Kinetic Rate Constants

Reactions can occur in a number of ways but we measure the processes under way in a common manner, by observing the change in the amount of reactants and/or products over time. This can be done by employing one or more of a wide variety of physical methods; the key requirement is that the technique selected involves a measurable change, and that the scale of the change with respect to the amounts of compounds is either essentially linear or of known variability in the range of interest. There are different types of behaviour for reactions; for example, in zeroth-order reactions the change in the amount of a reactant or product is essentially linear over time; in first-order reactions, it is exponential. The first-order reaction process is extremely common in coordination chemistry and this is the type that we shall restrict ourselves to here. In its simplest form, the concentration of a reacting species varies over time as (5.33):

where [S]t is the concentration at a particular time t during the reaction [S]o is the original concentration at time zero and k is the rate constant, which is the characteristic measure of the rate of the reaction. This equation may also be expressed as (5.34):

so that a plot of ln([S]t) versus time (t) will be linear with a slope equal to the negative rate constant (k). This rate constant, usually expressed in units of s-1 is a constant only under the experimental conditions operating (as it varies with temperature solvent and added electrolyte). Typically rate constants double for about every 5 ◦C rise in temperature.

Measurement of variation in rate constants with temperature allow determination of the activation parameters (activation enthalpy ΔH≠ and activation entropy Δ S≠) applying in the reaction which assist in elucidating the mechanism. In some reactions, the rate constant is seen to vary with concentration of one of the reactants (say molecule X). If a plot of the measured rate constant (kobs) determined under particular defined conditions versus [X] present is made, where reaction with various concentrations of X has been examined, and is found to increase linearly with increasing [X] then the behaviour can be represented as (5.35)

kobs = kX · [X]                                                      (5.35)

and kX is then called a second-order rate constant and will have units of M⁻¹s⁻¹ the experimentally observed rate constant (kobs. s⁻¹) being dependent on the molar concentration of X. This type of behaviour means that X is involved in the reaction in some way; it may appear as part of the product, although it may alternatively participate only in the transition state and not appear in the product. These are brief aspects of reactions that form a background to our discussion of the mechanisms of reactions below.

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مواضيع ذات صلة

Lability and Inertness in Octahedral Complexes

Thermodynamic and Kinetic Stability

Undergoing Change– Kinetic Stability

Overall Stability Constants

Factors Influencing Stability of Metal Complexes

Bedded Down– The rmodynamic Stability

Bedded Down– The rmodynamic Stability

Host–Guest Molecular Assemblies

Encapsulation Compounds

Compounds of Polydentate Ligands

Sophisticated Shapes

Isomer Preferences