Chemical Kinetics

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What is Chemical Kinetics?

In physical chemistry, the importance of chemical kinetics topics is at the top level.

Fundamentally, it helps the pupils to learn about the various features of a chemical reaction. Taking into consideration the word ‘kinetics’ which implies the rate of change of some quantity. 

Let’s take an illustration such as velocity. Velocity can be represented as the rate of change of displacement. Also, acceleration can be defined as it is the rate of change of velocity.

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Based on the factors of chemical reactions’ procedures, they are categorized as:

  1. Fast Reactions

  2. Moderate Reactions

  3. Slow (esterification) Reactions. 

We are about to learn the chemical kinetics and its purposes in chemical reactions and their rate with numerous factors and examples in this content.


Chemical Kinetics Definition

The chemical kinetics is also known as the reaction kinetics. These reactions assist us in knowing about the reactions’ rate as well as certain conditions about how these are conducted. 

Chemical kinetics also helps to gather and examine the data about the mechanism of the reaction and describe the features of a chemical reaction. 

The chemical kinetics is the division of chemistry which compacts mostly with the rates of chemical reactions. 


Chemical Kinetics Formula

  1. Rate of Reaction

The rate of reaction is depicted as an object in motion with a certain velocity; chemical reactions also happen with certain velocities. 

This process (the rate of reaction) shows how fast the product is starting. Also, it displays the speed at which the reactants are consumed concerning time. 

The expression given below is described mathematically:

[R]2 - [R]1 = Δ[R]

[P]2 - [P]1 = Δ[P]

 t2 - t1= Δt

∴ Rate of Reaction = \[\frac{\text{Increase in concentration of P}}{\text{Time taken}}\] = \[\frac{\text{Decrease in concentration of R}}{\text{Time taken}}\]

∴ Rate of Reaction = \[\frac{ΔP}{Δt}\] = \[\frac{-ΔR}{Δt}\]

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  1. Rate Expression and Rate Constant

General form of the reaction: aA + bB cC + dD

Some others also explain this equation as follows:

Rate = k[A]x[B]y

Here, 

x and y may or may not be stoichiometric coefficients. 

k = the rate constant.

  1. Order of the Reaction

The order of the reaction can be obtained by the addition of these components that is

The reaction’s order = x + y

The equation’s rate of a chemical reaction is given as:

Rate = k[A]x[B]y

k = the rate constant.


Units of Rate Constant

The rate constant can be obtained as;

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

The order of the reaction decides the unit of rate constant (k). The table given below is showing the units for the different orders of the reaction. 

Reaction Type

Order of Reaction

Units of Rate Constant

Zero-order reaction

0

mol L-1 s-1

First-order reaction

1

s1

Second-order reaction

2

mol-1L s-1


Chemical Kinetics Equations

i. Zero-order Reaction

The order of the reaction is relative to the power in which reactants’ concentration is zero in a zero-order reaction. 

The expression below states the rate equation:

[R]= -kt + [R]0

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ii. First-order Reaction

The first-order reaction states that the rate of reaction is relative to the first power of the reactants’ concentration. 

The expression below states the rate equation:

\[log\frac{[R]_0}{[R]}\] = \[\frac{k(t_2-t_1)}{2.303}\] 

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iii. The Half-life Reaction

At a certain time, when the concentration of the reactant is changed into half of its initial value, that type of reaction is known as the half-life of a reaction. 

Its symbol is t1/2

We can write it in a mathematical form such as:

t1/2 = \[\frac{0.693}{k}\]


Arrhenius Equation 

The Arrhenius equation can be explained as the rate of a chemical reaction which relies on the temperature. 

The rate constant grows to twice its value at every 100 rises in temperature. 

The tendency to depend on the rate of the chemical reaction is explained by the Arrhenius equation as given below: 

K = Ae-Ea / RT

Here,

A = the Arrhenius factor

Ea = the activation energy 

R = the gas constant

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Chemical Kinetics Examples

Let’s consider that the balanced equation for the reaction is not satisfied with the reaction's kinetic equation.

A clear illustration is given below, which is between hydrogen and iodine chloride. These reactants will help to form iodine and hydrogen chloride:

H2 + 2ICl → I2 + 2HCl.

It can be said that it is a second-order reaction as the rate is calculated to be proportional to both concentrations to the first power. 

Rate= k[H2][ICl]

Let’s break down the concept and explain it briefly. 

Assume that a sluggish reaction between one hydrogen molecule and one iodine chloride at the very beginning of the reaction;

Slow reaction is given by:

H2 + ICl → HI + HCl 

Immediately, after the slow reaction, a rapid reaction is conducted. It is between the newly formed hydrogen iodide and an additional molecule of iodine chloride;

Fast reaction: HI + ICl → HCl + I2 

As we explained in the previous example, the same procedure is also applied over here.

It is the reaction between hydrogen and bromine as written below;

H2 + Br2 → 2HBr

The reaction is generally slow at room temperature, but suddenly, it changes its rate at the increase of temperature.

FAQ (Frequently Asked Questions)

Q1. Illustrate the five kinds of Chemical Reactions.

Ans: The five basic types of chemical reactions are 

  1. Combination

  2. Decomposition

  3. Single-replacement 

  4. Double-replacement

  5. Combustion

The observer can place the chemical reaction into the given categories by analyzing the reactants and products of a given reaction.

Many kinds of changes have happened during the time of chemical reactions such as a change in temperature, energy wasted via the process, and so on.

Q2. What is your opinion about the Rate of Reaction?

Ans: We can explain the rate of reaction as the amount at which the concentration is altering or the ratio of change in concentration and change in time.

It is given by: r = Δ[C] / Δt

Where,

rate = r 

change in concentration = Δ[C] 

change in time = Δt

Q3. Mention a factor that affects the Rate Constant (k).

Ans: The factors that are affecting the rate constant are given below: 

Increasing the temperature of a reaction generally speeds up the process because the rate constant increases according to the Arrhenius Equation. 

With the rising value of T, the value of the exponential part of the equation becomes less negative. This results in an increased value of k.

Q4. What are the major reasons which cause problems for the Reaction Rate?

Ans: The major reasons which cause a problem for the reaction rate are:

  1. Concentration of reactants

  2. Temperature

  3. Phase and surface area of reactants

  4. Effect of solvent

  5. Catalyst