Concepts of 1st teaching period, Chemical Kinetics, 2016/17

Our course deals with the rate of chemical reactions.

Upon experimentation, one realizes that reaction rates may be stated as a function of the concentrations of the species appearing (reactants, products, intermediates...)

The rate of a chemical reaction is unique. However, depending on stoichiometry the rate at which reactants are consumed and products are generated may differ.

Sometimes the differential rate law (i.e., the rate of change of concentration) may be stated simply as a constant times a power of concentrations. Then one may think of reaction order

The rate constant does not change along the reaction for a given temperature. Its dependen on temperature follows, as a first approximation, Arrehnius law.

The simplest reaction, A->B, may also have the simplest expression (first-order) for the differential rate law: d[A]/dt = k [A]. It may be solved analytically by straight integration.

Reactions may be second-order, zeroth-order, fractionary order... And for reactions like A + B -> P expressions for the differential rate law may be complicated and even analytically unsolvable.

When a differential rate law can be solved analytically, then an explicit expression for [A] as a function of time is obtained, thus an integrated rate law. We have defined halflife for [A] and other concepts similar concepts.

Sometimes the method of initial rates may be useful when starting from experimental experiments.

We have considered thus first-order and higher-order reactions, with an only reactant, and more than one reactant.

Then, for more complex reactions, we have tackled:
- Reactions close to equilibrium, and its relationship to the concept of equilibrium constant.
- Reactions involving intermediates:
  • Consecutive reactions
  • Reactions involving preequilibrium

For more complex reactions, one must write down a differential equation for

To approximate the solution of those more complex processes, we have explained the so-called Steady State Approximation.

Instead of an analytical solution to differential rate laws (i.e., finding the integrated rate law), one may consider numerical solutions. For instance, a numerical solving of the differential equations may be found through the Runge-Kutta procedure (the simplest yet bad method, Euler procedure, has been explained).

Analytical solutions may be found in a different way, like Laplace Transforms.