Le Chatelier Virtual Lab: Color Shifts, Qc and Kc

This Le Chatelier virtual lab follows the AP Chemistry guided-inquiry lab in which students stress colored equilibria and explain each color change. Students work with two test tubes. In the first, Fe³⁺ and SCN⁻ form blood-red FeSCN²⁺; students add reactants, remove one with a precipitate, and dilute. In the second, pink [Co(H₂O)₆]²⁺ and blue [CoCl₄]²⁻ respond to heating and cooling. After every change the simulation shows the concentrations, and it compares Qc with Kc. Students leave with a data table, a rule for each kind of stress, and evidence for the sign of ΔH.

Curriculum links

• AP Chemistry Unit 7, topics 7.9 Introduction to Le Châtelier's Principle (TRA-8.A) and 7.10 Reaction Quotient and Le Châtelier's Principle (TRA-8.B), and the AP Chemistry guided-inquiry lab on Le Chatelier's principle.
• NGSS HS-PS1-6: refine the design of a chemical system by changing conditions to shift an equilibrium.
• It also fits the equilibrium topics of IB Chemistry and A-level Chemistry.

Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Show the red iron thiocyanate tube on the projector and ask:

"This red mixture is at equilibrium: Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺. You add a drop of silver nitrate. Ag⁺ removes SCN⁻ as a white solid. What happens to the red color?"

Many students say "nothing, silver isn't in the equation". Collect the votes and move on.

Method in the simulation

Part A: iron thiocyanate (Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺)

1. Open the first tab. Keep the starting values (Fe³⁺ 2.0 mM, SCN⁻ 1.5 mM). Wait about 10 seconds, until the readout says the system is at equilibrium. Record the three concentrations and the color.
2. Press Add Fe³⁺ once. Watch the gauge: Qc drops below Kc, then returns. Record the new equilibrium.
3. Press Reset, wait for equilibrium, then press Add AgNO₃ once. Record.
4. Press Reset, wait, then press Add water once (5 mL). Record. Calculate what [FeSCN²⁺] would be from dilution alone (× 10/15).

Part B: cobalt chloride ([Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O)

5. Open the second tab. Keep the starting values (Cl⁻ 6 M, bath 25 °C). Wait for equilibrium and record both cobalt concentrations and Kc.
6. Move Water-bath temperature to 60 °C. Wait until the thermometer reads 60 °C and Kc stops changing (about 40 seconds). Record.
7. Move the bath to 0 °C and record again once Kc stops changing (up to a minute).

Stress	Color after	Qc vs Kc just after the stress	Direction of shift	Key concentration (mM)
None (start)
+ Fe³⁺
+ AgNO₃
+ water
Bath 25 → 60 °C
Bath 60 → 0 °C

Expected results

All values are from the simulation with the settings above.

Part A step	[Fe³⁺] (mM)	[SCN⁻] (mM)	[FeSCN²⁺] (mM)	Color
Equilibrium	1.710	1.210	0.290	orange-red
+ Fe³⁺	2.577	1.091	0.394	deeper red
+ AgNO₃	1.791	0.796	0.199	paler, white AgSCN
+ water	1.190	0.857	0.143	paler

Qc returns to Kc = 140 after every stress. Dilution alone would leave 0.193 mM FeSCN²⁺. The measured 0.143 mM shows a shift to the side with more dissolved particles.

Part B bath	[Co(H₂O)₆]²⁺ (mM)	[CoCl₄]²⁻ (mM)	Kc	Color
0 °C	48.0	2.0	3.16·10⁻⁵	pink
25 °C	39.9	10.1	2.00·10⁻⁴	violet
60 °C	16.8	33.2	1.66·10⁻³	blue

Adding or removing a substance changes Qc but never Kc. Only temperature changes Kc. Heating raises Kc about 8.3 times and turns the tube blue, so the forward reaction is endothermic.

Questions for students

1. (Prediction, asked again after the lab) You add AgNO₃ to the red equilibrium mixture. What happens to the red color?
2. Which variables must stay the same between the 25 °C and 60 °C cobalt readings?
3. At the starting values, what is [FeSCN²⁺] at equilibrium?
4. By what factor does Kc for the cobalt system rise from 25 °C to 60 °C?
5. Is the forward cobalt reaction exothermic or endothermic? Explain using the color and Kc.

Answers for teachers: (1) It gets paler: removing SCN⁻ makes Qc > Kc, so FeSCN²⁺ breaks up. (2) The amount of Cl⁻ and the volume: add no HCl or water between readings. (3) 0.290 mM. (4) About 8.3 (1.66·10⁻³ ÷ 2.00·10⁻⁴; accept 8.0–8.6). (5) Endothermic: heating turns the tube blue and raises Kc, so the equilibrium shifts forward to absorb the added heat.

Common misconceptions

• "Adding Fe³⁺ uses up all the extra Fe³⁺." The shift only partly opposes the change: [Fe³⁺] still ends higher (2.577 vs 1.710 mM).
• "Kc changes when you add a reactant." The gauge shows Qc moving and Kc fixed at 140. Only temperature moves Kc.
• "Dilution just makes the color weaker." It also shifts the equilibrium, so the color fades more than the dilution factor alone.

Extension

• Check Kc yourself: students calculate Qc from the three readings after each stress, for example 0.394 ÷ (2.577 × 1.091) × 1000 = 140 L/mol.
• Vary the start: set different starting values for Fe³⁺ and SCN⁻ on the class link. The equilibrium concentrations change, but Qc settles at 140 every time.

FAQ

Why does equilibrium take several seconds?

Time is slowed down so students can watch the shift. In a real test tube these equilibria settle almost instantly.

Are the cobalt Kc and ΔH real values?

They are illustrative values chosen to give the observed colors. The direction of every shift is right, but don't compare the cobalt numbers with a data book.

Should I still run the wet lab?

If you can, yes: real colors are memorable. Cobalt(II) chloride is a hazardous substance, though, so many schools use the simulation for the cobalt part and keep the iron thiocyanate tubes for the bench.

Related simulations and guides

Chemical equilibrium – N₂O₄ ⇌ 2NO₂ shift Reaction energy diagram – exothermic vs endothermic

For more chemistry activities, see interactive chemistry lesson ideas. The prediction in question 1 works well as a Predict–Observe–Explain cycle: see predict, observe, explain with simulations.