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