Reaction Mechanisms in Inorganic Chemistry

Elementary Reaction Kinetics: A Review of the Fundamentals

The Arrhenius equation A + B  P k2k2 Experimental rate law Describes temperature dependence of the reaction rate constant. k 2 = A.exp[-E a /RT] ln k 2 = -(E a /RT) + constant

Activated Complex Theory (ACT) Transition state or activated complex K ‡ = k 1 /k -1

Principal aim of kinetics: Relating experimental (“macroscopic”) rate and equilibrium constants to theoretical (“microscopic”) rate and equilibrium constants

What is the relationship between k 2, k ‡ and K ‡ ?

and it can be shown (see notes) that Gibbs energy of activation Entropy of activation Enthalpy of activation The Eyring Equation

So: If  S ‡ increases, ln k 2 increases and reaction is entropy driven If  H ‡ decreases, ln k 2 increases and reaction is enthalpy driven

If  S ‡ increases, ln k 2 increases and reaction is entropy driven more disordered transition state

If  H ‡ decreases, ln k 2 increases and reaction is enthalpy driven less energy in total needed to break chemical bonds

Transition state theory Arrhenius equation In solution

Example (p. 1.5)

Excel Example (p. 1.5)

Excel Example (p. 1.5)

Excel Example (p. 1.5)

Excel Example (p. 1.5)

Excel Example (p. 1.5)

So report results as:  H ‡ = 84  2 kJ mol -1  S ‡ = 20  7 J K -1 mol -1 Example (p. 1.5)

The kinetic salt effect Define But So

If  i = 1 ∀ i, then K  = 1 and k 2 o = k ‡ K From Debye-Hückel theory: constant charge I = ½ m j z j 2 is the ionic strength

A = M -½ in aqueous solution at 25 o C

o + + z A = 0z B = +1

+ + + z A = 1z B = +1

The pressure dependence of rate constants

‡  V ‡ > 0

‡  V ‡ < 0