Chemistry 11–12 · Year 11

Galvanic cells with a salt bridge: measuring and predicting cell potentials

Module 3: Reactive Chemistry; no 2025 code: the Chemistry 11-12 Syllabus (2025) keeps galvanic cells, reduction potential tables and redox half-equations, but places them in Year 12 under CH-12-04, a year later than this Year 11 entry, so from 2028 the topic moves out of Year 11.

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

A spontaneous redox reaction split into two half-cells drives electrons through a wire; the measured voltage is the difference between the two half-cells' reduction potentials.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

lead(II) nitrate is toxic and a reproductive hazard
silver nitrate stains and is corrosive
copper(II) sulfate harmful

Controls

the teacher or technician makes up the lead(II) nitrate solution from the solid and sets up the lead half-cell; the bench is washed down with dilute detergent solution afterwards
gloves for lead and silver solutions
solutions stay in their beakers; wastes collected
wash hands
eye protection

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions. CSIS Volume 2 Appendix F lists lead and lead salts (solid) among the chemicals to be used with caution, largely by science teachers, and says lead salts are toxic by inhalation and absorbed through the skin; CSIS D.3(a) asks for the minimum quantity that shows the effect, so the lead(II) nitrate is 20 mL at 0.10 mol/L, not 50 mL at 1.0 mol/L.

What you need

Strips (about 1 cm by 5 cm) of zinc, copper, iron, magnesium and lead, cleaned with emery paper; silver wire if available
1.0 mol/L solutions, 50 mL each, of zinc sulfate, copper(II) sulfate, iron(II) sulfate and magnesium sulfate; 0.10 mol/L lead(II) nitrate 20 mL (0.66 g of the salt), made up by the teacher or technician from the solid; 0.10 mol/L silver nitrate 20 mL
100 mL beakers, five
Salt bridge: filter-paper strips soaked in saturated potassium nitrate solution (or a U-tube of 1 mol/L KNO3 in agar)
Digital voltmeter reading to 0.01 V (high input impedance), leads with crocodile clips
0.10 mol/L copper(II) sulfate 50 mL for the concentration test

How to do it

Set up the zinc half-cell (zinc strip in zinc sulfate) and the copper half-cell (copper strip in copper(II) sulfate); connect them with a fresh salt-bridge strip and the voltmeter; record the voltage and which electrode is negative.
Predict the voltage of each remaining pair (zinc-iron, zinc-lead, iron-copper, magnesium-copper, copper-silver) from a table of standard reduction potentials, correcting the zinc-lead and copper-silver pairs for their 0.10 mol/L solutions with the Nernst equation, then measure each with a fresh salt bridge.
Remove the salt bridge from one working cell and record the voltage; replace it and record again.
Replace the 1.0 mol/L copper(II) sulfate with 0.10 mol/L in the zinc-copper cell and record the voltage change.
Tabulate predicted and measured voltages, the polarity and the difference; write the half-equations and overall equation for each cell.
Use the simulation to see the predicted voltage for any pair and for the concentration change.

What you should see

Standard reduction potentials (CRC Handbook, 25 degrees Celsius, 1 mol/L) predict zinc-copper 1.10 V, zinc-iron 0.31 V, zinc-lead 0.64 V, iron-copper 0.79 V, magnesium-copper 2.71 V and copper-silver 0.46 V, with the more active metal as the negative electrode; with the 0.10 mol/L silver nitrate and lead(II) nitrate used here the Nernst equation lowers the copper-silver prediction to 0.40 V and the zinc-lead prediction to 0.61 V. Measured values come out somewhat below these, magnesium furthest below because its oxide coating and its reaction with water lower its potential; a zinc-copper reading close to 1.1 V shows the set-up works. With the salt bridge removed the reading falls to near zero. Diluting the copper solution tenfold lowers the zinc-copper voltage by about 0.030 V (Nernst equation), to 1.07 V.

What changes

What you change: the pair of metals (and, in the last step, the copper ion concentration)
What you measure: cell voltage (V) and polarity
What you keep the same: solution concentrations at 1.0 mol/L (0.10 mol/L for the lead and silver solutions)
temperature
fresh salt bridge each time
clean electrodes

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The salt bridge carries electrons (it carries ions; electrons travel only in the wire).
A bigger electrode gives a bigger voltage (voltage depends on the metals and concentrations, not size).
The reduction potential of a metal changes when the half-equation is doubled (potentials are intensive and do not scale with coefficients).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-10 CH11/12-3 CH11/12-4
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 3, which it allows in any moduleCH11/12-6
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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