HESS'S LAW OF CONSTANT HEAT SUMMATION

HESS’S LAW OF CONSTANT HEAT SUMMATION

                                                        Hess’s law stated that if the chemical reaction takes place in one step or in several step then amount of heat absorbed or evolved during any chemical reaction is always remain same                                                                     

                                                                       OR

                                                  ‘’The law stated that the change in enthalpy in a chemical reaction depend upon initial state of reactant and final state of product and it does not depend on a path by which reaction is brought about”.

Explanation: -

                                       Let consider a reaction in which substance change from A to D this reaction takes place by two methods

1)     Direct Method

            2)     Indirect Method

 Direct Method:-

                In this method A is directly converted to D

                          A        ----------------->     D           ΔH = H_D- H_A           ----------------------------       1 

Where H_D is enthalpy of product D   AND H_A is enthalpy of reactant A

Indirect Method:-

                                    In this method the A is converted into D in several step

                        Step (i):-         A ---------------->    B                    ΔH₁ = H_B –H_A

                       Step (ii):-        B   ----------------->  C                     ΔH₂ = H_C- H_B

                       Step (iii):-       C  -------------------> D                    ΔH₃ = H_D -H_C

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                  Add i+ii+iii            A ------------------->  D       ΔH₁ + ΔH₂+ ΔH₃ = H_D – H_A ------------   2

From equation 1 and 2

                                    ΔH = H_D- H_A

                                    ΔH₁ + ΔH₂+ ΔH₃ = H_D – H_A

       In general      ΔH = ΔH₁ + ΔH₂+ ΔH₃+-----------+ ΔH_n

    Illustration: Consider the formation of Methylene chloride. This reaction is carried out by two ways.

Method I: - Direct method

                            Methylene chloride can be prepared directly by the reaction of CH₄ with Cl₂ gas

CH₄(g) + Cl₂ (g) + Heat ----------->  CH₂ Cl₂ (g) + 2HCl(g) ∆H₁ = -202.3 KJ ------1

Method II: - Indirect method

 Step I -  CH₄(g) + Cl₂(g)--------> CH₃Cl (g) + HCl(g)     ∆H₂ =  -98.3KJ  ------  1                                                                                                                            

Step II - CH₃Cl(g)+ Cl₂(g)---------> CH₂Cl₂ (g) + HCl(g)  ∆H₃  = -104.0KJ  ---  2

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   CH_4(g) + 2Cl_{2(g)------------->}  CH₂Cl_2(g) + 2HCl_(g)∆H = ∆H₂ + ∆H₃ =-202.3 KJ ---3

From equation 1 & 2

                             ∆H₁ = ∆H₂   +∆ H₃

Therefore, whether the reaction take placed in one step or in two or more step enthalpy of Reaction is always same