Iodination of Acetone Experiment Four
Chemistry M01B Laboratory Manual pp. 12
Chemical Kinetics: The Iodination of Acetone
The rate at which a chemical reaction occurs depends on several factors: the nature of the
reaction, the concentrations of the reactants, the temperature, and the presence of possible
All of these factors can markedly influence the observed rate of reaction.
In this experiment, we will study a reaction which, in the vicinity of room temperature,
proceeds at a relatively easily measured rate.
For a given reaction, the rate typically increases with an increase in the concentration of
For the reaction aA + bB
cC, the rate can be expressed by the following
equation, which is called the rate law,
rate = k[A]
where m and n are generally, but not always, integers, 0, 1, 2 or possibly 3; [A] and [B]
are the initial concentrations of A and B (ordinarily in moles per liter); and k is the
specific rate constant for the reaction.
The numbers m and n are called the orders of the
reaction with respect to A and B.
If m is 1 the reaction is said to be first order with
respect to the reactant A.
If n is 2 the reaction is second order with respect to reactant B.
The overall order is the sum of m and n.
In this example, the reaction would be third
The rate of reaction is also significantly dependent on the temperature at which the
An increase in temperature increases the rate, an often cited rule being
that a 10
C rise in temperature will double the rate.
This rule is only approximately
correct; nevertheless, it is clear that a rise of temperature to say 100
C could change the
rate of a reaction appreciably.
As with the concentration, there is a quantitative relationship between reaction rate and
This relation is based on the idea that in order to react, the reactant species
must have a certain minimum amount of energy present at the time the reactants collide
in the reaction step.
This amount of energy, which is typically furnished by the kinetic
energy of the species present, is called the activation energy of the reaction.
The equation relating the rate constant, k, to the absolute temperature, T, and the
activation energy, E
where R is the gas constant (8.314 Joules/mole K for E
in Joules per mole).
measuring k at different temperatures we can determine graphically the activation energy
for a reaction.