Original post: Friday, May 18th, 2007
The study of kinetics is to find out the things that affect how fast reactions occur and then ultimately to explain the actual mechanics of the process (what actually happens and in which order)
On the basic level, simply observation shows us that changing the concentration of reactants affects the rate of formation of products (or disappearance of reactants) – the question is, “what exactly is the relationship between reactant concentration and rate?”
Mathematically speaking there are only three logical possibilities:
1. There is no relationship
2. There is a linear relationship
3. There is a non-linear relationship
All three possibilities can be covered by one generic equation, called the rate equation:
For a one component (A) system…
Rate = k [A]^x
for a two component (A + C) system
Rate = k[A]^x[C]^y
At the very beginning of a reaction the initial rate can be found (or a close approximation) by experiment. We also know the concentration of the reactants at this time (we choose them), so if two experiments are carried out on a one component system we have two unknowns (k and x) and two equations – simultaneous equations that can then be solved for k and x
For two component systems (A+C) things get a little more complicated – but only a little. Two experiments must now be carried out, keeping the concentration of one of the reactants constant (say, C). The second equation can then be written as:
Rate = constant [A]^x
where the constant is (k[C]^y)
and once again you have two simultaneous equations with two unknowns. The constant doesn’t concern us at this time so you just solve for x.
Now you carry out two reactions keeping [A] constant when the rate equation can be simplified to:
Rate = constant[C]^y
now solve for y
Now you can choose any of the experimental results to substitute for x and y to find k (the rate constant)
Why do we go through all of this?
The answer lies in the fact that the orders of reaction x and y give us hints about likely mechanisms as they show the molecularity (number of particles ) involved in the rate determining step (slowest step) of the mechanism (and possibly any prior equilibria)
For example, if the rate equation comes out to be
Rate = k[A]^2[C]^0
This tells us that the rate determining step (slowest) does not involve C in any way and probably involves a step with two particles of A..
2A –> A2 (slow step)
Kinetics is a wholly experimental branch of chemistry that seeks answers to what actually is happening during the course of a reaction. It does so by ’solving’ the rate equation – a general equation that can be adapted for 1, 2 or 3 component systems.
The orders (x and y) give clues as to the mechanism of the reaction.
Once the orders of reaction with respect to the individual components of the reaction are found, further investigations at different temperatures can be carried out to find the activation energy of the reaction using the Arrhenius equation.
Advantages? All the calculations in exam questions involve VERY simple maths and once the concepts are grasped it’s easy marks.