Cliffs AP Chemistry, 3rd Edition

Experiment 12: Determination of the Rate

of a Reaction and Its Order and an Actual

Student Lab Write-Up*

Background: Reaction rates depend on several criteria: the concentration of the reactants, the
nature of the reaction, temperature, and presence of catalysts. The rate of most reactions in-
creases when the concentration of any reactant increases. For the reaction

aA bB+ "cC

the rate can be expressed by the rate equation, rate = k(A)x(B)y. The values of x and y are usu-
ally whole number integers ranging from 0 to 3 and can only be determined by examining lab
data. These numbers represent the order of the reactant. If the order of the reactant is 0, then in-
creasing the concentration of that reactant has no effect on the rate. If the reactant order is 1,
then doubling the concentration of that reactant will double the rate of the reaction. If the reac-
tant order is 2, then doubling the concentration of that reactant will increase the rate 4 times.
And if the reactant order is 3, then doubling the concentration of that reactant will increase the
reaction rate eight times. It is possible to have reactant orders that are fractions or that are nega-
tive. To obtain the overall order of the reaction, simply add the reactant orders.

k is known as the rate-specific constant. It is a value, unique for each reaction, that allows the
equality to exist. It is constant for the reaction as long as the temperature does not change. It
depends principally upon the nature of the reactants and the temperature at which the reaction
occurs. For reactions between ions in aqueous solution, it is affected by the total concentration
of ions in the solution.

Svante Arrhenius proposed that a minimum amount of energy was necessary for a reaction to
proceed. This minimum amount of energy is called the activation energy, Ea. The Arrhenius
equation

k = Ae–Ea/RT

relates the temperature to the rate specific constant, k. A more useful derivation of this
equation is

lnk=+-RTEa lnA

where A is called the collision frequency factor constant and considers the collision frequency
and the geometry of colliding species, which in this experiment will be ignored. R is the uni-
versal gas constant. This equation follows the straight line relationship: y = mx + b. A plot of
the natural logarithm of k versus 1/T gives a straight line. The slope of the graph will be used to
determine the activation energy.

This experiment studies the kinetics or reaction mechanisms, and their rates, when iodine is
added to acetone:

CH C CH +++I "CH C CH I H I

OO

33232 _____aqiiiiiaq aq +-aq aq

Part III: AP Chemistry Laboratory Experiments