Molecularity of a Reaction

Contents

1 Molecularity of a Reaction
2 Mechanism and Rate Law

For a chemical reaction to occur, the reacting molecules must collide with each other. The number of reacting species (molecules, atoms or ions) which collide simultaneously to bring about a chemical reaction is called molecularity of a reaction.

If a reaction involves the decomposition of only a single species, the molecularity is one or it is called unimolecular reaction.

For example:

1) The decomposition of hydrogen peroxide involves single species which undergoes the change to form the products. Hence, it is a unimolecular reaction.

H₂O₂ ——–> H₂O + ½ O₂

2) Decomposition of ammonium nitrite

NH₄NO₂ ——-> N₂ + 2H₂O

If the reaction involves the collision of two species, it is bimolecular and if three species take part in a collision leading to the formation of the products it is called trimolecular and so on.

The examples of bimolecular reactions are given below:

1) Dissociation of hydrogen iodide is a bimolecular reaction because two molecules collide to bring about the reaction.

2HI (g) ——-> H₂ (g) + I₂ (g)

2) Combination of NO and O₃ is a bimolecular reaction

NO(g) + O₃ (g) ——-> NO₂ (g) + O₂ (g)

The examples of trimolecular reactions are

The reaction of nitric oxide and oxygen is a trimolecular reaction because it involves collision of three reacting molecules.

2NO(g) + O₂ (g) ——-> 2 NO₂ (g)

Reactions involving three or more molecules are uncommon because such reactions requires the simultaneous collision of three or more than three molecules.

Simultaneous collision of three molecules means that the third molecule must collide the other two molecules at the same time when they are in the process of collision. The chances of the occurrence of such collisions are very small.

Some reactions involving more than three molecules are quite fast.

For example:

a) The reaction of bromide ions with bromate ions in the presence of an acid:

5Br¯ (aq) +BrO₃‾ (aq) +6H⁺ (aq) ——-> 3 Br₂ (aq) + 3H₂O (l)

The experimentally measured rate law for this reaction is given as:

Rate = k[Br¯] [BrO₃¯] [H⁺]²

This rate is first order with respect to Br¯ and BrO₃¯ ion and second order with respect to H⁺ ions and the overall order of the reaction is 1+1+2=4.

b) The reaction of potassium chlorate with ferrous sulphate in the presence of sulphuric acid involves ten species.

KClO₃+ 6FeSO₄ + 3H₂SO₄ ——>KCl + 3Fe₂(SO₄)₃ +3H₂O

The above reaction appears to be of tenth order but actually it is a second order reaction. If this reaction were to take place in a single step, the 10 particles (1 KClO₃ , 6 FeSO₄ , and 3 H₂SO₄) would have to collide simultaneously.

But chances of such events are extremely small, so much so that a reaction which takes place by such collision will not occur at all.

Type of reactions which take place through a sequence of two or more consecutive steps are called complex reactions.

The detailed description of various steps by which reactants change into the products is called mechanism of the reaction.

The steps which contribute to the overall reaction are called elementary processes.

Mechanism and Rate Law

In multi-step reactions, since each elementary step involves quite different type of reactions, so each step will occur at its own distinctive rate. Some of the steps will be very fast while others will be slow. If one step takes place much more slowly then all other steps, it will definitely control the overall reaction rate. This means that all the steps have to wait for the occurrence of this step and the rate of the reaction cannot be less than the rate of this slowest step. But once this slowest step has occurred, the other steps will take place to form the products.

The rate of the reaction is the rate of the reaction which is determined by the slowest step in the sequence. The slowest step is called rate determining step in the proposed mechanism.

2NO₂ (g) + F₂ (g) ——-> 2 NO₂F (g)

The rate of the reaction is proportional to the product of the concentrations of nitrogen peroxide and fluorine. This indicates that the rate determining step in the mechanism of this reaction must be the reaction between NO₂ and F₂ only.

NO₂ + F₂ ——-> NO₂F + F slow step

NO₂ + F ——-> NO₂ F Fast step

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2 NO₂ + F₂ ——> 2 NO₂F
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The rate of the overall reaction is determined by the first step which is the slower of the two steps. Each of the steps is called an elementary process.

Rate = -dx/dt = k [NO₂][F₂]

This is the rate law for the reaction

(i) Reaction between NO₂ and CO to form CO₂ and NO₂

NO₂ (g) + CO (g) ——> NO (g) + CO₂ (g)

The rate of the reaction is proportional to the square of the concentration of nitrogen peroxide. The rate determining step in the mechanism of this reaction must be independent of the concentration of CO. A mechanism of the reaction may be suggested as:

Step 1 NO₂+ NO₂———> NO + NO₃

Step 2 NO₃+ CO ———–> CO₂+ NO₂

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NO₂+ CO ———-> CO₂+ NO
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The above reaction proceeds through two steps but the rate of the overall reaction is determined by the first step which is the slower of the steps.The experimentally observed rate of the reaction is given by the expression:

Rate = k[NO₂]²

This is the rate law for the reaction.

(ii) Thermal decomposition of dinitrogen pentoxide

2 N₂O₅ (g) ——-> 4 NO₂ (g) + O₂ (g)

Rate of reaction =k[N₂O₅]

If the reaction were to take place by the collisions of two N₂O₅ molecules as indicated by the balanced equation, it would be a second order reaction as

Rate = k[N₂O₅]²

But the observed rate law suggests that it is a first order reaction.The reaction is a complex reaction and proceeds by two or more successive steps.

Step 1 N₂O₅ —–> NO₂ + NO₃ slow

Step 2 N₂O₅ + NO₃ —–> 3 NO₂ + O₂ Fast

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2 N₂O₅ ———> 4 NO₂ + O₂_________________________________

Hence, the above reaction is unimolecular reaction or reaction of first order.

(iii) Reaction of NO and H₂ as

2NO (g) + 2 H₂ (g) ———-> N₂ (g) + 2 H₂O (g)

Rate of reaction = k[NO]² [H₂]

It is a complex reaction and is believed to proceed by two step mechanism in which the first step is the rate determining step as:

Step 1 2NO(g) + H₂ (g) ———-> N₂ (g) + H₂O₂ slow

Step 2 H₂O₂(g) + H₂(g) ———> 2 H₂O (g) Fast

Since the first step is slow and rate determining, the rate law is

Rate = k[NO]² [H₂]

(iv) Reaction of decomposition of hypochlorite (ClO‾)

3ClO‾ ——-> ClO‾ + 2 Cl¯

The various steps are :

ClO‾ + ClO‾ ——> ClO₂‾ + Cl¯ slow

ClO₂‾ + ClO‾ ——-> ClO₃‾ + Cl¯ fast

The first step is slow and hence rate determining. Thus, the rate law is

Rate =k [ClO‾]²

Molecularity	Order
Molecularity is the number of reacting species undergoing simultaneous collision in the elementary or simple reaction.	Order is the sum of the powers of the concentration terms in the rate law expression.
Molecularity is applicable only for elementary reactions. For complex reactions, molecularity has no meaning.	Order is applicable to elementary as well as complex reactions.
Molecularity is a theoretical concept.	Order of a reaction is determined experimentally.
Molecularity has whole number values only i.e., 1, 2, 3, etc. It cannot be a non-integer	Order of a reaction need not be a whole number i.e., it can have fractional values also.
Molecularity of a reaction cannot be zero.	Order of a reaction can be zero.
For complex reactions, molecularity is given for elementary steps. Molecularity of the slowest step is same as the order of the reaction.	For complex reactions, order is given by slowest step.