Notes of Thermochemistry

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 Chapter #05 ThermoChemistry  

 Prepared by:

Lecturer. S.Fayyaz Hussain

City Of Knowledge

              (Science Campus)

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THERMOCHEMISTRY

Thermo Chemical Reaction:      

"These chemical reactions, which are accompanied with the energy changes during the conversion of the reactants into the products as known as "Thermo chemical Reactions"

There are two types of thermo chemical reaction, which are as follows

1.         Exothermic reaction

2.         Endothermic reaction

1.         Exothermic Reaction:

"An exothermic reaction is one during which heat is liberated or released to the surrounding"

example

1.         C    +   O₂                                 CO₂ + Heat

2.         CH₄ + 2O₂                               CO₂ +   2H₂O + Heat

2.         Endothermic Reaction:

            "An endothermic reaction is one during which heat is absorbed from the surroundings".

example:

1.         C + 2S + Heat                               CS₂

2.         H₂ + I₂ + Heat                                     2HI

Thermodynamic & Its Terms:

            "The branch of science, which deals with the study of energy changes in a system based on the principle of conservation of energy, is known as "THERMODYNAMIC".

            In the study of the thermodynamics, it is convenient to identity the terms system, surrounding and state.

1.         System:

            "The part of universe, which is under study or anything, which is under consideration is called "SYSTEM".

The term system has a very wide application it may range from a small quantity      

     e.g. a cup of water up to the entire universe

            In the study of thermal decomposition of CaCO₃, the sample of this compound is a system similarly; the liquid whose boiling point is being determined is a system.

2.         Surrounding:

            "The environments in which a system is examined and which is not the part of the system are called "SURROULNDING".

            e.g. In the study of thermal decomposition of CaCO₃, the beaker, the source of heat, table etc. all constitute the surrounding of the system.

3.         State:

            "A system can be described by its properties e.g. volume, pressure, temperature etc. A system is said to be in a definite "state" when each of its properties has a definite value and thus the system is completely defined".

            The description of the system before it undergoes a change is known as the INITIAL STATE and the description of a system after the change is known as the FINAL CHANGE. Change in the system is therefore described by comparing the final and initial state of the system e.g. a system composed of a flask containing water of temperature is heated to temperature T₂, the change in temperature, therefore

T = T₂ - T₁

Similarly the change in volume will be

V = V₂ - V₁

Macroscopic Properties:

            The properties of a system in bulk, which are easily measurable as known as the "MACROSCOPIC PROPERTIES" These properties may be divided into two main group which are

1.         Intensive Properties

2.         Extensive Properties

(A)      Intensive Properties:

            "Those properties of a system, which are independent of the amount of material, concerned."

For Example:

Density, Pressure, Temperature, Viscosity, Surface, Tension, Melting and Boiling points.

(B)      Extensive Properties:

            "The properties of a system that depends upon the amount of the substance are called “EXTENSIVE PROPERTIES"

For Example

Mass, Volume, Mole, Enthalpy, Entropy, Internal energy etc.

FIRST LAW OF THERMODYNAMICS:

            This law is also known as “LAW OF CONSERVATION OF ENERGY” and may be defined as:

"Energy can neither be created nor destroyed although it may

 change from one form to another"

                                                            OR

"The total energy of the system and its surrounding must remains constant although it may change from one form to another"

Explanation

            Suppose a cylinder of a gas fitted with a frictionless and weightless position is present at constant atmospheric pressure. Let "q" is the quantity of heat supplied to the system this heat energy.

(i) Increases the internal energy of the system,

(ii) Perform the work of expansion by moving the piston.

Let the internal energy at initial state             = E₁  

& the internal energy at final state ` = E₂

\ The increase in internal energy       =E₂- E₁=DE

& the work done by the system          = W

There for according to the definition of Law,

q   = DE + W

OR                                                    DE = q - W

This is the mathematical expression of first law of thermodynamic.

Work done by the gas:

            Suppose,

                        A cylinder fitted weightless frictionless piston which the area of cross section is A, and piston covers a small distance “∆h” 

                        \ Work = Force x Displacement

                        \ W= F   x ∆h

            But                            

                        Þ 

                        Þ   F = PA

\                    W = P x A ∆h

But ∆h = h₂ –h₁

Change of volume = A ∆h

                        W = P DV

Hence first law of thermodynamic may also be written as

                        Q = DE + W

                Þ q = DE + PDV

                        Or

            DE = q - PDV

i) At Constant Volume:

            When a system has constant volume, the piston of the cylinder does not move or fixed, then the quantity of heat supplied will be qv

                        Therefore in this case

                        DV = O      &    q = q_v

The first law of thermodynamic becomes

                        q = DE + PDV

                        q_v = DE + O

Þ                                      q_v = DE

i.e. A constant volume, the heat absorbed by the system is completely utilized increase the internal energy of the system.

ii) At Constant Pressure:

            When the system is at constant pressure, then quantity of heat supplied to this system is q_p. It increase the internal energy change of the system and performs some work if expansion. Hence we have

                        q_p =  DE + p DV

            But      DE = E₂- E₁ 

            &         DV = V₂- V₁

                        q_p = E₂- E₁ + P (V₂- V₁)

            Þ        q_p = (E₂ + PV₂) – (E₁ + PV₁)

According to definition, E+ PV is the enthalpy of the system, which is denoted by H.

                        E₂ + PV₂ = H₂

                        E₁ + PV₁ = H₁

                                q_p = H₂- H₁

            Þ        q_p = DH

            This expression shows that at constant pressure the heat absorbed by the system is simply equal to the change in enthalpy.

Enthalpy:  

            Enthalpy is also called heat constant. For all exothermic reaction DH = -ve & for all endothermic reaction DH = +ve.

A summary of exothermic and endothermic reactions explained n terms of heat constant is diagrammatized as follows:

Exothermic Reaction

    HCl(aq) + NaOH(I)    energy level of reactants

 

                                                            Heat of reaction = DH

                                                                                      = -57.9 KJ/mole

                                                                                                  = heat lost

 

Endothermic Reaction

2H₂(g) + O₂(g)                   energy level of products

 

                                                       Heat of reaction   = DH

                                                                                = +575 KJ/mole

                                                                                            =  heat gained

 

Thermochemistry:

            "The branch of science which deals with the measurement or calculation of heat changes in a chemical reaction is called THERMOCHEMISTRY"

            Usually heat is measured in the units of kilo joules (KJ) and

Kilocalorie (K, cal)

Hess's Law Of Constant Heat Of Summation:

According to this law,

            "The resultant heat change in a chemical reaction is the same whether the reaction takes place in one or several stages"

OR

            "The enthalpy change of system depends upon its initial and final states only, no matter by which method the change is brought about"

Explanation

            Consider a chemical reaction in which reaction in which reactant "A" changes to the product "D" in a single step with

DH as the heat change

            A                                      D`                          DH

It may proceed through different inter mediate stages i.e. "A" , "B" then change "C" changes to "D" with heat change as DH₃

                                    A                      B                     DH₁

                                   

                                    B                      C                     DH₂

           

                                    C                      D                    DH₃

It follows from Hess's law that enthalpy changes accompanying changes from A to D

According to Hess's Law

DH = DH₁ + DH₂ + DH₃

It is the mathematical expression of Hess's Law.

VERIFICATION:

            To verify this law, we take the example of washing soda. This compound is prepared by tow methods. i.e.

            (a) One- step method

            (b) Tow- steps method.

(A) One Step Method:

            In this method one mole of CO₂ is reacted with two moles of NaOH to give one mole of sodium carbonate along with 90 K/mole energy released.

            CO₂ + 2NaOH                                             Na₂CO₃ + H₂O     DH = 90 KJ/mol

(B) Two Step Method:

            In this method firstly one mole of CO₂ is reacted with one mole of NaOH to give a mole of sodium bicarbonate along with 49KJ/mole energy.

                        i.e. CO₂ + NaOH                                        NaHCO₃          DH = -49 KJ/mol

            In the second step NaHCO3 is reacted with second mole of NaOH to form sodium carbonate and water along with -41KJ/mole energy

            NaHCO₃ + NaOH                                Na₂CO₃ +H₂O        DH₂ = -41 KJ/mol

                                   

According the two step equations we have

                        CO₂ + NaOH                             NaHCO₃            DH = -49 KJ/mol

                        NaHCO₃ + NaOH                      Na₂CO₃ + H₂O   DH = -41 KJ/mol

            CO₂ + 2NaOH                                        Na₂CO₃ + H₂O + 90 KJ/mole.

i.e.              DH₁ + DH₂= -49 + (-41) = -90KJ/mole = DH

                                                Þ    DH = DH₁ + DH₂

Hence the Hess's Law if verified.

Heat Of Formation:

            "It is the quantity of heat i.e. enthalpy change which is evolved or absorbed when one mole of the substance if formed from its elements"

Generally heat of formation is represented by, DHF

e.g.                  C + O₂                           CO₂       DH_f = - 393.7 KJ/mol