F. Electrochemistry · Electrochemistry

Galvanic Cells: Turning Redox into Electricity

F3.2 F3.3 F2.5 F2.1About 70 minutes

Learning goals

I can identify the components of a galvanic cell and explain what each one does.
I can identify the anode and cathode, and the direction of electron flow and ion flow.
I can explain why a salt bridge (or porous barrier) is needed.
I can write and read standard cell notation.

Words to know

galvanic cell
An electrochemical cell that uses a spontaneous redox reaction to produce electricity. Also called a voltaic cell. A battery is one or more galvanic cells.

half-cell
One half of an electrochemical cell: an electrode in contact with an electrolyte, where either oxidation or reduction happens.

electrode
A solid electrical conductor (a metal or graphite) where electrons enter or leave the cell.

anode
The electrode where oxidation happens. In a galvanic cell it is the negative electrode.

cathode
The electrode where reduction happens. In a galvanic cell it is the positive electrode.

electrolyte
A substance that conducts electricity because it contains mobile ions, such as a salt solution.

salt bridge
A tube or strip containing an unreactive electrolyte (such as KNO₃) that connects two half-cells and lets ions move between them so charge does not build up.

inert electrode
An electrode that conducts electrons but does not react, such as platinum or graphite (carbon).

cell notation
A shorthand for a cell, e.g. Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s), with the anode on the left.

Catching the electrons

When zinc sits in copper(II) sulfate solution, electrons jump straight from zinc atoms to copper ions. The energy is released as a little heat, and nothing useful happens.

Now imagine putting the zinc and the copper(II) ions in separate containers, joined by a wire. The only way for electrons to get from zinc to copper ions is to travel through the wire, and electrons moving through a wire are an electric current. That is the whole idea of a galvanic cell: separate the two half-reactions and make the electrons take the long way round, doing useful work on the way.

Alessandro Volta built the first battery (a stack of zinc and copper or silver discs separated by brine-soaked cloth) in 1800. In 1836, John Frederic Daniell built the zinc–copper cell you will study today, which gave a much steadier current and was used to power telegraph systems.

A zinc–copper (Daniell) cell. Zinc is oxidized at the anode; electrons travel through the wire and the voltmeter to the copper cathode, where copper(II) ions are reduced. In the salt bridge, anions (NO₃⁻) drift toward the anode and cations (K⁺) toward the cathode.

The parts and their jobs

Two half-cells. Each has an electrode in an electrolyte. Here: a zinc strip in Zn(NO₃)₂(aq), and a copper strip in Cu(NO₃)₂(aq).
The anode is where oxidation happens: Zn(s) → Zn²⁺(aq) + 2e⁻. Electrons are released here, so in a galvanic cell the anode is the negative electrode.
The cathode is where reduction happens: Cu²⁺(aq) + 2e⁻ → Cu(s). Electrons are used up here, so the cathode is the positive electrode.
The external circuit (wire, plus a voltmeter, light or motor) carries electrons from the anode to the cathode. Electrons never travel through the solution.
The salt bridge carries ions between the half-cells so the circuit is complete.

Two memory aids: "An Ox, Red Cat" (anode: oxidation; reduction: cathode) and "electrons flow from A to C" (anode to cathode, in alphabetical order).

Why the salt bridge matters

As the cell runs, the anode compartment fills with extra Zn²⁺ ions and the cathode compartment loses Cu²⁺ ions. Without a salt bridge, the anode solution would quickly become positively charged and the cathode solution negatively charged. That charge build-up would stop the electrons almost immediately.

The salt bridge contains an electrolyte whose ions do not react, such as potassium nitrate. Anions (NO₃⁻) move out of the bridge toward the anode to balance the new Zn²⁺. Cations (K⁺) move toward the cathode to replace the Cu²⁺ being used up. Ions also drift through the solutions themselves: cations toward the cathode and anions toward the anode.

In place of a salt bridge, many cells use a porous barrier (such as unglazed porcelain) that lets ions pass slowly but stops the solutions mixing. Remove the bridge or barrier and the voltmeter reading drops to zero: the circuit is open.

Signs in galvanic cells

In a galvanic cell the anode is negative and the cathode is positive. In the lesson Electrolytic Cells and Electrolysis the signs flip, but the definitions do not: the anode is always where oxidation happens.

What changes as the cell runs

The zinc anode loses mass (zinc atoms become ions).
The copper cathode gains mass (copper plates onto it).
[Zn²⁺] increases; [Cu²⁺] decreases, so the blue colour slowly fades.
The voltage stays fairly steady until one reactant runs low; then it falls. A "dead" battery is one in which the reaction has nearly reached its end (equilibrium).

Inert electrodes

Some half-reactions have no solid metal to use as an electrode, for example Fe³⁺(aq) + e⁻ → Fe²⁺(aq), or Cl₂(g) + 2e⁻ → 2Cl⁻(aq). Then we dip an inert electrode into the solution. Platinum and graphite (carbon) are common choices. The inert electrode just carries electrons to or from the ions; it is not used up.

Cell notation

Drawing a full diagram every time is slow, so chemists use a one-line cell notation:

Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)

The anode is written on the left, the cathode on the right.
A single line | shows a boundary between phases (solid electrode and solution).
A double line || shows the salt bridge or porous barrier.
When two species are in the same solution, separate them with a comma: Pt(s) | Fe²⁺(aq), Fe³⁺(aq). Inert electrodes go at the outer ends.

Reading cell notation from left to right follows the electrons: from the anode, through the solutions and salt bridge (ions), to the cathode. Single lines are phase boundaries; the double line is the salt bridge.

Worked example

Example 1: Analyse a silver–copper cell

A copper strip in 1.0 mol/L Cu(NO₃)₂ is connected to a silver strip in 1.0 mol/L AgNO₃ by a wire and a KNO₃ salt bridge. Identify the anode and cathode, write the half-reactions and the overall equation, state the direction of electron flow and ion flow, and write the cell notation.

Use the redox table from the lesson Predicting Redox Reactions and Investigating Them in the Lab: the strongest oxidizing agent present is Ag⁺; the strongest reducing agent is Cu(s). Ag⁺ is above Cu, so the reaction is spontaneous: Ag⁺ is reduced and Cu is oxidized.
Reduction happens at the cathode: the silver electrode. Ag⁺(aq) + e⁻ → Ag(s).
Oxidation happens at the anode: the copper electrode. Cu(s) → Cu²⁺(aq) + 2e⁻.
Overall (×2 for silver): Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s).
Electrons flow through the wire from copper (anode, −) to silver (cathode, +). In the salt bridge, NO₃⁻ moves toward the copper half-cell and K⁺ toward the silver half-cell.
Cell notation: anode on the left.

Cu(s) | Cu²⁺(aq) || Ag⁺(aq) | Ag(s). The copper anode loses mass and the silver cathode gains mass.

Worked example

Example 2: A cell with an inert electrode

Write the half-reactions and the overall reaction for the cell Zn(s) | Zn²⁺(aq) || Fe³⁺(aq), Fe²⁺(aq) | Pt(s). What does the platinum do?

The anode is on the left: zinc is oxidized. Zn(s) → Zn²⁺(aq) + 2e⁻.
The cathode is on the right: iron(III) is reduced to iron(II) at the platinum surface. Fe³⁺(aq) + e⁻ → Fe²⁺(aq) (×2).
Overall: Zn(s) + 2Fe³⁺(aq) → Zn²⁺(aq) + 2Fe²⁺(aq).

Platinum is an inert cathode: it passes electrons from the wire to Fe³⁺ ions but does not react, because neither iron ion is a solid that could act as an electrode.

Just for fun

Galvanic cells are named after Luigi Galvani, who made frog legs twitch with two different metals in the 1780s. He thought the electricity came from the frog. Volta proved it came from the metals, and then invented the battery to win the argument.

Hands-on activity

The salt-bridge test

You need: 2 small glasses, warm water, table salt, a strip of aluminium foil (about 2 cm × 10 cm, folded lengthwise for stiffness), a piece of bare copper wire (about 15 cm, from a hardware store), a paper towel, and a digital multimeter if you have one (a basic one costs about the same as a pizza). Optional: alligator-clip leads.

Fill both glasses about two-thirds full of water. Stir 1 tablespoon of salt into each.
Stand the foil strip in one glass and the copper wire in the other. Keep the dry tops of the metals out of the water so you can clip to them.
Set the multimeter to DC volts (2 V or 20 V range). Connect the black lead to the foil and the red lead to the copper. Record the reading. It should be zero or flicker near zero.
Twist a strip of paper towel into a rope, soak it in salt water, and drape it so one end is in each glass: a home-made salt bridge. Record the reading again.
Lift the bridge out. What happens? Put it back.

What to look for: with the bridge, you should see a steady reading of a few tenths of a volt; without it, the reading falls to about zero. The positive reading with the red lead on copper tells you copper is the cathode (+) and aluminium is the anode (−). The aluminium is being oxidized; at the copper, the main reduction is of oxygen dissolved in the water.

Explain in your notebook: why does the reading drop when the bridge is removed? (No path for ions, so charge builds up and the circuit is open.)

Safety: this is safe with table salt and low voltage. Do not use any other chemicals. Dry your hands before handling the meter, and pour the salt water down the drain afterwards.

Your turn

At which electrode does oxidation occur?

In a galvanic cell, which electrode is positive?

In the zinc–copper cell, which electrode gains mass, and why?

In which direction do electrons flow in the external circuit of Cu(s) | Cu²⁺(aq) || Ag⁺(aq) | Ag(s)?

In a cell with a KNO₃ salt bridge, toward which electrode do the nitrate ions move?

Write the cell notation for a cell with a magnesium anode in Mg²⁺(aq) and a copper cathode in Cu²⁺(aq).

Why is platinum used as the electrode in a Cl₂/Cl⁻ half-cell?

More practice: warm-up

What does "An Ox, Red Cat" stand for?

What does the double line || stand for in cell notation?

Do electrons travel through the salt bridge?

Name two common inert electrodes.

What is another name for a galvanic cell?

More practice: on your own

A cell is built from a nickel electrode in Ni²⁺(aq) and a silver electrode in Ag⁺(aq). Use the redox table to identify the anode and cathode, and write the half-reactions, overall equation and cell notation.

In the cell in the previous question, describe what happens to the mass of each electrode and the concentration of each metal ion over time.

A student builds a zinc–copper cell but forgets the salt bridge. The voltmeter reads 0 V. Explain.

Write the overall reaction for Pt(s) | H₂(g) | H⁺(aq) || Cu²⁺(aq) | Cu(s).

Why can't you put a copper electrode into a solution of AgNO₃ and use it as one half-cell of a galvanic cell?

Challenge

A salt bridge made of KCl is used in a cell whose cathode half-cell contains AgNO₃(aq). What problem will occur, and what would you use instead?

Draw (in words) a labelled diagram for Al(s) | Al³⁺(aq) || Pb²⁺(aq) | Pb(s): name each electrode and its sign, the solution in each beaker, the direction of electron flow, and the direction of cation and anion flow in a KNO₃ bridge. Then write the balanced overall equation.

In a zinc–copper cell, 0.020 mol of zinc dissolves. By how much do the masses of the two electrodes change? (Zn 65.38 g/mol, Cu 63.55 g/mol)

Check your understanding

Quiz

In a galvanic cell, the anode is:

What is the main job of the salt bridge?

In the cell Mg(s) | Mg²⁺(aq) || Ni²⁺(aq) | Ni(s), electrons flow:

Which electrode would be used for the half-cell Fe³⁺(aq) + e⁻ → Fe²⁺(aq)?

In a zinc–copper cell, what happens to [Cu²⁺] as the cell operates?

In a cell with a KNO₃ salt bridge, K⁺ ions move:

For parents and teachers

Big idea (F3.2, F3.3, F2.5): a galvanic cell separates the oxidation and reduction half-reactions, so electrons must travel through a wire to get from one to the other. That current is electricity. The salt bridge lets ions move so the circuit is complete.

How to help: Ask your teen to draw a zinc–copper cell from memory and label anode, cathode, signs, electron flow and ion flow. Check it against the diagram in this lesson. Then ask: "Where do electrons go? Where do ions go?" They should never say electrons go through the solution.

Common difficulties:

Thinking electrons travel through the salt bridge.
Mixing up the signs. In a galvanic cell the anode is negative.
Getting ion directions backwards: anions go to the anode, cations to the cathode.
Writing the cathode on the left in cell notation.

Safety: the salt-bridge activity uses only table salt, water and low voltage. It is safe; just keep the multimeter dry.