Electrochemistry
Electrochemistry Lecture-5
Gibb’s free energy
Δ G r x n = w e l = − Q Δ E \Delta G_{rxn}=w_{el}=-Q\Delta E Δ G r x n = w e l = − Q Δ E
= − n F Δ E =-nF\Delta E = − n F Δ E
Z n + C u S O 4 → Z n S O 4 + C u ↓ Zn+CuSO_4 \rarr ZnSO_4 + Cu \darr Z n + C u S O 4 → Z n S O 4 + C u ↓
Δ G < 0 \Delta G < 0 Δ G < 0
Electrolysis will take place
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Electrolytic cell
Electrolysis of molten alkali halides (NaCl(Na-metal))
Carbon electrodes
Gibb's free energy → \rightarrow → Electrolytic cell → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Electrolytic cell
Reaction of electrolytic Cell
Cathod reaction : N a + + e − → N a ( s ) E ° = − 2.71 Na^+ + e^-\rarr Na(s) \hspace{3 mm} E\degree= -2.71 N a + + e − → N a ( s ) E ° = − 2.71
Anode reaction : C l − → 1 2 C l 2 ( g ) + e − E ° = − 1.36 Cl^- \rarr \frac{1}{2} Cl_2(g)+e^- \hspace{3 mm} E\degree=-1.36 C l − → 2 1 C l 2 ( g ) + e − E ° = − 1.36
Overall reaction : N a + + C l − → N a ( s ) + 1 2 C l 2 ( g ) E ° = − 4.1 Na^+ + Cl^- \rarr Na(s)+\frac{1}{2}Cl_2(g) \hspace{3 mm} E\degree=-4.1 N a + + C l − → N a ( s ) + 2 1 C l 2 ( g ) E ° = − 4.1
Gibb's free energy → \rightarrow → Electrolytic cell → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Electrolysis
Ag Solution of N i C l 2 NiCl_2 N i C l 2
Cathode reaction : N i 2 + + 2 e − → N i ( s ) E ° = − 0.24 Ni^{2+}+2e^-\rarr Ni(s) \hspace{3 mm} E\degree=-0.24 N i 2 + + 2 e − → N i ( s ) E ° = − 0.24
Anode reaction : 2 C l − → C l 2 ( g ) + 2 e − − 1.36 2Cl^- \rarr Cl_2(g)+2e^- \hspace{3 mm} -1.36 2 C l − → C l 2 ( g ) + 2 e − − 1.36
N i 2 + + 2 C l − → 2 N i ( g ) + C l 2 ( g ) E ° n e t = − 1.6 Ni^{2+}+2Cl^- \rarr 2Ni(g)+Cl_2(g) \hspace{3 mm} E\degree_{net}=-1.6 N i 2 + + 2 C l − → 2 N i ( g ) + C l 2 ( g ) E ° n e t = − 1.6
Anode reaction : H 2 O → 1 2 O 2 ( g ) + 2 H + + 2 e − E ° = − 1.23 H_2O\rarr \frac{1}{2}O_2(g)+2H^+ +2e^- \hspace{3 mm} E\degree=-1.23 H 2 O → 2 1 O 2 ( g ) + 2 H + + 2 e − E ° = − 1.23
Cathode reaction : 2 H 2 O + 2 e − → H 2 ( g ) + 2 O H − E ° = − 0.83 2H_2O +2e^- \rarr H_2(g)+2OH^- \hspace{3 mm} E\degree=-0.83 2 H 2 O + 2 e − → H 2 ( g ) + 2 O H − E ° = − 0.83
Gibb's free energy → \rightarrow → → \rightarrow → Electrolysis → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Electrolysis
Cathode reaction:
2 H 2 O + 2 e − → H 2 ( g ) + 2 O H − E ° = 0.4 2H_2O +2e^- \rarr H_2(g)+2OH^- \hspace{3 mm} E\degree=0.4 2 H 2 O + 2 e − → H 2 ( g ) + 2 O H − E ° = 0.4 [ O − H ] ∼ 10 − 7 M [O^-H]\sim 10^{-7}M [ O − H ] ∼ 1 0 − 7 M
Anode reaction:
C l − + H 2 O → 1 2 C l 2 + e − E ° = − 0.95 Cl^- +H_2O \rarr \frac{1}{2}Cl_2 + e^- \hspace{3 mm} E\degree=-0.95 C l − + H 2 O → 2 1 C l 2 + e − E ° = − 0.95
C l − + H 2 O → 2 H 2 ( g ) + 1 2 C l 2 + 2 O H − → E = − 0.95 Cl^- +H_2O \rarr 2H_2(g)+\frac{1}{2}Cl_2 + 2OH^- \rarr E=-0.95 C l − + H 2 O → 2 H 2 ( g ) + 2 1 C l 2 + 2 O H − → E = − 0.95
N a + + e − → N a ( s ) E ° = − 2.7 Na^+ +e^- \rarr Na(s) \quad E \degree = -2.7 N a + + e − → N a ( s ) E ° = − 2.7
Gibb's free energy → \rightarrow → → \rightarrow → Electrolysis → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Electrolysis of pure-high resistance
Difficult to undergo electrolysis → \rarr →
Cathode reaction:
2 H 2 O + 2 e − → H 2 ( g ) + 2 O H − E ° = − 0.83 2H_2O+2e^-\rarr H_2(g)+2OH^- \hspace{3 mm} E\degree=-0.83 2 H 2 O + 2 e − → H 2 ( g ) + 2 O H − E ° = − 0.83
Anode reaction:
H 2 O → 1 2 O 2 ( g ) + 2 H + + 2 e − E ° = − 1.29 H_2O \rarr \frac{1}{2}O_2(g)+2H^+ +2e^- \hspace{3 mm} E\degree=-1.29 H 2 O → 2 1 O 2 ( g ) + 2 H + + 2 e − E ° = − 1.29
3 H 2 O ( l ) → H 2 + 1 2 O 2 + … … E ° = − 2.06 3H_2O(l)\rarr H_2+\frac{1}{2}O_2+……E\degree=-2.06 3 H 2 O ( l ) → H 2 + 2 1 O 2 + …… E ° = − 2.06
Faraday’s Law of electolysis (Michael Faraday) in 1832.
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → Electrolysis of pure-high resistance → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Faraday’s law
Weight of substance formed at the electrode during electrolysis in directly proportional to the quantity of electricity that passes through the electrolyte.
m ∝ Q → m \propto Q \rarr m ∝ Q → → m = Z Q \rarr m=ZQ → m = ZQ Z → Z \rarr Z →
The weight of different substances formed by the passage of same quantity of electricity are proportional to the equivalent weight of each substance.
W 1 W 2 = m 1 m 2 = E 1 E 2 \frac{W_1}{W_2}=\frac{m_1}{m_2}=\frac{E_1}{E_2} W 2 W 1 = m 2 m 1 = E 2 E 1 Z 1 I t Z 2 I t = E 1 E 2 ⇒ Z 1 Z 2 = E 1 E 2 \frac{Z_1 It}{Z_2 It}=\frac{E_1}{E_2}\rArr \frac{Z_1}{Z_2}=\frac{E_1}{E_2} Z 2 I t Z 1 I t = E 2 E 1 ⇒ Z 2 Z 1 = E 2 E 1
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Faraday's law → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Electrolysis of brine solution
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Electrolysis of brine solution → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Types of cell
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Types of cell → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Lead-acid storage cell (1859)
P b ( s ) ∣ P b S O 4 ( s ) ∣ H 2 S O 4 ( a q ) ∣ ∣ P b S O 4 ( s ) , P b O 2 ∣ P b ( s ) Pb(s)|PbSO_4(s)|H_2SO_4(aq)||PbSO_4(s),PbO_2|Pb(s) P b ( s ) ∣ P b S O 4 ( s ) ∣ H 2 S O 4 ( a q ) ∣∣ P b S O 4 ( s ) , P b O 2 ∣ P b ( s )
Net cell reaction:
P b ( s ) + P b O 2 ( s ) + 2 H 2 S O 4 ( a q ) → 2 P b S O 4 ( s ) + 2 H 2 O Pb(s)+PbO_2(s)+2H_2SO_4(aq) \rarr 2PbSO_4(s)+2H_2O P b ( s ) + P b O 2 ( s ) + 2 H 2 S O 4 ( a q ) → 2 P b S O 4 ( s ) + 2 H 2 O
↔ c h a r g i n g d i s c h a r g i n g \xleftrightarrow[{charging}]{discharging} d i sc ha r g in g c ha r g in g
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Lead-acid storage cell (1859) → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Lead-acid storage cell (1859)
P H 2 S O 4 ∼ 2 P H 2 O P_{H_2SO_4} \sim 2P_{H_2O} P H 2 S O 4 ∼ 2 P H 2 O
[ H 2 S O 4 ] = 6 m o l / d m 3 [H_2SO_4]=6mol/dm^3 [ H 2 S O 4 ] = 6 m o l / d m 3
Normal cell voltage ~ 2.1v at 298K
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Lead-acid storage cell (1859) → \rightarrow → → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Problems
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Problems → \rightarrow → → \rightarrow →
Electrochemistry Lecture-5
Dry cell
Le clanche’ dry cell 1866
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Dry cell → \rightarrow →
Electrochemistry Lecture-5
Modern version of primary storage (1949)
Voltage 1.5-1.65
Net reaction:
Z n + 2 M n O 2 → Z n O + M n 2 O 3 Zn+2MnO_2\rarr ZnO+Mn_2O_3 Z n + 2 M n O 2 → Z n O + M n 2 O 3
Primary/Secondary Storage
Gibb's free energy → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → → \rightarrow → Modern version of primary storage (1949)
