Chemical Kinetics

Chemical Kinetics studies the speed of chemical processes and determines the factors which influence on it. It aims is to find mathematical expression that relates the speed of a chemical reaction with the various factors on which it depends.

Reaction speed: The derivative with respect to time, of the concentration of any reactant or product divided by their respective stoichiometric ratio and converted into positive number. Its units are: mol / L·s

Speed Equation or differential speed law.

It is called kinetic equation to the mathematical expression that relates the reaction speed with the molar concentrations of all substances involved in the process.

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K = rate coefficient, rate constant or rate factor.

Reaction orders.

Reaction partial orders indicate the dependence of the reaction rate against the concentration of each of the reagents. They don’t have to coincide with the stoichiometric coefficients of the reaction.

α, β, φ, … = reaction order with respect to the reagent A, B, C, …

α + β + φ + … = total order reaction.

Change in concentration over the time.

If we want to know how the concentration of a reactant or product varies with time, we need to perform a mathematical operation on the kinetic equation called integration. Thus we obtain the concentration depending on the reaction order:

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Mechanism chemical reactions.

A balanced chemical equation is the summary of multiple chemical processes, the mechanism of the reaction is the sequence of simple intermediate steps, corresponding to the advance of the chemical reaction on a molecular scale. Each intermediate step of the reaction mechanism is called elementary stage and the reaction orders of the elementary stages if are the same as the stoichiometric coefficients of the species involved on it unlike what happens with the global kinetic equation.

In the elementary stages appear new chemical species that are neither reactants and products, and therefore are not in the chemical equation. These species are called reaction intermediates and their life is very short, as they are highly reactive species that are consumed in the elementary stages following their training. They may be atoms, molecules, ions or free radicals. The total number of species involved as reagents in the elementary stage will be called molecular.

Factors affecting on the speed reaction.

-Nature chemistry of a process.

It is the factor that most affects to the rate constant. Typically, reactions that don’t involve links readjusting are usually rapid.

-Concentration of reagents.

His influence is so important that it is explicitly shown in the kinetic equation.

-Physical state of reagents.

Homogeneous reactions: All reagents are in the same phase. These are the fastest reactions.

Heterogeneous reactions: The reagents are in different phase. They are slower than homogeneous.

-Presence of catalysts and inhibitors.

A catalyst is a substance that is characterized by increase greatly the rate of reaction which it is catalyzing and recover the end of the process not appearing in the overall chemical equation.
Inhibitors, however, decrease the reaction rate. Usually they act on the first elementary stages of the reaction mechanisms, blocking the reaction intermediates. They may also disable the action of a catalyst.

-Temperature effect: Arrhenius’ equation

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Where A=frecuency factor; Ea= Activation energy; e^-Ea/RT=Boltzman factor or power factor.

If we know k at two temperatures we get:

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Chemical reactions theory.

-Collisions theory.

The collision theory relates the rate constant with the number of effective collisions between molecules.

To calculate the effective collisions it is performed in the following steps:

1. The total number of collisions between molecules of reagents is calculated.
2. The fraction of collisions that occur as proper orientation is estimated.
3. Colliding molecules must have sufficient energy to break bonds. The minimum energy that must be overcome is the activation energy.

-Transition state theory.

In the transition state theory a dynamic study of motion and collision of molecules is not done, but the energy profile of the reaction is studied. The existence of a species called activated complex is postulated. The activated complex, which is in equilibrium with the reactants, is very unstable due to their high energy and decomposes almost instantaneously causing the reaction products.

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Catalysis

Catalysts are outside substaces to a reaction whose presence changes the speed of it without they suffer a permanent alteration.

The basic characteristics of a catalyst are:

• It is neither reactive nor product. Therefore they do not appear in the global chemical equation. They are effective even in a very small amount.
• It recover at the end of the process in an state which is equal to the initial state, it is said, it can be re-used in a new catalysis cycle.
• It does not alter the process thermodynamic variables, because the catalyst don’t provide or consume energy system.
• It accelerates in the equally way forward and reverse reactions.
• Although there are valid catalysts for many processes, the catalysts are generally specific, it is to say, they activate a particular reaction and there are no other possible reactions.

The types of catalysts are:

• Carriers (homogeneous catalysis).
• Contact information (heterogeneous catalysis).
• Enzymes or biocatalysts.

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