CTEVT Chemistry note: Physical Chemistry(VII)- Chemical Kinetics and Thermodynamics.

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Chemistry complete note for HA, Pharmacy, Lab technician (CMLT), Dental science and other CTEVT diploma level courses.

CHAPTER- 16 : Chemical Kinetics:

Chemical kinetics is the branch of physical chemistry which deals with the study of reaction mechanism, rate of reaction and the factors affecting the rate of reaction.

Rate of reaction :

Rate of a chemical reaction is the speed or velocity with which the reactants are converted into products.

Let us consider a simple reaction :

Here, rate of reaction = rate of disappearance of A = rate of appearance of B.

Mathematically, Rate = -d[A] / dt = + d[B]/ dt

Hence, “The change in the concentration of reactant or product with change in time is called rate of reaction ”.

Its unit is molL-1S-1.

Factors affecting the rate of reaction :

 1. Temperature :

The rate of reaction increases with increase in temperature. It has been found that the rate of reaction nearly doubles for every 10oC rise in temperature.

 2. Surface area of reactant :

When the surface area of reactant is increased then the rate of reaction increases. For example: powder lime stone reacts more vigorously with dil.HCl than pieces of lime stone.

 3. Concentration of reactant :

When the concentration of reactant increases then rate of reaction also increases. For examples :- Lime stone reacts more vigorously with con. HCl than with dil. HCl.

 4. Catalyst

A catalyst itself doesn’t take part in reaction but affect the rate of reaction. By use of positive catalyst, the rate of reaction increases at particular temperature. Catalyst finds out the alternative new path for the reaction having lower activation energy. A negative catalyst decreases the rate of reaction.

Activation Energy (Ea)

The minimum amount of energy required by the reactant to change into product is called activation energy.

OR, The minimum amount of energy which must be supplied to the reactants to enable them to cross over the energy barrier is called activation energy.

         energy profile diagram- CTEVT chemistry

If the activation energy high in a reaction then slower will be the rate of reaction.

Catalyst/ Catalysis :

A catalyst is a substance which provides a new path for the reaction with lower activation energy and speeds up the rate of the reaction.

         Catalyst : CTEVT diploma chemistry

Enzyme catalysis : Enzymes are also called biological catalysts. Enzymes are protein molecules which act as catalysts to speed up biochemical reactions. Eg.

Order of reaction:

The sum of the powers of concentration in the rate law equation is called order of a reaction.

Let us consider a general chemical reaction,

A + B → Products

We know,

Rate(R) =k [A]a [B]b

Where a and b are the order of reaction with respect to A and B respectively.

Therefore overall order of reaction = a+b

If a+b = o then the reaction is zero order reaction.

If a+b = 1 then the reaction is first order reaction.

If a+b = 2 then the reaction is second order reaction and

If a+b = 3 then the reaction is third order reaction.

Rate law equation :

Consider a general reaction

A + B → C

Rate of reaction (R) α [A] [B]

R = K [A] [B] ………(i) where K is rate constant.

This equation (i) is called rate law equation.

If [A] = [B] = 1.

Then R = K

So rate constant (K) can be defined as the rate of reaction when the initial concentration of reactant is unit.

Molecularity of reaction

The total number of atoms, ions or molecules which take part in a chemical reaction is called molecularity of a reaction.

Consider a reaction,

2HI → H2 + I2

In this reaction, 2 molecules of reactants are involved in the reaction, so the molecularity of this reaction is 2.

If the molecularity of reaction is 1, the reaction is unimolecular. If the molecularity is 2, the reaction is bimolecular and so on.

Differences between order and molecularity of reaction:

Order of reaction

Molecularity of reaction
  • The sum of the powers of concentration in the rate law equation is called order of a reaction.
  • It is experimentally determined.
  • It may be zero, fractional or whole number.
  • It changes with change in temperature and pressure
  • The total number of atoms, ions or molecules which take part in a chemical reaction is called molecularity of a reaction.
  • It is theoretically determined.
  • It is generally small positive whole number.
  • It doesn’t change with change in temperature and pressure.
Le-Chatelier’s principle :

If a system in equilibrium is subjected to change in concentration, pressure or temperature then the equilibrium shifts in such a way so as to nullify the effect of the change.

Consider an exothermic reaction,

Total volume of reactant = 4

Total volume of product = 2

Effect of temperature :

It is exothermic reaction. Now, if the temperature is increased i.e. heat is supplied to the system, then according to Le-Chatelier’s principle equilibrium shift to the side that absorbs heat i.e. equilibrium shift in backward direction. Similarly decrease in temperature will shift the equilibrium in the forward direction.

Effect of pressure:

The volume of reactant is more than that of product. When the pressure is increased , the equilibrium shifts to the direction where volume is decreased. So, if the pressure is increased then equilibrium shifts to forward direction. But if pressure is decreased then equilibrium shifts to backward direction.

Effect of concentration :

According to Le-Chatelier’s principle, an increase in concentration of reactants (N2 and H2) would shift the equilibrium in that direction in which the reactants are consumed i.e. equilibrium shift in the forward direction. If concentration of NH3 is increased, the reaction shifts to the backward direction.

Law of mass action :

This principle states that, “the rate of reaction is directly proportional to the product of active masses (i.e. molar concentration) of the reactants.”

Let us consider a general chemical reaction,

At equilibrium, suppose the active masses of A, B, C and D are represented as [A], [B], [C] and [D] respectively. Now, applying the Law of mass action,

Rate of forward reaction (Rf) α [A] [B]

Rf = Kf [A] [B]

Rate of backward reaction (Rb) α [C] [D]

Rb = Kb [C] [D]

At equilibrium,

Rate of forward reaction (Rf) = Rate of backward reaction (Rb)

Kf [A] [B] = Kb [C] [D]

Where Kf is equilibrium constant for forward reaction and Kb is equilibrium constant for backward reaction.

Kf/ Kb =[C] [D] / [A] [B]

K = [C] [D] / [A] [B]

At constant temperature, as Kf and Kb are constant , then K is also constant and is called ‘Equilibrium constant’.

CHAPTER–17: Thermochemistry / Thermodynamics

( thermo=heat)

All the chemical reactions proceed with absorption or evolution of energy. The chemistry dealing with the chemical process and the energy absorbed or evolved during this process is called thermo – chemistry or simply called as chemical thermodynamic.

For example: – In the combustion of fuels like kerosene, coal, wood, etc. energy is evolved whereas in the dissolution of Ammonium chloride in water, heat is absorbed.

The various unit of energy are Joule, Ergs, Calorie, etc.

{1 Joule = 107 ergs , 1 Cal = 4.2 Joule}

Some Thermo – Chemical terms :

 1. System :

A system is the specific portion of universe in which the energy change is to be studied. Water taken in a beaker is an example of a system.

 2. Surrounding :

The remaining portion of universe which is not the part of system is called surrounding. System interacts with the surrounding.

 3. Boundary :

The part that separates system from the surrounding is called boundary. Boundary is that portion of universe through which system and surrounding can interact with each other. Boundary may be real or imaginary.

CTEVT chemistry: system, surrounding and boundry

Internal Energy (E) :

The total amount of energy stored in a system (substance) is called internal energy.

Enthalpy (H) :

Enthalpy is the total heat content of the of a system. It is equivalent to the sum of the internal energy and the product of the pressure and volume of the system.

H = E + PV

Where,

H = Enthalpy, E = Internal Energy, P = Pressure and V = Volume of the system.

Exothermic reaction :

The reaction in which heat is liberated /released from the system is called exothermic reaction.

For example:- In the reaction between a metal and dil. mineral acid, heat is liberated.

Zn (s) + 2HCl (aq.) → ZnCl2 (aq.) + H2 (g) + Heat

CTEVT diploma level chemistry: exothermic reaction

Endothermic reaction :

The reaction in which heat is absorbed from the surrounding is called endothermic reaction. For example:- In the dissolution of ammonium chloride in water, heat is absorbed from the surrounding.

NH4Cl (s) + H2O(l)  →  NH4Cl (aq.) – Heat

First law of thermodynamics (Law of Conservation of Energy) :

This law state that, “the total energy of the universe i.e. system and surrounding always remains constant, however it may changes from one form to another.” i.e. Energy can neither be created nor destroyed.

Let us consider a system having initial internal energy (E1). If heat (q) is supplied to the system and work (w) is done on the system then the internal energy in the final stage i.e. E2 is given as,

E2 = E1 + q + w

E2-E1 = q + w

Therefore, ∆E = q + w ………..(i)

This equation (i) is the mathematical statement of first law of thermodynamics which shows the relationship among internal energy, work and heat.

Hess’s law (of constant heat summation) :

“The total amount of heat evolved or absorbed in a reaction (i.e. enthalpy change) in a reaction is always same whether the reaction takes place in one step or in a number of steps. In other words, the total amount of heat change in a reaction depends only upon the nature of the initial reactants and the final products and is independent of the path or the manner by which this change is brought about.”

Consider a general reaction

Suppose the heat evolved in this reaction is Q1 Joules.

Now suppose the same reaction takes place in three steps as follows:

Suppose the heat evolved in these three steps are q1, q2 and q3 Joules respectively.

Thus the total heat evolved (suppose Q2) = q1 + q2 + q3 Joules.

Then, according to Hess’s law, we must have Q1 = Q2

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