13. How To Measure Disorder   Previous PageNext Page

Spontaneous reactions are those that will take place by themselves, given enough time. They do not have to be rapid; speed is not a factor in the definition of spontaneity. Explosions and many other spontaneous reactions are rapid, but other spontaneous processes, such as the precipitation of calcium carbonate in the stalactites of Mammoth Cave, require thousands of years.

We recognize the irrelevance of time to the idea of spontaneity when we use the term "spontaneous combustion" for the slow smouldering of paint-soaked rags. The oxidation of newsprint is spontaneous, although we do not worry about our morning paper bursting into flames as we read it. At 25° C, the reaction of newsprint with oxygen is exceedingly slow, but the gradual browning of old newspapers in library files shows us that the process is spontaneous nevertheless.

In contrast, the same reaction at the temperature of a lighted match is both spontaneous and rapid. By raising the temperature we have hastened the achievement of a chemical reaction, but the tendency for the reaction to take place was already there, even at room temperature. It is this tendency to react that we mean when we talk about spontaneity, and it is this tendency toward reaction that we would like to be able to predict.

One good reason for wanting to predict spontaneity is that, if a reaction is genuinely spontaneous but slow, we may be able to speed it up by changing the experimental conditions.

Changing the temperature is one way that is particularly effective for oxidations. Finding a suitable catalyst is another.

If a reaction is spontaneous, a catalyst will accelerate it.

If the reaction is not spontaneous to begin with, then looking for a catalyst is a waste of time.

This chapter is focused on one fundamental question: How can we tell in advance whether a reaction that has not been tried will be spontaneous?

  Page 1 of 45 HomeGlossary