Galvanic cells

Principle of operation and difference from conventional reactions

The operation of any galvanic cell is based on a oxidation-reduction (redox) reaction. However, it proceeds under specific conditions.

If an oxidizing agent and a reducing agent are simply mixed in a single container, their direct contact leads to a random exchange of electrons. There is no directed motion of charged particles in such a system, so no electric current arises, and all the released chemical energy turns into heat and dissipates into space.

In a galvanic cell, the oxidizing agent and reducing agent are spatially separated. Direct contact between them is excluded. Because of this, electrons cannot transfer from one substance to another directly. Their only path is to pass through an external metallic conductor (the electrical circuit) connecting the two poles of the cell. This very directed flow of electrons constitutes the electric current. The energy of chemical bonds is thus realized as electrical energy.

Structure of a zinc-carbon cell (Leclanché cell)

Let us examine the classical structure of a dry galvanic cell using a system that employs zinc and manganese. This cell consists of several key components, each performing its own function:

• Anode (reducing agent, negative pole "−"): Represented by an outer zinc container (Zn). During operation, the surface of this container accumulates a negative charge.
• Cathode (oxidizing agent, positive pole "+"): The central graphite rod (C) acts as the cathode. A mixture of carbon powder and manganese(IV) oxide (MnO_2) is densely pressed around it.
• Electrolyte (conducting medium): The internal space between the electrodes is filled with a thick paste. It consists of water, ammonia (NH_3), ammonium chloride (NH_4Cl), and starch, which acts as a thickener to prevent the liquid from leaking.
• Sealing: The entire system is tightly sealed at the top with bitumen, which prevents the electrolyte from drying out and air from entering.

Reaction mechanism: processes at the electrodes

When we close the external circuit (e.g., connect a light bulb with a conductor to the zinc container and the graphite rod), potential jumps occur at the phase boundaries (solution/metal and solution/oxide), and the directed movement of electrons from zinc to graphite begins. The process can be divided into two half-reactions.

1. Process at the anode (oxidation of zinc) Zinc atoms on the container surface donate their electrons, which enter the external electrical circuit. The resulting zinc cations pass into the electrolyte layer. There, they instantly bind with ammonia molecules, forming a complex ion. The half-reaction equation is as follows: Zn + 4NH_3 → [Zn(NH_3)_4]^{2+} + 2e^-

2. Process at the cathode (reduction of manganese) Via the external circuit, electrons reach the graphite rod and are transferred to the manganese(IV) oxide layer. Manganese atoms accept these electrons, lowering their oxidation state from +4 to +3. Simultaneously, oxygen is released in the form of an O^{2-} ion, which passes into the electrolyte paste and binds with protons donated by ammonium ions. The cathodic half-reaction equation is: 2MnO_2 + 2NH_4^+ + 2e^- → Mn_2O_3 + 2NH_3 + H_2O

Overall reaction equation Adding both half-reactions together, we obtain the final equation of the chemical process that drives the zinc-carbon galvanic cell. During this reaction, a zinc complex ion, manganese(III) oxide, and water are formed: Zn + 2MnO_2 + 2NH_4^+ + 2NH_3 = [Zn(NH_3)_4]^{2+} + Mn_2O_3 + H_2O

Briefly: answers to topical questions

What else to study on this topic

• Mechanism of the potential jump at phase boundaries
• Complexation of zinc ions in ammonia solution
• Thermodynamics of redox reactions
• The role of ammonium chloride in the electrolyte

Other topics in "Chapter 7. Classification of chemical reactions by electronic processes"

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