Primary (Voltaic) Cells

Full Notes

A voltaic (or galvanic) cell is a type of electrochemical cell in which a spontaneous redox reaction generates an electric current.

A voltaic cell can be constructed using two half-cells connected by a salt bridge and an external wire.

Half-Cells

A simple half-cell consists of:

• A metal electrode (solid metal)
• An electrolyte (a solution containing ions of that metal)

A redox equilibrium is established between the metal atoms and their ions:

Mⁿ⁺(aq) + ne⁻ ⇌ M(s)

This sets up a potential difference between the metal and the solution. The position of equilibrium – and hence the total charge or ‘potential’ of the electrode – depends on how readily the metal loses or gains electrons.

The electrode potential of a half-cell cannot be measured directly, but we can compare it to a standard reference (like the standard hydrogen electrode) to determine its relative value.

Components of a Voltaic Cell

The solid metal in each half-cell acts as an electrode, with one being the anode and the other the cathode.

• Anode: site of oxidation.
  • Metal loses electrons and enters solution as ions.
  • Electrons flow away from the anode.
• Cathode: site of reduction.
  • Metal ions in solution gain electrons and are deposited as metal.
  • Electrons flow into the cathode.

Salt Bridge Function

In a voltaic cell, the salt bridge allows ion exchange between half-cells. This prevents charge buildup by allowing:

• Cations to move toward the cathode
• Anions to move toward the anode

Salt bridge ions must be inert to avoid interfering with redox reactions, which is why potassium nitrate (KNO₃) is often used.

Because K⁺ and NO₃^- ions don’t easily get oxidised or reduced, meaning they don’t affect the redox processes occurring. They just simply allow charge to flow.

How a Simple Cell Works

Example: Zn–Cu Voltaic Cell

At the anode, zinc metal is oxidised: Zn(s) → Zn²⁺(aq) + 2e⁻

The released electrons travel through the external circuit to the cathode, where copper ions are reduced: Cu²⁺(aq) + 2e⁻ → Cu(s)

The salt bridge maintains charge balance by allowing ions to move between the two half-cells, preventing charge buildup.

Cell Notation

Electrochemical cells can be written using shorthand notation:

• Single vertical line (|): separates different phases
• Double vertical line (||): salt bridge
• Anode (oxidation) on the left, cathode (reduction) on the right

Zn–Cu cell notation:

Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)

Non-Rechargeable Cells (Primary Cells)

In primary cells, the redox reactions are not reversible.

Once the active materials are used up, the cell can no longer generate current.

Example: Leclanché Cell (Zn/NH₄⁺)

• Anode: Zinc casing oxidised to Zn²⁺
• Cathode: Carbon rod where NH₄⁺ is reduced to NH₃
• Ammonia reacts with Zn²⁺ to form [Zn(NH₃)₄]²⁺, preventing gas buildup

Linked Course Question

Summary

• Voltaic cells use spontaneous redox reactions to produce electricity.
• Electrons flow from anode (oxidation) to cathode (reduction).
• Salt bridge maintains electrical neutrality by ion movement.
• Zn–Cu cell is a classic example of a voltaic system.
• Primary cells are non-rechargeable and stop working once reactants are used.