Order and molecularity of the reaction

Chemical kinetics is a fascinating field of chemistry that deals with the speed or rate of chemical reactions. Key concepts in chemical kinetics include the terms "order" and "molecularity" of reactions, which help us understand the mechanism and behavior of reactions.

Order of reaction

The order of a reaction shows how the rate of a chemical reaction depends on the concentration of the reactants. It can be determined experimentally and is represented as the sum of the exponents in the reaction's rate equation.

Understanding the rate law

The rate law for a reaction describes how the concentration of the reactants affects the rate of the reaction. It is usually expressed as:

Rate = k [A]^x [B]^y

Here, k is the rate constant, [A] and [B] are the concentrations of the reactants, and x and y are the orders of the reaction relative to A and B, respectively.

The total order of the reaction

The overall order of the reaction is the sum of the powers to which the reactant concentrations are raised in the rate law. For the rate law given above, the overall order of the reaction is x + y.

Examples of reaction orders

Zero order reaction

In a zero-order reaction, the rate is constant and independent of the concentration of the reactants. This means that the rate law is expressed as:

Rate = k

The rate of the reaction remains constant as time progresses. A common example of this is the decomposition of ammonia on a platinum surface.

First order reaction

In a first-order reaction, the rate depends linearly on the concentration of one reactant. The rate law for such a reaction is:

Rate = k [A]

An example of this is the radioactive decay of isotopes, where the concentration of the isotope decreases rapidly over time.

Second order reaction

In a second order reaction, the velocity depends on the square of the concentration of one reactant or on the product of the concentrations of two reactants:

Rate = k [A]^2

Or

Rate = k [A][B]

An example of this is the reaction between hydrogen and iodine, forming hydrogen iodide.

Partial and mixed orders

Some reactions may involve fractional or mixed orders. These can be observed in more complex reactions where the behaviour does not follow simple integer orders. For example, enzyme-catalysed reactions can exhibit fractional orders.

Determining the reaction order experimentally

The reaction order is usually determined by:

• Initial rate method: By studying how the initial reaction rate varies with different initial concentrations.
• Integrated rate laws: By observing concentration-time data and fitting it to different integrated rate laws, we decide which law is most appropriate.

Molecularity of the reaction

Molecularity refers to the number of molecules or atoms participating in an elementary reaction step. Unlike reaction order, which can be fractional or determined experimentally, molecularity is always a whole number and is theoretical.

Types of molecularity

Unimolecular reactions

In a unimolecular reaction, one molecule undergoes a transformation to form a product. An example of this is the isomerization of cyclopropane to propylene:

C3H6 → C3H6

Bimolecular reactions

Bimolecular reactions involve two reactant molecules. This is common and can be represented as follows:

A + B → Products

Or

2A → Products

Examples of bimolecular reactions

A typical bimolecular reaction is the formation of hydrogen bromide from hydrogen and bromine:

H2 + Br2 → 2HBr

Thermomolecular reactions

There are reactions in which three molecules collide together. However, these reactions are rare because of the low probability of three particles colliding together. An example of this is:

2NO + O2 → 2NO2

Comparison of order and molecularity

Though both order and molecularity give information about the nature of the reaction, yet there are clear differences between the two:

• Definition: Order is an experimental quantity that describes the effect of reactant concentrations on reaction rates, while molecularity is a theoretical concept based on the elementary stage of the reaction.
• Values: The order can be fractional or zero, but molecularity is always a whole number.
• Determination: The order is determined experimentally while the molecularity is determined from the reaction mechanism.

Examples illustrating the differences

Consider the decomposition of hydrogen peroxide catalyzed by iodide ions, which is a reaction of order one with respect to hydrogen peroxide:

2H2O2 → 2H2O + O2

For this reaction:

• The experimentally determined rate law may be first order in hydrogen peroxide.
• The molecularity of the initial step, where decomposition occurs, may involve as few as two molecules, but the overall reaction is more complex.

Conclusion

Both order and molecularity are essential for understanding the dynamics and mechanisms of chemical reactions. They provide scientists with tools to analyze and predict the behavior of reactions under different conditions. While molecularity provides theoretical understanding from an atomic perspective, reaction order provides experimental verification, which enriches our knowledge about chemical processes.

The study of chemical kinetics, which focuses on reaction rates, order, and molecularity, is an important aspect of chemistry that has applications in many fields, including pharmaceuticals, environmental science, and industrial chemistry, making it increasingly important and useful in scientific research and application.

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