E. Chemical Systems and Equilibrium · Chemical EquilibriumInvestigation: Testing Le Châtelier's Principle in the Lab
E2.2 E3.3 A1.1 A1.2 A1.4 A1.5 A1.6 A1.8 A1.10 A1.11About 75 minutes
Learning goals- I can write a testable prediction for an equilibrium using Le Châtelier's principle or Q.
- I can plan safe procedures for changing concentration and temperature, and identify hazards using WHMIS information.
- I can record qualitative observations in an organized table.
- I can explain each observation in terms of the direction of the shift, and evaluate my predictions.
Words to know
qualitative observation
An observation that describes a quality (colour, cloudiness, formation of a solid) rather than measuring a number.
control
A sample left unchanged so that the changed samples can be compared with it.
WHMIS
The Workplace Hazardous Materials Information System, Canada's system of labels, pictograms and safety data sheets for hazardous products.
safety data sheet (SDS)
A document that gives the hazards, safe handling, first aid and disposal information for a chemical product.
precipitate
An insoluble solid that forms when two solutions are mixed.
Can you make it change colour on command?
A solution of cobalt(II) chloride in water can be pink, violet or blue, and it switches between them when you add certain chemicals or warm it up. In this lesson you will plan an investigation like the one done in Grade 12 labs, predict each colour change with Le Châtelier's principle, look at a full set of results, and decide whether the evidence supports the predictions. Then you will run a safe model version at home.
The system
In water, cobalt(II) ions are surrounded by water molecules, forming the pink complex ion [Co(H₂O)₆]²⁺. Chloride ions can replace the water to form the blue complex ion [CoCl₄]²⁻:
[Co(H₂O)₆]²⁺(aq) (pink) + 4 Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) (blue) + 6 H₂O(l) ΔH > 0 (endothermic forward)
A solution with similar amounts of both looks violet. The colour tells you which side is favoured, so the colour is your evidence.
Planning the investigation
Question: How do changes in [Cl⁻], the amount of water, the removal of Cl⁻ and the temperature affect this equilibrium?
Predictions (with reasons):
- Adding concentrated HCl (a source of Cl⁻): a reactant is added → shift right → blue.
- Adding water: the solution is diluted; the side with more dissolved particles (left: 5 ions versus 1) is favoured → shift left → pink.
- Adding silver nitrate, AgNO₃: Ag⁺ removes Cl⁻ as solid AgCl → shift left → pink, with a white precipitate.
- Hot-water bath: heat is a reactant (endothermic forward) → shift right → blue.
- Ice-water bath: removing heat → shift left → pink.
Variables: the independent variable is the change made to each tube; the dependent variable is the colour; controlled variables include the volume of starting solution in each tube and the drops added. One tube is kept unchanged as a control for colour comparison.
Procedure outline: place 2 mL of the violet starting solution in each of six test tubes. Keep tube 1 as the control. To tube 2, add concentrated HCl drop by drop; to tube 3, add water a few millilitres at a time; to tube 4, add a few drops of AgNO₃ solution; place tube 5 in a hot-water bath (about 70 °C) and tube 6 in an ice-water bath for 3 minutes. Compare each tube with the control against a white background.
Safety (WHMIS): read the SDS for each chemical before the lab.
- Cobalt(II) chloride is a health hazard (it may cause cancer and allergic reactions): wear gloves, avoid dust and skin contact.
- Concentrated hydrochloric acid is corrosive and gives off irritating fumes: use it in a fume hood, with goggles and gloves; add it slowly.
- Silver nitrate is corrosive and stains skin dark.
- Hot water baths can scald.
- All cobalt and silver wastes go in the labelled heavy-metal waste container, not down the drain.
This lab is done only in a school laboratory with a teacher. Never try it at home.
Sample results
Here are typical observations from this investigation.
| Tube | Change | Observation | Shift | Prediction supported? |
|---|
| 1 | none (control) | violet | — | — |
| 2 | conc. HCl added | turned blue | right | yes |
| 3 | water added | turned pink | left | yes |
| 4 | AgNO₃ added | white precipitate; solution pink | left | yes |
| 5 | hot-water bath | turned blue | right | yes |
| 6 | ice-water bath | turned pink | left | yes |
Worked exampleExample 1: Analysing a result
Explain the result in tube 4 fully, including the white solid.
- Silver ions react with chloride ions: Ag⁺(aq) + Cl⁻(aq) → AgCl(s), a white precipitate.
- This removes Cl⁻, which is a reactant in the cobalt equilibrium, so Q = [CoCl₄²⁻] ÷ ([Co(H₂O)₆²⁺][Cl⁻]⁴) becomes larger than K.
- The system shifts left to replace some Cl⁻, converting blue [CoCl₄]²⁻ into pink [Co(H₂O)₆]²⁺.
Removing Cl⁻ as solid AgCl shifts the equilibrium left; the solution turns pink. The prediction is supported.
Worked exampleExample 2: Using temperature results to find the sign of ΔH
Suppose you did not know the sign of ΔH. How do tubes 5 and 6 let you decide?
- Heating turned the solution blue: a shift right. Le Châtelier says heating favours the endothermic direction.
- So the forward reaction (toward blue) must be endothermic: ΔH > 0.
- Cooling turned it pink (shift left), consistent with this.
- Because the temperature changed, K changed too: K is larger at higher temperature for this reaction.
The forward reaction is endothermic (ΔH > 0), and K increases with temperature. Colour changes with temperature reveal the sign of ΔH.
Evaluating the investigation
- Sources of error: colour judgments are subjective (different people describe violet differently); lighting affects colour; the tubes cool or warm as soon as they are removed from the baths. Improvements: compare each tube with the control against the same white background, have two people record colours, photograph tubes, or measure colour intensity with a colorimeter (which turns a qualitative result into numbers).
- Conclusion: all five predictions were supported, so the evidence is consistent with Le Châtelier's principle. A conclusion should refer back to the question and to the scientific explanation (shifts caused by changes in Q, or in K for temperature).
Writing a strong prediction
Use the form "If ... then ... because ...". For example: If concentrated HCl is added, then the solution will turn blue, because adding Cl⁻ (a reactant) makes Q < K, so the equilibrium shifts right toward blue [CoCl₄]²⁻.
Just for funCobalt chloride is so sensitive to water that it is used in humidity indicator cards and moisture-test papers: blue when dry, pink when damp. It is the original "mood ring" of the chemistry lab.
Hands-on activityHome inquiry: testing predictions with the straw model
You need: the two tall straight-sided glasses, ruler and two straws of different widths from the dynamic-equilibrium lesson, a third straw that is wider than both (or a second wide straw), water, paper.
Question: Does the straw model behave the way Le Châtelier's principle predicts?
- Run the model (wide straw from A to B, narrow straw from B to A) from 12 cm in A until the heights stop changing. Record the final heights and the ratio h(B) ÷ h(A) (this ratio plays the part of K).
- Stress 1 (add a reactant): pour enough water into glass A to raise it by 3 cm. Predict: which way will water flow, and will the final ratio change? Run until steady; record.
- Stress 2 ("change the temperature"): replace the wide straw with an even wider one. Predict what happens to the ratio. Run until steady; record.
- Make a table like the sample results table above: change, observation, "shift", prediction supported?
What to look for: after stress 1, water flows from A to B (a "shift right"), both heights end up higher, and the ratio h(B) ÷ h(A) returns to the same value: adding a substance does not change K. After stress 2, the ratio changes, just as K changes with temperature, because you changed the "rates" themselves rather than the amounts. Sources of error: reading the ruler at an angle, and different depths of straw dipping.
Your turn
1Write an "If ... then ... because ..." prediction for adding water to the cobalt(II) chloride equilibrium.
If water is added, then the solution will turn more pink, because dilution lowers all ion concentrations and favours the side with more dissolved particles (Q > K), shifting the equilibrium left toward pink [Co(H₂O)₆]²⁺.
2What is the purpose of tube 1 in the investigation?
It is a control: an unchanged sample to compare the colours of the other tubes against.
3Name two WHMIS hazards in this investigation and one precaution for each.
Concentrated HCl is corrosive (use goggles, gloves and a fume hood; add slowly). Cobalt(II) chloride is a health hazard (gloves, avoid skin contact, dispose of in heavy-metal waste). Silver nitrate is corrosive and stains skin (gloves, goggles).
4A student adds solid NaCl to the violet solution and it turns blue. Explain.
NaCl dissolves to give Cl⁻, a reactant. Adding Cl⁻ makes Q < K, so the equilibrium shifts right toward blue [CoCl₄]²⁻. (Na⁺ is a spectator.)
5Why is colour a good dependent variable here, and what is its main weakness?
Each complex ion has a distinct colour, so colour directly shows which side is favoured. Its weakness is that judging colour by eye is subjective and qualitative.
6In the iron(III) thiocyanate equilibrium, Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺ (red), predict the observation when a few crystals of KSCN are added, and give the reason.
The red colour deepens, because adding SCN⁻ (a reactant) shifts the equilibrium right.
More practice: warm-up
1What colour is [CoCl₄]²⁻?
2What colour is [Co(H₂O)₆]²⁺?
3Is the forward reaction (pink to blue) endothermic or exothermic?
4What is the white precipitate formed when AgNO₃ is added?
More practice: on your own
1Identify the independent, dependent and controlled variables in the temperature part of the investigation.
Independent: temperature of the bath. Dependent: colour of the solution. Controlled: volume and concentration of solution in each tube, time in the bath, background and lighting used to judge colour.
2A student's tube 5 was blue in the hot bath but violet by the time it was compared with the control. Explain and suggest an improvement.
The tube cooled once removed from the bath, so the equilibrium shifted back left. Compare colours while the tube is still in the bath, or photograph it immediately.
3Explain why adding water shifts the cobalt equilibrium even though water is a pure liquid not included in K.
Water itself is not in K, but adding it dilutes all the dissolved species. Q = [CoCl₄²⁻] ÷ ([Co(H₂O)₆²⁺][Cl⁻]⁴): halving every concentration makes the denominator fall by 2⁵ = 32 and the numerator by 2, so Q becomes 16 times larger than K, and the system shifts left.
4How could a colorimeter improve this investigation?
It measures how much light of a chosen colour is absorbed, giving a number related to concentration. This replaces subjective colour judgments with quantitative data and allows K to be estimated.
5Write a conclusion for the investigation in two or three sentences.
Example: All five predicted colour changes were observed, so the evidence supports Le Châtelier's principle for the cobalt(II) chloride equilibrium. Adding Cl⁻ or heat shifted it right (blue); adding water, removing Cl⁻ with Ag⁺, or cooling shifted it left (pink). The temperature results show that the forward reaction is endothermic.
Challenge
1A student predicts that adding a catalyst would change the colour of the cobalt solution. Design a short test and predict the result.
Prepare two identical violet tubes; add the proposed catalyst to one and an equal volume of water to the other (to keep dilution the same). Compare colours. Prediction: no colour difference, because a catalyst speeds up both directions equally and does not shift an equilibrium.
2Explain why the result for tube 3 (water added) is weaker evidence than tube 2, and how you could make it stronger.
Adding water also simply dilutes the colour, so a paler colour could come from dilution alone. Evidence of a shift is the change of hue from violet to pink, not just lightness. To strengthen it, dilute a control tube of pure blue solution and pure pink solution the same amount, or use a colorimeter at two wavelengths.
3The cobalt equilibrium is used in moisture indicators. Using Le Châtelier's principle, explain why the indicator is blue in a dry container and pink in humid air.
In dry conditions water is scarce, which favours the side with fewer water molecules attached: blue cobalt chloride forms (in the solid indicator, the blue form has lost its water). In humid air, water is abundant, shifting the equilibrium toward the pink water-bound form [Co(H₂O)₆]²⁺.
Quiz
1. Adding concentrated HCl to a violet cobalt(II) chloride solution turns it:
2. The cobalt solution turns blue in a hot-water bath. This shows that the forward reaction is:
3. Why does adding AgNO₃ turn the solution pink?
4. What is the main purpose of the unchanged control tube?
5. Which is the best improvement to reduce subjective error in colour observations?
6. Where should cobalt(II) chloride waste go?
Big idea (E2.2, A1): an equilibrium investigation tests Le Châtelier predictions with careful, safe procedures, a control, organized observations and an evaluation of error. The colour of the cobalt complex ions shows which side is favoured.
How to help: the real lab uses hazardous chemicals and is done only at school. At home, run the straw-model inquiry with your teen and ask them to write predictions in "If ... then ... because ..." form before each step.
Common difficulties:
- Predictions without reasons.
- Explaining the water-addition result as "water is a reactant" (it is dilution that matters).
- Forgetting that temperature changes K while the other stresses do not.
- Vague observations ("it changed"); encourage specific colours and comparisons with the control.
Safety: cobalt(II) chloride, concentrated HCl and silver nitrate are school-lab chemicals only.