Qhot = Qcold + W Efficiency: e = W/Qhot

Second Law and Efficiency But second law says that there can be no perfect heat engine due to non-idealities of gases and friction. That means there is a maximum efficiency for every heat engine, called the Carnot efficiency. eideal = (TH – TL)/TH = 1 – TL/TH