Stability and the 2nd law

Welcome to thermodynamics in energy engineering week 3 Before getting into the 2nd law of thermodynamics, let’s think about the thermodynamic stability of the two status. Compare the stability of the two bricks below. We say that the first brick is metastable and the second brick is stable. The metastable status means that it is not the most stable state but does not change status easily since it needs stimulus or additional energy for status change. But when we compare the stability in this case, we only considered energy. The most stable state is the lowest energy state. It is stable only in terms of energy. Here’s another example.

White tin is a tetragonal structure and it transforms into grey tin with a cubic structure at 13.2 C. The enthalpy of this transformation is -8.5 cal/mol, exothermic. The transformation evolves heat. This means that at this temperature, grey tin has the lowest energy. Does it mean that grey tin is the most stable? The answer is no. When we think about stability, we have to consider Entropy also. The 2nd law of thermodynamics is the entropy principle. The 2nd law is concerned with the conversion of heat into work. Traditionally people obtained the mechanical work from the thermal energy, the heat using heat engine, so the main concern is the efficiency of the conversion from heat to work.

The 2nd law states that 100 % conversion from heat to work is impossible and there’s always a limitation. Other expression for 2nd law is that the useful energy, the energy that can do work decrease over time. So the energy of the isolated system becomes less usable over time. Here, special caution is required. The reverse statement is not true. There’s no limitation on the conversion of work into heat. We can convert 100% electrical work into heat in electrical heater. There are various statement describing the second law. The fist statement is the most general statement for the second law. The entropy of an isolated system always increases or remains constant.

The other statement is that every system left to itself changes in such a way to approach a definite final state of rest, that is the equilibrium status. The equilibrium state is the state of rest, and it is the lowest free energy state. So every system is approaching the lowest free energy states without additional stimulus. That’s because the free energy decrease is equivalent to entropy increase. So the free energy decreases until it gets minimum. We will see that in week 5 of this course. So this statement is nothing but the entropy increase principle in an isolated system. Another statement is for the reversible process, the total entropy of the universe does not change. It is the definition of the reversible process.

If the total entropy increases, it is the spontaneous process. Do not confuse“spontaneous” with“fast”. Not every spontaneous process is fast process. Spontaneous is not related with the speed of the process. It simply indicates a process that occurs without the need for additional energy.