Exhaustive Guide: Galvanic Cells & Electrode Potential | Class 12 Chemistry | ChemCA

Electrochemistry: Galvanic Cells

Module 1 | CBSE Class 12 Chemistry | Electrochemistry Chapter

1. Introduction to Electrochemistry

Electrochemistry is the study of the production of electricity from the energy released during spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous chemical transformations.

This field is highly relevant in real life: batteries, fuel cells, electroplating, and the extraction of metals like aluminium and sodium all rely on electrochemical principles. Moreover, sensory signals transmitted to our brain also have an electrochemical origin.

2. Electrochemical Cells (The Daniell Cell)

An electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy is called a Galvanic cell or a Voltaic cell.

The most common example is the Daniell Cell. It is based on the redox reaction between zinc and copper sulphate:

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Construction and Working:

Anode (Oxidation half-cell): A zinc rod is dipped in an aqueous solution of zinc sulphate (ZnSO₄). Zn loses electrons to become Zn²⁺. (Remember: Anode is Negative in Galvanic cells).
Cathode (Reduction half-cell): A copper rod is dipped in an aqueous solution of copper sulphate (CuSO₄). Cu²⁺ ions accept electrons to become solid Cu. (Remember: Cathode is Positive).
Salt Bridge: A U-tube containing a semi-solid paste of an inert electrolyte like KCl, KNO₃, or NH₄NO₃ in agar-agar gel. It connects the two half-cells internally.

Crucial Role of the Salt Bridge:
1. It completes the electrical circuit by allowing the flow of ions.
2. It maintains the electrical neutrality of the two half-cells. Without it, the accumulation of Zn²⁺ at the anode and SO₄^2- at the cathode would immediately stop the flow of electrons.

The electrical potential (voltage) of the Daniell cell is 1.1 V when the concentration of Zn²⁺ and Cu²⁺ ions is exactly 1 mol dm^-3 (1 M).

2.1 Effect of Opposing External Potential (Highly Tested in Board Exams)

What happens if an external voltage (E_ext) is applied in the opposite direction to the Daniell cell? The NCERT textbook highlights three specific cases:

Condition	Current Flow & Electron Flow	Chemical Reaction & Behavior
E_ext < 1.1 V	Electrons flow from Zn to Cu. Current flows from Cu to Zn.	Zn dissolves at anode and Cu deposits at cathode. (Acts as a normal Galvanic Cell).
E_ext = 1.1 V	No flow of electrons or current.	No chemical reaction occurs. The cell is at equilibrium.
E_ext > 1.1 V	Electrons flow from Cu to Zn. Current flows from Zn to Cu.	Reaction reverses: Zn is deposited at the zinc electrode, Cu dissolves at the copper electrode. (Acts as an Electrolytic Cell).

3. Galvanic Cells & Cell Representation

Any Galvanic cell consists of two half-cells. The reactions occurring in these half-cells are called half-cell reactions.

Oxidation Half-Reaction (at Anode): Loss of electrons. (e.g., Zn → Zn²⁺ + 2e^-)
Reduction Half-Reaction (at Cathode): Gain of electrons. (e.g., Cu²⁺ + 2e^- → Cu)

Mnemonic: LOAN (Left side, Oxidation, Anode, Negative pole).

IUPAC Convention for Representing a Cell

To avoid drawing a full diagram every time, chemists use a shorthand notation:

Anode (Solid) | Anode Electrolyte (Conc.) || Cathode Electrolyte (Conc.) | Cathode (Solid)
Zn(s) | Zn²⁺(aq, 1M) || Cu²⁺(aq, 1M) | Cu(s)

A single vertical line ( | ) represents a phase boundary (solid to liquid).
A double vertical line ( || ) represents the salt bridge.

4. Electrode Potential and EMF of a Cell

When a metal is placed in a solution of its own ions, a potential difference develops between the electrode and the electrolyte. This is called the Electrode Potential.

Standard Electrode Potential (E°): When the concentration of all the species involved in a half-cell is unity (1 M), the electrode potential is known as standard electrode potential. According to IUPAC, standard reduction potentials are now called standard electrode potentials.

Cell Potential (EMF)

The potential difference between the two electrodes of a galvanic cell is called the Cell Potential. It is measured in volts. When no current is drawn from the cell, this potential difference is called the Electromotive Force (EMF) of the cell.

E°_cell = E°_cathode - E°_anode

E°_cell = E°_right - E°_left

Note: Both values used in the formula must be Standard Reduction Potentials.

5. Measurement of Electrode Potential (SHE)

It is impossible to measure the absolute potential of a single half-cell because a reaction cannot occur in isolation. We can only measure the difference between two half-cells. To overcome this, a reference electrode is chosen.

Standard Hydrogen Electrode (SHE): A reference electrode whose standard reduction potential is arbitrarily assigned a value of exactly 0.00 V at all temperatures.

Construction of SHE:

It consists of a platinum wire sealed in a glass tube, attached to a platinum foil coated with finely divided platinum (platinum black).
The foil is immersed in an acidic solution having 1 M concentration of H⁺ ions (e.g., 1 M HCl).
Pure hydrogen gas (H₂) at 1 bar pressure is continuously bubbled over the platinum foil at 298 K.

Cell Representation of SHE: Pt(s) | H₂(g, 1 bar) | H⁺(aq, 1 M)

Half-Cell Reaction: H⁺(aq) + e^- → ½ H₂(g)

How to use SHE: To find the E° of zinc, we connect a zinc half-cell to the SHE. The voltmeter reads 0.76 V. Since zinc undergoes oxidation (electrons flow from Zn to SHE), Zn is the anode.
E°_cell = E°_cathode(SHE) - E°_anode(Zn)
0.76 V = 0.00 V - E°_Zn &implies; E°_Zn = -0.76 V.

6. Electrochemical Series and its Applications

The arrangement of various elements in the increasing or decreasing order of their standard reduction potentials is called the Electrochemical Series.

Key Applications (Very Important for MCQs):

Comparing Oxidizing and Reducing Power:
- A higher (more positive) standard reduction potential means the species has a high tendency to get reduced. Thus, it is a strong oxidizing agent. (Fluorine has the highest E° = +2.87 V, making it the strongest oxidizing agent).
- A lower (more negative) standard reduction potential means the species has a high tendency to get oxidized. Thus, it is a strong reducing agent. (Lithium has the lowest E° = -3.05 V, making it the strongest reducing agent).
Predicting Spontaneity of a Redox Reaction: A reaction is feasible if the calculated EMF of the cell (E°_cell) is positive.
Displacement of Hydrogen from Acids: Metals with a negative reduction potential (placed above hydrogen in the series) can displace hydrogen gas from dilute acids (e.g., Zn, Mg, Fe). Metals with a positive E° (e.g., Cu, Ag) cannot.

7. NCERT Solved Examples (Step-by-Step)

NCERT Intext Question: Can you store copper sulphate solutions in a zinc pot?

Solution:
To determine this, we check the standard reduction potentials from the electrochemical series.
E°(Cu²⁺/Cu) = +0.34 V
E°(Zn²⁺/Zn) = -0.76 V

Since the reduction potential of Cu is higher, Cu²⁺ ions have a greater tendency to get reduced than Zn²⁺. Therefore, if CuSO₄ is placed in a zinc pot, zinc will undergo oxidation (dissolve) and copper will be displaced (reduced).
Reaction: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s).
Conclusion: No, we cannot store copper sulphate solution in a zinc pot because a spontaneous chemical reaction will occur, destroying the pot.

8. Previous Year Questions (PYQs) & Exhaustive Question Bank

Part A: Conceptual & Assertion-Reason (1 Mark)

[CBSE 2017, 2020]

Q1. What is the function of a salt bridge in a galvanic cell?

Answer: The salt bridge serves two main functions:
1. It completes the inner electrical circuit by allowing ions to flow from one half-cell to the other.
2. It maintains the electrical neutrality of the solutions in the two half-cells, preventing the buildup of charge that would otherwise stop the flow of electrons.

[CBSE Sample Paper 2023]

Q2. Assertion (A): E° for Cu²⁺/Cu is positive (+0.34 V) whereas for Zn²⁺/Zn is negative (-0.76 V).
Reason (R): Copper is a stronger reducing agent than zinc.

Answer: Assertion is Correct but Reason is Incorrect.
The Assertion is a factual statement. However, a negative E° indicates a greater tendency to undergo oxidation. Since Zn has a lower (more negative) reduction potential than Cu, Zinc is the stronger reducing agent, not Copper.

[CBSE 2018, 2019]

Q3. Under what condition does a galvanic cell behave like an electrolytic cell?

Answer: A galvanic cell behaves like an electrolytic cell when an external opposing voltage applied to it is greater than the cell's own internal potential (i.e., E_ext > E_cell). This causes the chemical reaction to reverse, turning a spontaneous process into a non-spontaneous one driven by external electrical energy.

Part B: Application Based Problems (2-3 Marks)

[CBSE 2016]

Q4. Depict the galvanic cell in which the reaction Zn(s) + 2Ag⁺(aq) → Zn²⁺(aq) + 2Ag(s) takes place. Further indicate:
(i) Which of the electrodes is negatively charged?
(ii) The carriers of the current in the cell.
(iii) Individual reaction at each electrode.

Answer:
Cell Representation: Zn(s) | Zn²⁺(aq) || Ag⁺(aq) | Ag(s)

(i) The Zinc (Zn) electrode is the anode, where oxidation takes place, and hence it is negatively charged.
(ii) In the external circuit, electrons carry the current from the Zn electrode to the Ag electrode. Within the cell (inner circuit), ions in the salt bridge and the electrolyte carry the current.
(iii) At Anode (Oxidation): Zn(s) → Zn²⁺(aq) + 2e^-
At Cathode (Reduction): 2Ag⁺(aq) + 2e^- → 2Ag(s)

[CBSE 2015]

Q5. Given the standard electrode potentials:
E°(K⁺/K) = -2.93 V, E°(Ag⁺/Ag) = +0.80 V, E°(Hg²⁺/Hg) = +0.79 V, E°(Mg²⁺/Mg) = -2.37 V, E°(Cr³⁺/Cr) = -0.74 V.
Arrange these metals in their increasing order of reducing power.

Answer:
The reducing power of a metal depends on its tendency to get oxidized (lose electrons). A lower (more negative) standard reduction potential means a higher tendency for oxidation, thus a stronger reducing power.
Let's arrange the E° values from most positive to most negative:
Ag (+0.80 V) < Hg (+0.79 V) < Cr (-0.74 V) < Mg (-2.37 V) < K (-2.93 V).
Increasing order of reducing power: Ag < Hg < Cr < Mg < K.

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Exhaustive Guide: Galvanic Cells & Electrode Potential | Class 12 Chemistry