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F2.4 · Build a galvanic cell and measure its potential

Learn to build a galvanic cell and measure its potential through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Electrochemistry

Ontario Grade 12 Chemistry — F2.4

A battery can make a small bulb light or cause a voltmeter reading because a chemical reaction can transfer energy to moving electrons. In this lesson, you will connect that observation to the particles involved, assemble a simple galvanic cell, and measure the potential difference between its electrodes. A galvanic cell is a device in which a spontaneous chemical reaction produces an electrical potential.

What you will learn

Describe how a galvanic cell turns a spontaneous chemical reaction into an electrical potential.
Build a simple cell using two metal electrodes, their ion solutions, a salt bridge, and a voltmeter.
Identify the anode, cathode, and direction of electron flow in the cell.
Measure and report a cell potential with its polarity and units.

1. From particle changes to electrical energy

A metal strip placed in a solution containing ions of another metal may change. For example, a zinc strip in a copper(II) ion solution can gradually develop a coating of copper. This visible change is evidence that particles are changing form: zinc atoms become zinc ions, while copper(II) ions become copper atoms.

Recall two ideas from earlier chemistry. An atom that loses electrons is oxidized. An ion or atom that gains electrons is reduced. These changes happen together in a redox reaction. Electrons carry negative charge, so separating the two changes can make electrons travel through a wire rather than transfer directly at one surface.

In a galvanic cell, oxidation occurs at the anode. The anode is the electrode where oxidation happens. Reduction occurs at the cathode, the electrode where reduction happens. The electrons travel through the external wire from the anode to the cathode. A voltmeter connected across the electrodes measures their electrical potential difference, called the cell potential. Potential is measured in volts, symbol

\mathrm{V}

The solutions also need a way to maintain charge balance as the reaction proceeds. A salt bridge is a tube or strip containing an electrolyte, a substance that forms mobile ions in solution. The bridge lets ions move between the half-cells. A half-cell is one electrode and the solution that contains its ions. Ions in the bridge balance charge; electrons travel through the wire, not through the bridge.

\text{oxidation at anode} \qquad \text{reduction at cathode}

Oxidation is electron loss; reduction is electron gain.
The anode is the oxidation electrode, and the cathode is the reduction electrode.
Electrons travel through the external wire from anode to cathode.
The salt bridge allows ions to move and helps maintain charge balance.

2. Build a zinc–copper galvanic cell

A zinc–copper cell uses a zinc strip in a solution containing zinc ions and a copper strip in a solution containing copper(II) ions. The two solutions are separate. Connect the strips to the two terminals of a voltmeter with wires. Connect the solutions with a salt bridge containing a suitable inert electrolyte. Inert here means that the bridge ions do not take part in the cell’s redox reaction.

Use clean electrodes and make sure each strip contacts only its own solution. The salt bridge must touch both solutions, but its contents should not spill into them. Follow your teacher’s directions for handling solutions and disposing of them. Do not handle chemicals by tasting or touching them.

Set the voltmeter to measure direct-current voltage and choose a range that can display the expected reading. Connect the red lead to the copper electrode and the black lead to the zinc electrode. A positive reading means the red lead is at the higher potential. If the meter gives a negative reading, check the lead connections and electrode identities before interpreting it.

When the circuit is complete, the cell reaction can proceed. Zinc atoms at the zinc electrode lose electrons and enter the solution as zinc ions. The released electrons travel along the wire to the copper electrode. Copper(II) ions in the copper solution gain those electrons and form copper metal on the copper electrode. The cell potential is the voltmeter reading between the electrodes while they are connected to the meter.

Write down the electrode materials, the meter polarity, the reading, and its unit. A measured potential depends on the cell as assembled and the conditions during measurement. Do not call a classroom meter reading a standard potential unless the required standard conditions have been established.

The zinc electrode is the anode; the copper electrode is the cathode.
Connect the voltmeter across the two electrodes, with red at copper for a positive reading in this cell.
Record the measured value in volts and include which electrode is connected to each meter lead.

3. Represent the cell reaction

The particle model can be written as two half-reactions. At the zinc anode, each zinc atom loses two electrons. At the copper cathode, each copper(II) ion gains two electrons. The electrons cancel when the half-reactions are added, so the overall reaction conserves both atoms and net charge.

The equations show the substances involved, their states, and the electrons transferred. The symbol

(s)

means solid, and

(aq)

means dissolved in water. The charge on each ion is written as a superscript. In the overall reaction, the zinc atoms replace copper atoms in the solution: zinc ions enter solution and copper metal forms.

These equations help explain what the meter measures, but the equations alone do not give the actual reading for a particular setup. The voltmeter measures a potential difference. The sign depends on which electrode is connected to the red lead, and the magnitude is reported in volts.

\mathrm{Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-} \qquad \mathrm{Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)}

Half-reactions show electron loss and electron gain separately.
The overall reaction has no electrons because they cancel when the half-reactions are added.
A voltmeter reading is a measured potential difference, not a count of electrons.

4. Read, record, and report a measurement

A voltmeter compares the potential at its red lead with the potential at its black lead. In the zinc–copper setup, the red lead is connected to copper and the black lead to zinc. A positive display therefore reports that the copper electrode is at higher potential than the zinc electrode.

In the worked example, the meter reading is supplied as a hypothetical value for practice. It is not a claim that an experiment was performed. The task is to report what that reading means, with the correct polarity and units.

Use the digits shown by the meter and do not add extra decimal places. Include the unit because a number without a unit does not fully describe a potential. Also state the lead connections: this makes the sign understandable to another reader.

If a reading changes or is unstable, check that the wires and salt bridge make contact and that the voltmeter is set correctly. Record what you actually observe in a real investigation; do not replace an uncertain reading with an expected value.

E_{\text{cell}} = V_{\text{red lead}} - V_{\text{black lead}}

The red and black leads determine the sign of the displayed reading.
Report the meter’s displayed precision and include volts.
A supplied practice value is not an experimental result.

Worked example

Interpreting a meter reading

A zinc–copper cell is connected with the voltmeter’s red lead on the copper electrode and its black lead on the zinc electrode. For practice, suppose the display reads

+1.07\ \mathrm{V}

. State the measured cell potential and explain what the sign means.

Identify the leads
The red lead is connected to copper, and the black lead is connected to zinc. The meter reports the red-lead potential relative to the black-lead potential.
Interpret the sign
The positive sign means the copper electrode is at higher potential than the zinc electrode for these lead connections. It does not mean that the electrons travel toward the zinc electrode.
$V_{\text{Cu}} - V_{\text{Zn}} = +1.07\ \mathrm{V}$
Report the measurement
Keep the three significant digits shown in the supplied reading and include the unit. State the electrode connections so the polarity is clear.
$E_{\text{cell}} = 1.07\ \mathrm{V}$

Answer: The measured potential is

1.07\ \mathrm{V}

, with the copper electrode connected to the red lead and the zinc electrode connected to the black lead. The positive sign means copper is at higher potential than zinc.

Check: The value is a hypothetical practice reading, not a reported laboratory result. In this galvanic cell, electrons flow through the wire from zinc to copper.

Common mistakes and how to avoid them

Saying that electrons travel through the salt bridge.

Correction: Electrons travel through the external wire. Ions move through the salt bridge to help maintain charge balance.

Calling the cathode the oxidation electrode.

Correction: Oxidation occurs at the anode; reduction occurs at the cathode.

Ignoring which voltmeter lead is connected to each electrode.

Correction: Record the red and black lead connections. The sign of the reading is meaningful only with that information.

Reporting a classroom reading as a standard potential without checking the conditions.

Correction: Call it a measured cell potential unless standard conditions have been established.

Lesson summary

A galvanic cell uses a spontaneous redox reaction to produce an electrical potential.
Oxidation takes place at the anode; reduction takes place at the cathode.
Electrons flow through the wire from anode to cathode, while ions move through the salt bridge.
Connect a voltmeter across the electrodes, note its lead polarity, and report the reading in volts.

Check your understanding

Question 1

In a zinc–copper galvanic cell, which way do electrons travel through the external wire?

From zinc to copper
From copper to zinc
Through the salt bridge from zinc to copper
correctIndex: 0

Show answer and explanation

From zinc to copper

Zinc is oxidized at the anode and releases electrons. Those electrons travel through the wire to the copper cathode.

Question 2

Where does reduction occur in a galvanic cell?

At the anode
At the cathode
In the voltmeter
correctIndex: 1

Show answer and explanation

At the cathode

The cathode is the electrode where reduction, or electron gain, occurs.

Question 3

A meter’s red lead is on copper and its black lead is on zinc. What does a positive reading indicate?

The copper electrode is at higher potential than the zinc electrode.
The zinc electrode is at higher potential than the copper electrode.
Electrons travel through the salt bridge.
correctIndex: 0

Show answer and explanation

The copper electrode is at higher potential than the zinc electrode.

A voltmeter reading compares the red-lead potential with the black-lead potential. A positive reading means the red-lead electrode is at higher potential.

Key terms

Galvanic cell: A device in which a spontaneous chemical reaction produces an electrical potential.
Anode: The electrode where oxidation occurs.
Cathode: The electrode where reduction occurs.
Salt bridge: A connection containing mobile ions that helps maintain charge balance between half-cells.
Cell potential: The electrical potential difference between the two electrodes, measured in volts.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation F2.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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