Chemical Kinetics�Lecture 4
CML101
August 02, 2024
Reversible Reactions
k1= 2.25 x 10-1 s-1
k-1= 1.50 x 10-1 s-1
The value of [A] decreases from
to
Relaxation methods are fundamentally different from the experimental approaches that we have discussed so far. The general idea is to start with a chemical system initially in equilibrium at some specified temperature and pressure. The conditions are then suddenly changed so that the system is no longer at equilibrium. There are many different ways to shift the equilibrium. Temperature, pressure, pH, and pOH jump methods have been developed and used effectively to study kinetic processes.
Here, we examine the most common relaxation method used to study reaction kinetics in solution, the temperature-jump relaxation technique. In a temperature-jump experiment, the temperature of the equilibrium reaction mixture is suddenly changed at constant pressure. Following the sudden change in temperature, the chemical system responds by relaxing to a new equilibrium state that corresponds to the new temperature.
We will see that the rate constants for the forward and reverse reactions are related to the time required for the system to relax to its new equilibrium state.
Relaxation Method – Temperature Jump Technique
Relaxation Method – Temperature Jump
The time-dependent change in [B] following a temperature-jump experiment for the chemical system given by the equation where the rate laws for the forward and reverse reaction
are first order in the reactants. The plot assumes that ~ ΔHo < 0, whereby [B]2,eq < [B]1.eq . Following the temperature jump, the value of [B] decays exponentially from [B]1.eq to [B]2,eq
The time constant for this exponential decay is given by 1/(k1 + k-1)
Trigger of
temperature rise through laser heating or capacitor discharge
Fast temperature increase by ~9.6 °C
Now temperature is T2
Start responding – relaxation starts
Starting line
System at equilibrium at T1
Reached Finish Line
Full relaxation at T2