If all combustions with oxygen
liberate free energy, and the atmosphere is full of oxygen, then
why doesn't everything that is potentially flammable burn at once,
including ourselves? The answer is that these decompositions and
combustions, although thermodynamically spontaneous, occur at miniscule
rates at room temperature. The rates of chemical
reactions and the factors that affect them are the subjects
of this chapter.
The central theme to be developed in this chapter is that the rate
of a chemical reaction depends on its reaction
mechanism . Two molecules coming together must collide and rearrange
their atoms to make product molecules. The intermediate arrangements
of atoms may have a high energy, and if so, the reaction will be
slow because not all colliding molecules will have enough energy
to rearrange properly.
The concept of an "activation-energy
" barrier to reaction is illustrated with the mountain analogy
on the previous page. The boulder cannot roll off the edge of the
mountain without first surmounting the activation barrier crowned
by a double dagger symbol (the conventional indication of an activated
makes a chemical reaction go faster by providing an alternate
path with a lower activation-energy barrier. This is symbolized
by the winding path down the side of the mountain.