Factors Affecting the Rate of a Reaction

Three conditions, above all others, determine how fast a given reaction proceeds: the

concentration of the reactants, the temperature at which the reaction is carried out, and whether a

catalyst is present.

Concentration. Raising the concentration of a reactant packs more of its molecules into the same

volume, which increases how often reactant molecules encounter one another (or encounter the surface

of a solid reactant/catalyst). Since a reaction can only occur when reacting particles actually meet, a

higher collision frequency generally means a faster observed rate -- this is the qualitative basis for

every rate law discussed in this chapter, and it is developed rigorously in the next section.

Temperature. Raising the temperature increases the rate of almost every reaction, often quite

dramatically for only a modest rise in $T$ -- as a rough rule of thumb, many reactions roughly double

or triple in rate for every $10\,^\circ\text{C}$ rise near room temperature. This effect is

surprisingly large given that the average kinetic energy of the molecules (which is proportional to

$T$ in absolute terms) increases only slightly over such a small temperature range. The real

explanation, developed fully in the sections on collision theory and the Arrhenius equation later in

this chapter, is that only molecules whose energy exceeds a certain threshold (the activation energy)

can react at all, and the fraction of molecules that clear this threshold rises exponentially, not

linearly, with temperature -- so even a small rise in $T$ can sharply increase the population of

sufficiently energetic, reaction-capable molecules.

Catalyst. A catalyst is a substance that increases the rate of a reaction without being permanently

consumed by it, typically by providing an alternative reaction pathway with a lower activation energy

than the uncatalyzed route -- a much larger fraction of molecules can then clear this lower barrier at

the same temperature. Catalysis is developed in full, with its two main types and the special case of

enzyme catalysis, later in this chapter.

Beyond these three, a reaction's rate can also be influenced by the physical state and surface area of …