Chemistry 11–12 · Year 12

Le Chatelier's principle with iron(III) thiocyanate: adding and removing ions, and heating

Module 5: Equilibrium and Acid Reactions

PracticalMedium risk

School laboratory, not for home

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

The idea

Adding a reactant deepens the colour and removing one fades it, showing that the position of equilibrium moves to counter each change in concentration.

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

silver nitrate corrosive and staining
sodium hydroxide corrosive
iron(III) nitrate irritant
thiocyanate harmful if swallowed; never mix with acid (releases toxic gas)

Controls

dropper bottles only
no acids at this bench
gloves for silver nitrate
eye protection
silver waste collected

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.

What you need

0.1 mol/L iron(III) nitrate (or chloride) 10 mL and 0.1 mol/L potassium thiocyanate 10 mL
Deionised water 200 mL, 250 mL beaker
Test tubes, six, in a rack; dropper bottles of 0.1 mol/L iron(III) nitrate, 0.1 mol/L potassium thiocyanate, 0.5 mol/L silver nitrate, 1 mol/L sodium hydroxide, 0.5 mol/L sodium hydrogen phosphate
Hot and iced water baths, white card

How to do it

Make the stock equilibrium mixture: 2 mL of each 0.1 mol/L solution into 200 mL water; the solution is pale orange-red.
Pour 10 mL into each of six tubes; keep tube 1 as the reference.
Tube 2: add five drops of iron(III) nitrate. Tube 3: five drops of potassium thiocyanate. Tube 4: five drops of silver nitrate. Tube 5: five drops of sodium hydroxide. Record each colour against the card and relative to tube 1.
Tube 6: warm in the hot bath for one minute, record, then cool in ice and record.
For each tube state which ion's concentration changed, the direction of the shift in Fe3+ + SCN- reversible [FeSCN]2+, and the sign of the enthalpy change deduced from tube 6.
Repeat tube 4 with phosphate instead of silver nitrate and explain the fading in terms of a competing complex.

What you should see

Tubes 2 and 3 turn a deeper blood-red than the reference (added Fe3+ or SCN- pushes formation of [FeSCN]2+). Tube 4 fades to pale yellow with a white precipitate of silver thiocyanate (SCN- removed). Tube 5 fades with a rust-brown precipitate of iron(III) hydroxide (Fe3+ removed). Tube 6 fades on warming and deepens on cooling, so the forward reaction is exothermic. Phosphate fades the colour without a precipitate (it binds Fe3+ as a colourless complex).

What changes

What you change: the ion added or removed, or the temperature
What you measure: colour intensity of the mixture
What you keep the same: volume in each tube
drops added
reference tube kept beside each test

Common misconceptions

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

Adding more thiocyanate makes the solution more concentrated, that is all (the colour deepens because the equilibrium shifts to make more complex).
Removing an ion stops the forward reaction (both reactions continue; the position moves to replace some of the removed ion).
Heating deepens all coloured reactions (this one fades; the direction depends on the sign of delta H).

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)CH12-12 CH11/12-4 CH11/12-5
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-01 CH-12WS-04 CH-12WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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