Entropy & spontaneity

A spontaneous reaction is a reaction that, given the necessary activation energy (`E_"a"`), proceeds without continuous outside assistance (e.g., the reaction between sodium and water). A non-spontaneous reaction requires energy to be continually supplied to it (e.g., the decomposition of water).

Entropy (`S`) is a measure of randomness or disorder. One analogy for this is a deck of cards: when they are stacked in a pile, there is only a certain number of ways they can be arranged. When the cards are thrown into the air, entropy increases greatly because there is nearly infinitely more ways that they can be arranged when they land. You can see this visually because they appear much more chaotic and disorganized when they land compared to when they were stacked neatly.

We can determine the entropy change associated with a reaction using standard entropy, which is defined as the entropy of one mole of a substance at SATP, measured in joules per mole kelvin (J/mol·K). The equation is very similar to Hess’s law type 2:

`Delta S = sum n S_"products" - sum n S_"reactants"`.

Unlike standard enthalpies of formation, the standard entropy for elements already in their standard states is not zero.

These physical changes increase entropy:

These chemical changes also increase entropy:

Enthalpy change and entropy change together determine whether a reaction is spontaneous: