Tuning Reactivity of Platinum(II) Complexes

= 23.76 +
R
Hence, a plot of ln
⎛ 2 ⎞
⎜ ⎟
⎝ T ⎠
k
S #
-
R
H #
14
. 1/T (2.27)
against 1 is linear from which the slope gives
T
the enthalpy of activation, ΔH # and the y-intercept provides the value of the activation
entropy, ΔS # . This plot is usually referred to as the Eyring plot. 6,16,24
The important featuring arising out of the thermodynamic activation parameters are:
• The activation parameters are thermodynamic state functions.
• A fast reaction can have a higher enthalpy of activation (ΔH #), if ΔS # is more
positive to compensate.
• The activation entropy (ΔS #) associated with k2 for a second-order reaction is
particularly large and negative, reflecting loss of entropy from the union of two
reaction particles into a single transition-state in an associative reaction
mechanism dominated by bond formation process; the ΔS # is usually large and
positive for dissociative activated reactions.
• The values of ΔH # determined from the slope is more reliable than that of ΔS #
that are subjected to large relative errors since the data is calculated from values
extracted to infinite temperatures on the y-intercept.
• The uncertainty in ΔS # is approximately
amounts to a factor of 3. (
kilojoules for ΔH #).
2.3.5 Effect of Pressure on Rate Constant
T
1000 times that of ΔH
T
# , which
1000 , involves conversion from joule for ΔS # to
A more reliable parameter in diagnosing reaction mechanisms is the activation volume,
ΔV # since it is particularly less sensitive to temperature. Therefore comparison or
contrast between values determined at different temperatures for different reactions
can be monitored. The activation volume (ΔV #) can be determined from a series of
experiments in the observed rate constants, , measured as a function of applied