Concentration Cells 363f57

Electrochemistry (Concentration Cells) Dr.S.SURESH Assistant Professor Email:avitsureshindia@gm ail.com

CONCENTRATION CELLS In concentration cells, the EMF arises because of a difference in the concentration of the species involved. Concentration cells are of two types.

(a)Electrode concentration cell. (b)Electrolyte concentration cell.

Electrode concentration cell In these cells, two like electrodes at different concentration are dipping in the same solution. Two hydrogen electrodes at unequal gas pressure immersed in the same solution of hydrogen ions constitute an electrode-concentration cell. This may be represented by PtH ;2( P1 )

/ Solution of HH+ 2 (ions / P2 )

; Pt

Electrolyte concentration cells In these cells, both the electrodes are of the same metal (Zn) and these are in with solutions of the same ions (Zn2+). The concentrations and hence activities of the ions are, however different. Let (a1) and (a2) be the activities of zinc ions in the two electrolytes surrounding the electrodes. One such cell is represented as Zn; // ; Zn

Zn (a21 )

Zn (a22 )

Electrolyte concentration cells are of two types (a)Concentration cells without transference (b)Concentration cells with transference

Concentration cells without transference To understand the setting up of such a cell, let us consider two simple cells. Such as Pt, H, / AgCl(s) , Ag HCl 2 ( g) (a 1 ) Pt, , HCl / AgCl(s) , Ag H 2 ( g) (a 2 ) The two electrolytes are thus not in direct with one another. Let the activity of H+ ions in the two solutions be (a1) (a2). The cells are combined together in such a way that they oppose each other

Concentration cells without transference Let us consider the cell Pt,

,

/ AgCl(s) , Ag

Anode H (oxidation half 2 ( g) HCl (a 1cell ) reaction) H2

+ e‒

Cathode (Reduction half cell reaction)

1  HAg (a )1(s) 2 The net cell reaction is AgCl(s) + e‒    H2 + AgCl(s)

+

Ag(s) +   

1 2

  

Cl (a )1 HCl (a )1

Concentration cells without transference

The cells are connected together and it is represented as

Pt,H2(g)

HCl (a1 )

HCl (a 2 )

,AgCl(s) , Ag(s) / Ag(s) AgCl(s),

H2(g),Pt  Cell reactions

1 Left side cell reaction 2 H2 + AgCl(s)  Ag(s) + 1 cell reaction Right side H22 + AgCl(s)  Ag(s) + Subtract eq (2) minus eq (1) Overall reaction

HCl (a 2 )

HCl (a1 ) ---- (1)

HCl (a 2 ) HCl (a1 ) 

----- (2)

Concentration cells without transference The overall reaction of the combined cell for the age of one faraday of electricity, will be obtained as ⇌ HCl (a ) HCl (a ) Hence EMF of such a cell is given by 2

Ew.o.t =

1

RT a 2 ln F a1

Concentration cells with transference Consider a concentration cell formed by combining two hydrogen gas electrodes in with HCl solutions of different concentrations. The two solutions are in direct with each other. Pt, H2(g), HCl (a1 ) / HCl (a ) , H2(g), Pt 2

H+

  

Cl‒

HCl (a1)

Hydrogen electrode

HCl (a2)

Concentration cells with transference The following changes are involved for the flow of one faraday of electricity Left hand side electrode ‒ H2(g)1 ⇌ -------- (i) H+ e (a1 ) 2 Right hand side electrode

e‒ H⇌ H2(g) ---------- 1(ii) (a 2 ) 2 Thus H+ ions are generated at the left hand electrode and consumed at the right hand electrode, The solutions are in direct with each other and the ions are free to move from one solution to the other, when current flows through the cell. +

Concentration cells with transference Let t‒ be the transport number of Cl‒ ion and t+ that of H+ ion in HCl. The cell reaction involves the transport of t+ moles of HCl from the LHS to the RHS of the cell. Hence, t+ equivalent of H+ ions will be transferred from the solution of activity a1 to that of activity a2, which may be represented as HCl (a ) HCl (a ) t+ ⇌ t+ 1

2

Concentration cells with transference Since, t+ = 1 ‒ t‒ Hence the changes are represented as (1 ‒HCl t‒(a) ) ⇌ HCl (1(a‒) t‒) 1

t‒ (a1 ) HCl

2

⇌ HClt(a‒ ) The mean ionic activity of ions is 2 a () defined asa (H( )a (Cl ) = ), Hence 

2



The EMF of concentration cell is given by Ew.t =

RT a 2 2 t‒ ln F a1

Liquid Junction potential

Liquid Junction Potential Ew.t =

RT a 2 2tln ‒ F a1

RT a 2 Ew.o.t F=ln a 1

Hence liquid junction potential (El) is given by

El = Ew.t ‒ Ew.o.t = (2t‒

RT a 2 ‒ 1)ln F a1

We know that (t+ + t‒ = 1 ; Then t‒ = 1 - t+) RT a 2 = (t‒ + (1- tF+)ln‒a 1) 1 RT a 2 ln = (t‒ ‒F t+)a 1