called an activated complex or transition state. Reactants must form an
activated complex before products can be made.
Finally: H–H and I–I bonds are completely broken, and H–I bonds are formed.
The chemical reaction has produced hydrogen iodide.
H–IH–IAlthough this was a somewhat simplified account of what happens during a
chemical reaction, the actual process does involve molecular collisions, bond
breaking and making, and the formation of an activated complex.
Factors That Affect Reaction Rate
The test writers will expect you to be familiar with several key factors that
influence the rate of a reaction. All of these factors impact the reaction rate by
affecting the rate of molecular collisions, the energy of the collisions, or both.
Concentration of Reactants Reactant molecules must collide in order to form
products. If the rate at which reactant molecules collide is increased, then the
reaction rate will also increase. One way to increase the rate of reactant
collisions is to increase the amount of reactant present, or in other words, to
increase the concentration of reactants. For example, wood burns much faster in
a pure (100 percent) oxygen environment than in air (which is only about 20
percent oxygen by mass). An increase in the concentration of the reactant
oxygen causes an increase in the rate of combustion. However, this is true only
of reactants that are gaseous or in solution, whose concentrations can be
changed. For instance, a gas can be compressed into a smaller volume
(increasing the concentration of gas molecules per volume). Since the molecules
in a pure solid or liquid are relatively close together, they cannot be significantly
compressed, so their concentration is essentially constant. To sum up: If one or
more reactants are gaseous or in solution, the reaction rate can be increased by
increasing the concentration of those reactants. Obviously, a decrease in reactant
concentration will produce a decrease in reaction rate. If the reactants are in
gaseous phase, then increasing their pressure will also increase their
concentration, thereby accelerating the reaction rate.