Q. 53.8( 46 Votes )

Write the Nernst equation and emf of the following cells at 298 K:

(i) Mg(s)|Mg2+ (0.001M)||Cu2+ (0.0001 M)|Cu(s)

(ii) Fe(s)|Fe2+ (0.001M)||H+ (1M)|H2(g)(1bar)| Pt(s)

(iii) Sn(s)|Sn2+ (0.050 M)||H+ (0.020 M)|H2(g) (1 bar)|Pt(s)

(iv) Pt(s)|Br2(l)|Br(0.010 M)||H+ (0.030 M)| H2(g) (1 bar)|Pt(s).

Answer :

Ecell = ?


(i) Mg + Cu2 + Mg2 + + Cu (n = 2)


E0Cu2 + /Cu + = 0.34V


E0Mg2 + /Mg = - 2.37 V


E0cell = E0R - E0L


E0cell = 0.34 - ( - 2.37) Equation 1


Using Nernst equation, we get,


Equation 2


Substituting Equation 1 in equation 2, we get,


Ecell = 0.34 - ( - 2.37) -


= 2.71 -


= 2.71 - 0.02955


Ecell = 2.68 V


The e.m.f of the cell, Ecell is 2.68 V


ii) Fe + 2H + Fe2 + + H2 (n = 2)


E0H + /H2 = 0 V


E0Fe2 + /Fe = - 0.44 V


E0cell = E0R - E0L


E0cell = 0 - ( - 0.44) Equation 1


Using Nernst equation, we get,


Equation 2


Substituting Equation 1 in equation 2, we get,


Ecell = 0 - ( - 0.44) -


= 0.44 -


= 0.44 - 0.0887


Ecell = 0.5287 V


The e.m.f of the cell, Ecell is 0.5287 V


iii) Sn + 2H + Sn2 + + H2 (n = 2)


E0H + /H2 = 0 V


E0Sn2 + /Sn = - 0.14V


E0cell = E0R - E0L


E0cell = 0 - ( - 0.14) Equation 1


Using Nernst equation, we get,


Equation 2


Substituting Equation 1 in equation 2, we get,


Ecell = 0 - ( - 0.14) -


= 0.14 -


= 0.14 - 0.0295×2.0969


Ecell = 0.08 V


The e.m.f of the cell, Ecell is 0.08 V


iv) 2Br - + 2H + Br2 + H2 (n = 2)


E0H + /H2 = 0 V


E0Br2/Br - = 1.08V


E0cell = E0R - E0L


E0cell = 0 - (1.08) Equation 1


Using Nernst equation, we get,


Equation 2


Substituting Equation 1 in equation 2, we get,


Ecell = 0 - (1.08) -


= - 1.08 -


= - 1.08 - 0.208


Ecell = 1.288 V


The e.m.f of the cell, Ecell is 1.288 V


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