Chemistry Reference
In-Depth Information
a
ctivated state
activation
energy
reactants
reaction
energy
products
Reaction coordinate
Figure 3.28
A reaction coordinate, defining the barrier to reaction (activation energy) as well as the energy
difference between reactants (initial state) and products (final state) or reaction energy.
state (represented in Figure 3.28). Reactants convert to products through this transition or
activated state, which is a short-lived 'half-way house' between the two forms.
A simple macroscopic example may illustrate this. Consider a thin flat piece of board
painted white on one side and red on the other. Now arrange to turn this over on a tabletop
with the board touching the table throughout. You can do this, of course, by turning the
board onto its edge, then continuing the motion until it lies flat on its other side. In doing this
transformation, the position where it is precariously standing on its edge can be considered
a transition (or activated) state, and if you let go of it in that position it may fall back so the
white side is up (equivalent to no reaction) or fall forward so the red side is up (equivalent
to a completed reaction). To get it from lying flat to on its side costs you effort, or energy -
what we would call the activation energy at the molecular level. Molecules acquire this
activation energy mainly through collision as a result of their motion; if the collision energy
suffices to take the reactants to an arrangement where they can proceed to products without
further energy, they have achieved the status of an activated or transition state species. In
our macroscopic example, your physical effort takes the board to the upright activated state.
For a very heavy piece of board, the amount of effort is significant and you may not get
it into an upright position every time you try. This amounts to a high activation barrier or
large activation energy, which equates with a slower rate of reaction. If the board is small
and light, the effort required is small and the task easy and rapidly completed; this amounts
to a low activation barrier or small activation energy, consistent with a fast rate of reaction.
If you next consider your board starting on a different level to where it finishes (table
to floor, for example), it is obvious that one is the lower level; the board could fall from
table to floor, but not easily the reverse way. On the molecular level, being on the 'floor'
would be a thermodynamically more stable arrangement for the molecule than being on
the 'table'. The difference in energy between the reactants and products is the reaction
energy, and the stability of a complex depends on the size of this energy difference. This
additional consideration (thermodynamics) doesn't change the process by which you turn
the board over (kinetics), although there is a relationship between them at the molecular
level, which we won't explore here. At the molecular level, however, raising temperature
