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  Mechanism of Simple Reactions, Chemistry tutorial

Introduction:

Chemical kinetics is basically the study of rates and mechanisms of chemical reactions. The rate of a reaction based on numerous factors like the concentration of the reactants, temperature, catalysts and so on. 

Some Reaction Mechanisms:

Most of the chemical reactions occur via a sequence of steps. Each step is termed as an elementary reaction. A reaction method is a sequence of elementary reactions proposed for describing the rate law for the overall reaction. The elementary reactions are written as chemical equations. These chemical equations give a possible description for the reaction path.

For an elementary reaction, the molecularity is similar as the order of reaction.

The rate law for each and every elementary reaction can be written by using molecularity. The molecularity is the number of reactant molecules or atoms in an elementary reaction. If there is merely one reactant molecule (or atom) in an elementary reaction, the reaction is stated to be Unimolecular. The elementary reaction in which the two molecules (or atoms) react altogether is bimolecular reaction. Most of the reaction methods consider mostly Unimolecular and bimolecular reactions. The chance of termolecular reactions (that is, where three species are to combine) taking place is much less, as the probability of three species colliding concurrently is quite low. An illustration each for Unimolecular and bimolecular reactions is described below.

Unimolecular Reaction:

O3 (g) → O2 (g) + O (g)

A Unimolecular reaction consists of a first order rate law; therefore the rate of decomposition of O3 could be symbolized as follows:

Rate = k [O3]

Bimolecular Reaction:

2O3 (g) → O2 (g) + 2O2 (g)

A bimolecular reaction consists of an overall second order rate law, being first order in each reactant. Therefore, for the elementary reaction, rate can be deduced as follows:

Rate = k [O] [O3]

A few of the guidelines followed in recommending reaction mechanisms are described below:

1) The elementary reactions whenever added should be equivalent to the coverall balanced chemical equation for the reaction.

For illustration, the overall reaction in the decomposition of O3 in the upper atmosphere is,

2O3 (g) → 3O2 (g)

This reaction could be considered as the result of the given two elementary reactions:

Step (i): O3 (g) → O2 (g) + O (g)

Step (ii): O3 (g) + O (g) → 2O2 (g)

Overall reaction: 2O3 (g) → 3O2 (g)

2) While writing such a procedure, one possible support is proving the presence of intermediates. For illustration, in the procedure recommended above, atomic oxygen is the intermediate. These intermediates can be detected via chemical or physical methods. They are usually reactive species. Moreover an intermediate is produced and finally used up.

3) The procedure should agree by the overall rate law determined experimentally. In another words, the rate laws for the elementary reactions should be combined in such a manner that the overall rate law is illustrated. In order to complete this, we should be capable to decide the rate determining step. Out of the elementary reactions recommended, the slowest one is known as the rate determining step. The overall reaction rate can't be faster than the slowest step in a mechanism. The rate finding out step decides the rate of the overall reaction.

For illustration, in the mechanism recommended for the decomposition of ozone, Step (ii) (that is, equation O3 (g) + O (g) → 2O2 (g)) is probably the rate determining step.

4) The possibilities of both forward and reverse reactions taking place fast should as well be considered! That is, the possibility of a dynamic equilibrium should as well be examined. This is one of the manners to:

  • determine an appropriate relationship for deducing the concentration of an intermediate and
  • removing the term representing the concentration of the intermediate from the rate expression for the overall reaction.

5) Kinetic information can merely support a proposed mechanism; it must not be taken as a proof since a mechanism can't be proved completely.

Just a few guidelines are provided here for proposing a reaction mechanism. Though, these are adequate for studying the reaction mechanisms of the simple reactions.

The studies on organic and inorganic reaction mechanisms have led to the growth of separate streams of chemistry. Now, we shall talk about the reaction mechanism comprising: 

  • A fast equilibrium followed via a slow step
  • A slow step followed via a fast step
  • A chain reaction.

We shall as well define the following kinds by an illustration in each case without talking about the reaction mechanisms.

  • Consecutive reasons
  • Opposing reactions
  • Parallel reactions

Free-Radical Reactions:

In H2-Br2 reaction, H and Br atoms have unpaired elections and these are free-radicals. In the year 1929, Paneth and Hofeditz reported the formation of polyatomic free radicals (that is, CH3 radicals) by the thermal decomposition of lead tetramethyl. It was found that lead was deposited as a mirror, in the hot part of a tube via which hydrogen gas carrying lead tetramethyl vapor was passed.

C2H6 → (k1) → 2CH3

CH3 + C2H6 → (k2) → CH4 + C2H5

C2H5 → (k3) → C2H4 + H

H + C2H6 → (k4) → H2 + C2H5

H + C2H5 → (k5) → C2H6

The free-radicals like CH3 and C2H5 are detected via direct experimental methods or from the products they give. Experimentally acquired rate law is represented by the equation.

Rate = k [C2H6]

Here, 'k' is the overall rate constant: k is the complex combination of the rate constants of the individual elementary reactions.

Consecutive Reactions:

We have hypothesized the existence of intermediates. In most of the cases, the intermediate in one step is the reactant in the next. These reactions are known as consecutive reactions. The rates of consecutive reactions could be stated in terms of the concentrations of the reactants taken initially and the products formed in each phase. Illustration is the acid hydrolysis of diethyl adipate. 

The radical intermediates can be eliminated by using substances such as NO. As NO molecule has an unpaired electron, it joins by a radical intermediate which as well consists of an unpaired electron. This could yield in chain termination. Here, NO molecule is termed as the radical scavenger and it is stated to quench the chain reaction. To verify the chain mechanism, these radical scavengers are utilized.  

Just as we can terminate a chain reaction by employing radical scavengers, we can begin a chain reaction by using free radical sensitisers like Pb(CH3) or Hg(CH3)2. To raise the decomposition rate of an organic compound, Pb(CH3) or Hg(CH3)2 is added. Such substances decompose and introduce CH3, radicals to the system. This begins the decomposition of the organic compounds via a chain reaction. Pb(CH3)4 and Hg(CH3)2 are stated to sensitise the decomposition of organic compounds.

Opposing Reactions:

In the opposing reactions, rates of forward and reverse reactions are both appreciable. While in proposing a mechanism, both the reaction rates should be considered.

Illustration:

The formation of butyrolactone from γ-hydroxybutyric acid goes via the below procedure:

1594_Formation of butyrolactone from γ-hydroxybutyric acid.jpg

Fig: Formation of butyrolactone from γ-hydroxybutyric acid

Parallel Reactions:

Whenever a reactant can undergo more than one reaction, the resultant reactions are known as parallel reactions. The rates of a set of parallel reactions can be evaluated as the concentrations of the products formed in each and every case.

Illustration:

The nitration of phenol resulting o-nitrophenol and p-nitrophenol,

836_Nitration of phenol.jpg

Fig: Nitration of phenol

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