# Chapter 4 – Chemical Kinetics

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

Chemical kinetics is an important discipline in physical chemistry that deals with the rate & mechanism of chemical reactions. If you are working on solving a chemical reaction then you need to understand chemical kinetics first. Defined by its rate, chemical reactions can be fast, slow, or moderate. Many factors affect a reaction and make it slow or fast and thus altering the rate of the reaction. Chemical kinetics will help you in understanding the nature in which reactants  react and their mechanism. In this article, you will learn the key concepts of Chemical kinetics and these notes will help you revise in the exams as well.

#### Rate of a Reaction

The rate of any particular reaction defines whether the reaction is fast, slow, or moderate. For example, magnesium ribbon when burnt with a bunsen burner immediately gets combusted making it a fast reaction. The rate of a chemical reaction can be expressed in the form of:

• The rate of decrease in concentration/volume of any of the reactants.
• The rate of increase in concentration/volume of any of the products.

Assume that ‘R’ is the reactant & ‘P’ is the product & the volume of the system remains constant.

Now if,

à P (chemical reaction of first order)

The rate of the above equation can be expressed as:

Rate of disappearance of R = Concentration decrease of R/Time Taken(t) = Δ[R]/Δt and similarly,

Rate of appearance of P = Concentration increase of P/Time taken(t) = Δ[P]/Δt.

The rate expressions can also be termed as integrated rate equations as they represent the concentration of products/reactants as a function of time (t).

Note: Δ[R] will be negative as the rate of reactants will always decrease to form products. You will have to multiply it with ‘-1’ to make the rate of reaction positive.

#### Unit of the Rate of Reaction

As you saw above, the rate expression of any reaction is expressed as the rate of change in concentration of reactant/product divided by the time taken. The unit of the rate expression will be concentration time -1.

For example,

If the concentration is measured in moles per litre & the time is measured in seconds in any rate expression, then the unit of the rate expression would be (mol L-1 s-1) where ‘mol’ is moles, ‘L’ is litres & ‘s’ is seconds.

#### Order of a Reaction

The order of a reaction is said to be the sum of exponential value, i.e. the concentration of reactants in the rate law expression. For example, if the rate expression of any reaction is stated as:

Rate = k[A]1[B]1 where ‘A’ & ‘B’ are reactants, the order of this reaction will be the sum of coefficients, i.e. (1+1=2). We can also call this reaction a second-order reaction since its order is 2.

#### Molecularity of a Reaction

The molecularity of a reaction is defined as the number of reacting agents (atoms, ions, or molecules) in the reaction which must react to produce products. For example, in 2HI à H2 + I2, two different species are colliding simultaneously, thus making it a bimolecular reaction. The number of reacting species colliding simultaneously to complete a chemical reaction can also be termed as the molecularity of a reaction.

Half-life of Reactions

The half-life of a reaction is the time in which the concentration of the reactant is reduced to exactly half.

#### Factors Affecting the Rate of a Reaction

The rate of any reaction can change when subjected to a few factors. The factors influencing the rate of a reaction are:

• Temperature – If the temperature of the system is increased, then the rate of the equation will also increase. It happens because of the increased collision between reactants/molecules per second in a system. In some reactions, changing the temperature can even change the products– like when ethanol reacts with dilute sulphuric acid at 100 degrees produces diethyl ether, but when the same reaction is carried out at 180 degrees, it produces ethylene. The temperature dependence of the rate of a reaction is directly proportional.
• Concentration of Reactants – The more you increase the concentration of reactants, the more molecules will collide simultaneously hence increasing the rate of the reaction.
• Surface Area & Phase of Reactants -If you increase the surface area of reaction, the rate of the reaction will increase. If the reactants are in the same phase, then the reaction rate will also increase. The rate of reaction in homogeneous mixtures is generally more than the rate of reaction in heterogeneous mixtures. If the reactants are in the same phase, i.e. solid, liquid, or gas, they will collide more often with each other during a reaction.
• Solvents -Changing a solvent in any particular reaction can affect the rate of that reaction. For example, the reaction of sodium acetate & methyl iodide is generally done in the presence of methanol as a solvent, but when the same reaction is done in the presence of dimethylformamide (organic solvent), the reaction happens rapidly.
• Catalysts -The promoters will increase the rate of reaction, and poisons will decrease the rate of a reaction.

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