Chemistry · Lesson

Galvanic cells, electrodes and cell potentials

Follow oxidation and reduction through a galvanic cell’s two conduction paths. Calculate a relative cell potential consistently while separating voltage from reaction amount and operating current.

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By the end, you can

Identify anode/cathode reactions and galvanic-discharge polarity.
Distinguish external electron flow from internal ionic conduction.
Subtract compatible reduction potentials without stoichiometric scaling of voltage.

Separate half-reactions connect through two paths

For the model Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), zinc oxidation is Zn → Zn²⁺ + 2 e⁻ and copper reduction is Cu²⁺ + 2 e⁻ → Cu. The anode is the oxidation electrode and the cathode is the reduction electrode.

During this galvanic discharge, the zinc anode is negative and the copper cathode positive. Electrons travel through the external conducting circuit from zinc to copper. Conventional current in that external circuit is opposite to electron motion.

The electrolyte carries ions, not the external electrons

An internal ionic connection maintains charge balance while the electrode reactions proceed. A salt bridge is one design; it is not a requirement that every device have a visible U-shaped bridge. Dissolved ions carry ordinary solution current rather than electrons traversing that bridge as though it were a metal wire.

For the stated zinc/copper reactions, zinc-ion production and copper-ion consumption require appropriate ionic charge compensation. The migrating species and compatibility depend on the actual electrolyte and cell design. A conductive inert electrode can also support a redox couple without being a net reactant.

Keep the electrode reactions and conduction paths distinct

Part of the model	Reaction or carrier	Direction/role	Meaning
Zinc electrode	Zn → Zn²⁺ + 2 e⁻	Oxidation; negative galvanic anode	Supplies external electrons
Copper electrode	Cu²⁺ + 2 e⁻ → Cu	Reduction; positive galvanic cathode	Consumes external electrons
External conductor	Electrons	Zinc → copper	Conventional current is opposite
Internal electrolyte connection	Compatible mobile ions	Maintains charge balance	Electrons are not the normal salt-bridge carriers

Specified Zn/Cu galvanic discharge model. Electrons move through the external circuit; solution charge transport is ionic. Anode/cathode names identify reaction roles, and signs refer to this discharge operation.

Subtract two reduction potentials on one reference scale

Individual half-cell potentials are relative values on a common reference scale. The standard hydrogen electrode is conventionally assigned zero; this is a reference choice, not an absolute physical zero. Use the same reference, temperature and activity conventions when comparing two entries.

Rounded teaching reduction potentials near 25 °C are +0.34 V for Cu²⁺/Cu and −0.76 V for Zn²⁺/Zn. For the stated standard cell convention, E°cell = E°cathode,red − E°anode,red = 0.34 − (−0.76) = 1.10 V. Standard states specify activities and references; the word standard alone does not require 298.15 K or make every 1 mol L⁻¹ real solution an activity of 1.

Voltage does not scale with equation coefficients

Potential is an intensive quantity. Multiplying a half-reaction to balance electrons does not multiply its tabulated potential. For the supplied Cu + 2 Ag⁺ → Cu²⁺ + 2 Ag model with reduction potentials +0.80 and +0.34 V, E° = 0.46 V; doubling silver’s half-reaction does not turn its potential into +1.60 V.

A reversible no-current potential and a loaded terminal voltage are different descriptions. Resistance, kinetic overpotentials and transport can affect a working cell’s voltage and current. Electrode role is defined by its reaction; polarity must be stated for the operating mode rather than assigned permanently from the name anode.

Worked examples

Subtract compatible reduction potentials

Use rounded teaching reduction potentials +0.34 V for Cu²⁺/Cu and −0.76 V for Zn²⁺/Zn on the same reference near 25 °C. Find the stated Zn/Cu standard cell potential.

Copper reduction is the cathode reaction.
Zinc is oxidized at the anode, but its tabulated −0.76 V entry is a reduction potential.
Use E°cell = E°cathode,red − E°anode,red.
Calculate 0.34 − (−0.76) = 1.10 V.

E°cell ≈ +1.10 V under the supplied standard-state/reference conditions. Do not multiply potentials by electron-balancing coefficients.

Try it yourself

Which reaction defines an anode?
Show the answer and reasoning
Oxidation.
The operating sign additionally depends on galvanic or driven operation.
Are electrons the ordinary carriers through a solution salt bridge?
Show the answer and reasoning
No.
Compatible mobile ions provide the internal solution conduction path.
For a standard cell using Ag⁺/Ag reduction at +0.80 V and Cu²⁺/Cu at +0.34 V, silver is reduced and copper oxidized. What is E° when the silver half-reaction is doubled to match electrons?
Show the answer and reasoning
+0.46 V.
Subtract the compatible reduction potentials: 0.80 − 0.34 = 0.46 V. Doubling the silver half-reaction does not double its intensive potential.
Does a positive reversible cell potential specify the actual operating current?
Show the answer and reasoning
No.
Kinetics, resistance, transport and circuit conditions also determine operation.

What to carry forward

Anode means oxidation and cathode means reduction.
State the operating mode when assigning electrode polarity.
Separate external electron transport from internal ionic conduction.
Use compatible reduction potentials and keep voltage intensive.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.