NCERT Solutions
Class 12 Chemistry
Electrochemistry

Q.5
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).
(i) For the given reaction, the Nernst equation can be given as:
Ecell = E cell (-) – (0.0591/n) log [Mg2+]/ [Cu2+]
= {0.34 – (-2.36)} – (0.0591/2) log (.001/.0001)
=2.7 – (0.0591/2) log10
= 2.7 − 0.02955
= 2.67 V (approximately)
(ii) For the given reaction, the Nernst equation can be given as:
Ecell = E cell (-) – (0.0591/n) log [Fe2+]/ [H+] 2
= {0-(0.44)} – (0.0591/2) log [0.001]/ [12]
= (0.44 – 0.02955) (-3)
=0.52865 V
=0.53 V (approx.)
(iii) For the given reaction, the Nernst equation can be given as:
Ecell = E cell (-) – (0.0591/n) log [Sn2+]/ [H+] 2
= {0-(0.14)} – (0.0591/2) log (0.050)/ (0.020) 2
= 0.14 − 0.0295 × log125
= 0.14 − 0.062
= 0.078 V
= 0.08 V (approximately)
(iv) For the given reaction, the Nernst equation can be given as:
Ecell = E cell (-) – (0.0591/n) log [1]/ [Br-] 2/ [H+] 2
= (0- 1.09) – (0.0591/2) log (1)/ [(0.010) 2(0.030)]
=-1.09 -0.02955 x log (1/0.0000009)
=-1.09 – 0.02955 x log (1/9 x 10-8)
=-1.09 -0.02955(0.0453 +7)
=-1.09 – 0.208
=-1.298V