S products - S reactants As you can see, if the S products is larger than S reactants then the value of S is positive. A positive value of S means an increase in the randomness of the system during the change, and we have seen that this kind of change tends to be spontaneous. Any event that is accompanied by an increase in the entropy of the system tends to occur spontaneously. It is often a relatively simple matter to predict whether a particular change will become more or less random. One of the things to look for is the state of the reactants and products.
Gases have more entropy than liquids which have more than solids.
During chemical reactions, the freedom of movement of the atoms often changes because of changes in the complexity of the molecules. Among the reactants are six atoms combined into two molecules of NO 2 . Among the products, these same six atoms are confined to one molecule. In the reaction vessel, dividing the six atoms between two molecules allows them to spread out more ad gives greater freedom of movement than when the atoms are combined into just one molecule. Therefore, we can conclude that as this reaction occurs, there is an entropy decrease.