Chemical Kinetics: Quick Revision

  1. Chemical kinetics: The branch of chemistry deals with the rates of reactions, the factors that influence it, and the mechanism by which the reaction occurs.
  2. Rate of reaction: It is the speed with which the reactants art converted into products. It may be expressed in terms of the rate of any of the products. It may be represented in terms of the rate of disappearance of any of the reactants or the rate of appearance of any of the products.
  3. Law of mass action: At a given temperature, the rate of a reaction at a particular instant is proportional to the product of the active masses of the reactants at that instant raised to the powers which are numerically equal to the numbers of their respective molecules in the stoichiometric equation describing the reaction. For the following reaction: m1A1+m2A2+…….—-> Products the rate of reaction is given by the following expression: rate of reaction =K[A1]m1[A2]m2[A3]m3 …….where K is constant.
  4. Rate constant: It is equal to the rate of reaction when the concentration of each reactant is unity. It is a constant for a particular reaction at a given temperature and also known as specific rate constant or specific reaction rate. k=dx/dt
  5. Collision theory of reaction rate:
    • A chemical reaction occurs when the existing bonds of the reactants are broken down and new bonds come into existence in the molecules of the products.
    • The basic requirement for a reaction to occur is that there should be physical contact between reacting species. They must collide with each other.
    • The number of effective collisions which bring chemical changes are few in spite of a large number of actual collisions.
    • Effective Collisions: The collisions which bring a chemical change are known as effective collisions. The following two conditions must be satisfied for a collision to be effective:
      • Reacting species must possess adequate energy to overcome the energy barrier.
      • Reacting molecules must be properly oriented at the time of the collision.
    • Threshold energy: The minimum amount of energy which the colliding particles must possess to make the chemical reaction occur.
    • Activation Energy: The excess amount of energy required by the reacting species to undergo chemical reactions is called activation energy Ea.
    • Activation energy= Threshold energy -Average kinetic energy of the reacting molecules.
    • The effective collision increases with an increase in temperature.
  6. Factors influencing the reaction rate:
    1. Nature of reactant: The reactant with weak bonds reacts quickly whereas the reactants with strong bond reacts slowly. The molecular reactions are generally slow as compared to the ionic reactions.
    2. Concentration of reactants: The rate of reaction increases with the increase in the concentration of reactants as the number of collisions increases.
    3. Catalyst: Positive catalyst increases the rate of reaction by providing an alternative path involving a lesser amount of activation energy.
    4. Temperature: The rate of reaction increases with an increase in temperature. The rate of many reactions are approximately doubled or tripled for every 100C rise in temperature.
  7. Molecularity and order of reactions:
    1. Molecularity of reaction: The minimum number of reacting particles that come together or collide in a rate-determining step to form a product or products is called the molecularity of the reaction. The reactions are called unimolecular, bimolecular, and trimolecular, etc. depending on the number of reacting particles as 1,2,3…etc in the rate-determining step of the reactions.
    2. Order of reaction: Rate equation or rate law is an experimentally determined mathematical expression relating the molecular concentrations of the reactants to the actual reaction rate. Considering a general reaction:

n1A+n2B+n3C+……..——>Products

The rate law or rate equation is given by :

Rate=-dx/dt= k[A]x[B]y[C]z…….

the order of the reaction is the sum of the exponents(Powers) to which molar concentration terms are raised in the rate law. Order of reaction = x+y+z

8. Reaction of various order:

What is a Zero Order Reaction?

Zero-order reaction is a chemical reaction wherein the rate does not vary with the increase or decrease in the concentration of the reactants. Therefore, the rate of these reactions is always equal to the rate constant of the specific reactions (since the rate of these reactions is proportional to the zeroth power of reactants concentration).

What is a First-Order Reaction?

A first-order reaction can be defined as a chemical reaction in which the reaction rate is linearly dependent on the concentration of only one reactant. In other words, a first-order reaction is a chemical reaction in which the rate varies based on the changes in the concentration of only one of the reactants. Thus, the order of these reactions is equal to 1.

Examples of First-Order Reactions

  • SO2Cl2 → Cl2 + SO2
  • 2N2O5 → O2 + 4NO2
  • 2H2O2 → 2H2O + O2

What is a Second Order Reaction?

From the rate law equations given above, it can be understood that second order reactions are chemical reactions which depend on either the concentrations of two first-order reactants or the concentration of one-second order reactant.

Since second order reactions can be of the two types described above, the rate of these reactions can be generalized as follows:

r = k[A]x[B]y

Where the sum of x and y (which corresponds to the order of the chemical reaction in question) equals two.

Examples of Second Order Reactions

A few examples of second order reactions are given below:H++OH−→H2O C+O2→CO+O

The two examples given above are the second order reactions depending on the concentration of two separate first order reactants.2NO2→2NO+O2 2HI→I2+H2

These reactions involve one second order reactant yielding the product.

What is Third-order reaction:

A third-order reaction is a chemical reaction where the rate of reaction is proportional to the concentration of each reacting molecule. In this reaction, the rate is usually determined by the variation of three concentration terms.

For example, let us consider a chemical reaction where,

A+2B ⇢ C+D

According to rate formula,

Rate, R= k [A]ˣ [B]ʸ

Here, the order with respect to A is given by x and order with respect to B is given by y. So the complete order of the reaction is the sum of x and y.

For a third-order reaction, the order of the chemical reaction will be 3.

Let aA+bB+cC ⇢ Product

By rate formula, R= k [A]ˣ [B]ʸ [C]□

The third-order reaction for the above chemical reaction is given by,

Order = x+y+z

To summarize, the order of reaction can be defined as the sum of the exponents of all the reactants present in that chemical reaction. If the order of that reaction is 3, then the reaction is said to be a third-order reaction.

Different Cases in Third Order Reaction

When we are dealing with a third-order reaction there can be different cases involved. It could be;

(i) All three reactants have the same level of concentrations.

(ii) Two reactants have equal concentrations but one has a different concentration.

(iii) All three reactants have different or unequal concentrations.

Third Order Reaction Examples

● Let us consider the reaction between nitric oxide and chloride

2 NO +Cl2 ⇢ 2 NOCl

Rate, R= k[NO]2 [Cl2]

Order of above reaction =Sum of exponent of nitric oxide and chloride Order = 2 + 1 = 3

● Let us consider the reaction between nitric oxide and oxygen

2 NO + O2 ⇢ 2 NO2 R= k[NO]2 [O2]

Order = 2 + 1 = 3

The unit of third-order reaction when a rate is constant is given by, Rate of reaction= k[Reactant]³

Unit of rate is given by,

R = mol/ Ls = mol L⁻¹ s⁻¹ mol L⁻¹ s⁻¹ = k ( mol L⁻¹ )³ k =L² mol⁻² s⁻¹

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