KINETICS: RATES AND
MECHANISMS OF CHEMICAL REACTIONS
Changes in concentrations of reactants (or products) as functions of time are measured to determine the reaction
Rate is proportional to concentration. An increase in pressure will increase the number of gas molecules per unit
volume. In other words, the gas concentration increases due to increased pressure, so the
reaction rate increases
Increased pressure also causes more collisions between gas molecules.
The addition of more water will dilute the concentrations of all solutes dissolved in the reaction vessel. If any of
these solutes are reactants, the
rate of the reaction will decrease
An increase in solid surface area would allow more gaseous components to react per unit time and thus would
increase the reaction rate
An increase in temperature affects the rate of a reaction by increasing the number of collisions, but more
importantly the energy of collisions increases. As the energy of collisions increase, more collisions result in
reaction (i.e., reactants
products), so the
rate of reaction increases
The second experiment proceeds at the higher rate. I
in the gaseous state would experience more collisions with
The reaction rate is the change in the concentration of reactants or products per unit time. Reaction rates change
with time because reactant concentrations decrease, while product concentrations increase with time.
a) For most reactions, the rate of the reaction changes as a reaction progresses. The instantaneous rate is the rate
at one point, or instant, during the reaction. The average rate is the average of the instantaneous rates over a period
of time. On a graph of reactant concentration versus time of reaction, the instantaneous rate is the slope of the
tangent to the curve at any one point. The average rate is the slope of the line connecting two points on the curve.
The closer together the two points (shorter the time interval), the more closely the average rate agrees with the
b) The initial rate is the instantaneous rate at the point on the graph where time = 0, that is when reactants are
The calculation of the overall rate is the difference between the forward and reverse rates. This complication may
be avoided by measuring the initial rate, where product concentrations are negligible, so the reverse rate is
negligible. Additionally, the calculations are simplified as the reactant concentrations can easily be determined
from the volumes and concentrations of the solutions mixed.
At time t = 0, no product has formed, so the B(
) curve must start at the origin. Reactant concentration (A(
decreases with time; product concentration (B(
)) increases with time. Many correct graphs can be drawn. Two
examples are shown below. The graph on the left shows a reaction that proceeds nearly to completion, i.e.,
[products] >> [reactants] at the end of the reaction. The graph on the right shows a reaction that does not proceed
to completion, i.e., [reactants] > [products] at reaction end.