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T-1
EXPERIMENT T:
EFFECT OF TEMPERATURE ON RATE OF REACTION
OBJECTIVES
Experiment S dealt with the effect of changing reactant concentrations on the rate of a reaction at room
temperature. In Experiment T the same reaction is studied; however, this time the initial concentrations
of the reactants will be kept constant for each run: however, the temperature will be changed. You will
interpret your results using the transition state theory model. You will also examine the effect of a
catalyst on the rate of reaction.
THEORETICAL CONSIDERATIONS
Transition State Theory
For a chemical reaction to occur, reactant molecules must collide with enough energy for their electronic
orbitals to overlap enough to form (temporarily) an unstable grouping of molecules known as an
activated complex or transition state. The activated complex is unstable and undergoes further
change, including the rearrangement of the molecular structure, the breaking of existing bonds and the
making of new ones. The high energy requirement is to overcome the repulsion of the negative charges
of the electrons. In most cases when molecules collide, the molecules simply rebound like billiard balls
and remain unchanged. However, if the molecules collide with sufficient energy to push the molecules
closely together, their electronic orbitals will overlap sufficiently to form an activated complex. The
minimum energy required to form an activated complex from reactant molecules is called the activation
energy, E
a
. This minimum energy is supplied by the kinetic energy of motion of the reactant species
which becomes converted to vibrational energy upon collision.
At room temperature, the reactant molecules collide together with a wide range of energies. Very few
of the molecules collide with sufficient energy to form an activated complex. Forming an activated
complex can be compared to an attempt to roll a ball up an incline, over a hump, and down the other
side to a lower more stable level. The activation energy is analogous to the energy needed to drive the
ball to the top of the hump. This hump then forms an energy barrier between reactants and products.
The larger the value of E
a
, the higher the energy barrier, and the fewer the number of molecular collisions
