PROTON TRANSFER THEORY (OR) GLASSTONE - LAIDLER - EYRING THEORY

The energy of activation for proton transfer reaction of the area of 10 - 20 kcal. Which is energies with the Ea for H₂ evolution process. The order of 10 - 20 kcal expressions for current is given by following questions.

i = Fkc e∝Fη/RT

Where 'c' refers to the concentration of species which supplies H⁺ to the cathodes.

The Overpotential for H₂ evolution is independent of the PH of the medium. This shows that H⁺ supplied by acid doesn't get deposited at the cathode even though [H ⁺] is higher in acid medium. Therefore it's suggested that H⁺ from weaker is getting deposited at the cathode. As the concentration of water will be in large excess. It's concentration will remain almost the same or "c" will be a constant at all PH. Hence ηH₂ remains to be the same.

The proton gets transferred from the OHP and it is an activation process. Only a portion or part of the emf or part of η is attributed to this proton transfer process.

The ∝ above in the equation is a fraction

0< 2< 1. To find out the value of ∝ . The following mechanism has been proposed.

Chain growth theory mechanism :-

From the water molecule present near the cathode H⁺ is transferred and to compensate for the loss, a proton migrates from neighbouring water molecules. After deposition of proton from the primary stealths protons from secondary stealths migrates. The OH⁻ formed interact or taken by the solvent. As the proton transfer occurs between two H₂O molecules, the transition state is highly symmetrical. Hence values of ∝ is assumed to be 0.5.

                     This theory is called Growths chain theory.

η = a+ 2.303    RT   log i

                          ∝F

∝ = 0.5 according to the above theory. Hence the slope of the straight line obtained from a graph of η vs log i should be 2.303    RT   

                   ∝F 

and, so slope ≈ 0.118

Merits from this theory :-

(1) Using symmetric transition state involving proton proton transfer between two H₂O molecules, the value of ∝ is given as 0.5 molecules, the value of ∝ is given as 0.5 and the slope called for η vs log i graph is 0.118 which agree with experimental value for most of the metals except platinised Pt.

(2) This theory brings out the iii rity in the mechanism of ηH₂ and ɳO₂

(3) Nature of the electrode :-

       Metals which from stronger M⁺ - H bond will pull the H⁺ readily from H₂O. Hence H⁺ transfer, will be faster on these metals (e.g) Pt.

Over metals forming stronger M-H bonds, catalytic action is slow step and they will have lower ηH₂.

Metals (Hg) which form weak M- H bonds will repel H⁺ from water or H⁺ transfer from H₂O over Hg will be very slow.

                          Over metals forming weak M-H bond, the proton transfer is the slow step and they will have higher ηH₂.