The Second and Third Laws of Thermodynamics


By the end of this section, you will be able to:

  • State and explain the second and third laws of thermodynamics
  • Calculate entropy changes for phase transitions and chemical reactions under standard conditions

The first law of thermodynamics is essentially the conservation of energy ($\Delta U = Q – W$), as explored in the first half of this chapter.

The second and third laws relate to how reactions progress, and how we define reaction progress, respectively. Both of these rely on an understanding of entropy, as discussed in the previous section. In the next few pages, we will explore these more.

(There is also a “zeroth law” that defines heat flow: If two systems are in thermal equilibrium with a third system – i.e. they are at the same temperature – they must also be in thermal equilibrium with each other. In other words, if A is at the same temperature as B, and B is at the same temperature as C, then A and C are also at the same temperature. Sometimes this law is also stated to include the direction of heat flow – if a system’s temperature decreases, it is losing heat).