Takes place very slowly, if at all, unless the reaction is started by a spark.
The reason for this is that although a great deal of energy is released, there
is no low energy pathway for the reaction to follow. In order for water to from,
molecules of H2 and O2 must react, and their bond energies are about 435 and 490
kI/ mol, respectively.
Thermodynamics is concerned with the overall energy change for a process. If
necessary , this change can result after an infinite time. Accordingly,
thermodynamic does not deal with the subject of rates of reactions, at least not
directly. The example above shows that the thermodynamics of the reaction favors
the production of water; however, kinetically the process in unfavorable. We see
here the first of several important principles of chemical kinetics. There is no
necessary correlation between thermodynamics and kinetics. Some reactions that
are energetically favorable take place very slowly because there is no low
energy pathway for the reaction.
One of the features of the study of reaction rates is that a rate can not be
calculated. Theory is not developed to the point where it is possible to
calculate how fast most reactions will take place. For some very simple gas
phase reaction, it is possible to calculate approximately how fast the reaction
should take place, but details of the process must still be determined
The rate of a chemical reaction is expressed as a change in concentration of
some species with time. Therefore, the units of the rate must be those of
concentration/time . A reaction such as has a rate which can be expressed either
in terms of the disappearance of a or the appearance of B. Because the
concentration of B is increasing with time, the rate is expressed as +(change in
[B]/change in time) or +d [B]/dt.