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Chemistry LibreTexts

A Brief Introduction to the Second Law and to Entropy

Energy of all types -- in chemistry, most frequently the motional energy (KE) of molecules (but also including the phase change/potential energy of molecules in fusion and vaporization, as well as radiation) -- changes from being localized to becoming more spread out, dispersed in space


if that energy is not constrained from doing so.

The simplest example is the expansion of an ideal gas from one bulb to occupy both that bulb and an attached evacuated bulb in the stereotypical textbook illustration:

The initial motional/kinetic energy (and potential energy) of the molecules in the first bulb is unchanged in such an isothermal process, but it becomes more widely distributed in the final larger volume.  Further,  this concept of energy dispersal equally applies to heating a system:  a spreading of molecular energy from the volume of greater-motional energy (“warmer”) molecules in the surroundings to include the additional volume of a system that initially had “cooler” molecules.  It is not obvious, but true, that this distribution of energy in greater space is implicit in the Gibbs free energy equation and thus in chemical reactions.  

“Entropy change is the measure of how more widely a specific quantity of molecular energy is dispersed in a process, whether isothermal gas expansion, gas or liquid mixing, reversible heating and phase change, or chemical reactions.”


There are two requisites for entropy change.

  1. Iit is enabled by the above-described increased distribution of molecular energy. 
  2. It is actualized if the process makes available a larger number of arrangements for the system’s energy, i.e., a final state that involves the most probable distribution of that energy under the new constraints. 

Thus, “information probability” is only one of the two requisites for entropy change.  Some current apporaches regarding “information entropy” are either misleading or truly fallacious, if they do not include explicit statements about the essential inclusion of molecular kinetic energy in their treatment of chemical reactions.



**This literal greater spreading of molecular energy in 3-D space in an isothermal process is accompanied by occupancy of more quantum states (“energy levels”) within each microstate and thus more microstates for the final macrostate (i.e., a larger W in RlnW). Similarly, in any thermal process higher energy quantum states can be significantly occupied – thereby increasing the number of microstates in the product macrostate as measured by the Boltzmann relationship.


I. N. Levine, Physical Chemistry, 6th ed. 2009, p. 101, toward the end of “What Is Entropy?”