Lab, Le Châtelier & Equilibrium Shifts · AP Chemistry

CED Unit 7, Equilibrium (7–9% of the AP exam). In this lab you stress three real equilibria (by changing concentration and temperature) and watch each one shift. You will reason qualitatively (Le Châtelier) and finish with one quantitative piece: determining an equilibrium constant by colour.

Topics: 7.9 Le Châtelier's principle · 7.10 introduction to Q vs K · 7.1 / 7.6 equilibrium constants 1 double block

Background why colour tells you about equilibrium

A reversible reaction reaches equilibrium when the forward and reverse rates are equal, concentrations stop changing, but both reactions keep running. Le Châtelier's principle: stress an equilibrium and it shifts in the direction that partly relieves the stress.

Three equilibria today. Each has a coloured species, so a shift is visible as a colour change:

1 · Indicator (concentration stress)

HIn(aq) + H₂O(l) ⇌ In⁻(aq) + H₃O⁺(aq) HIn = yellow, In⁻ = blue (bromothymol blue)

Adding acid raises [H₃O⁺]; adding base removes H₃O⁺ via H₃O⁺ + OH⁻ → 2 H₂O.

2 · Cobalt complex (concentration + temperature stress)

Co(H₂O)₆²⁺(aq) + 4 Cl⁻(aq) ⇌ CoCl₄²⁻(aq) + 6 H₂O(l) left = pink, right = blue

Add concentrated HCl (raises [Cl⁻]); then heat and cool to test the temperature response.

3 · Iron thiocyanate (quantitative, Beer–Lambert)

Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) FeSCN²⁺ = deep amber/red

The deeper the colour, the more product, absorbance measures [FeSCN²⁺] directly.

Safety read before you start · goggles on

PPE (required the whole period): splash goggles, apron/lab coat, closed-toe shoes. Tie back long hair. Wash hands before leaving.

6.0 M HCl is corrosive: it burns skin, eyes, and clothing and gives off irritating fumes. Dispense at the fume hood / well-ventilated bench, keep it capped, never pipette by mouth. Flush any spill on skin with water for 5 min and tell Dr. Mulinta.
Heat the cobalt/HCl tubes only at the fume hood; concentrated HCl releases more fumes when hot.
0.1 M HCl and 0.1 M NaOH are irritants, rinse splashes off skin and eyes promptly.
Cobalt(II) chloride is toxic and a suspected carcinogen. No skin contact, no mouth contact; wash hands well. All cobalt waste goes to the labelled cobalt waste beaker, never down the sink.
AgNO₃ stains skin/clothes black (harmless but permanent for ~a week). Handle by the dropper only.
Hot-water bath / hot plate: the beaker and tongs are hot, move test tubes with tongs, not fingers. Keep the cord and water apart. The salted ice bath reaches about −5 °C; don't hold tubes in it bare-handed.
Glass: report any cracked test tube; do not use chipped glassware.

Waste: indicator and dilute acid/base solutions → sink with running water. All cobalt and all silver solutions → labelled waste beakers. Part 1 iron/thiocyanate → designated Part-1 waste bottle.

Materials & Equipment low-gear · Kathy pre-sets the bench

Per group (small): 4 × test tubes (18×150 mm) + rack · 1 × 10 mL graduated cylinder · 1 × 25 mL graduated cylinder · 2–3 × small beakers (100 mL, clean/dry) · 1 × 400 mL beaker (water bath) · 1 × 400 mL beaker (ice bath) · medicine dropper · tongs · test-tube brush · grease pencil or labels.

Shared at the table / front: hot plate + boiling chips · spectrophotometer + cuvettes (one set for the class) · ice and table salt · distilled water bottle.

Reagents (dropper bottles unless noted): bromothymol blue indicator · 0.1 M HCl · 0.1 M NaOH · 0.2 M CoCl₂ · 6.0 M HCl · 0.1 M AgNO₃ · 0.200 M Fe(NO₃)₃ (~30 mL) · 0.0020 M KSCN · 0.0010 M FeSCN²⁺ standard (by the spectrophotometer).

Lab tech Kathy will pre-set: the 6.0 M HCl at the fume hood with its own dropper/cylinder; the 0.0010 M FeSCN²⁺ standard already labelled beside the spectrophotometer with the spec warmed up and zeroed on distilled water; the cobalt and silver waste beakers labelled at each table; boiling chips and bagged ice + salt distributed. Everything else is everyday glassware students set up themselves.

Procedure: Part A · Le Châtelier (qualitative) CED 7.9 · do this first

Start your water bath now so it is boiling later: half-fill a 400 mL beaker with tap water, add 2–3 boiling chips, set the hot plate on high.

A1 · Indicator: concentration stress

Put ~15 mL tap water in a small beaker; add 5–8 drops bromothymol blue. Split evenly into two test tubes. Record the starting colour. (If your indicator starts yellow, add 1 drop of 0.1 M NaOH until green before step 3.)
Leave tube 1 as the control.
To tube 2, add 0.1 M HCl drop by drop, swirling, until the colour changes. Record drops and the new colour.
To the same tube 2, now add 0.1 M NaOH drop by drop until the colour changes again. Record drops and colour. Rinse both tubes (down the sink).

A2 · Cobalt: concentration stress

Using a graduated cylinder, put 1.0 mL of 0.2 M CoCl₂ into three test tubes. Record the starting colour. Leave tube 1 as the control.
At the fume hood, measure 2.0 mL of 6.0 M HCl into tube 2; repeat for tube 3. Record the colour of tubes 2/3 vs the control.

A3 · Cobalt: temperature stress

Make a cold bath: 400 mL beaker with ice + a spoon of table salt + a little tap water (≈ −5 °C).
Place tubes 2 and 3 in the boiling water bath. Watch for a colour change; record it.
When tube 2 has fully changed, lift it with tongs into the ice bath. Record what happens.
When tube 3 has fully changed, use tongs to transfer ~1 mL of the solution into the 4th test tube. Let it sit a few seconds, then add 2–3 drops of 0.1 M AgNO₃ and shake. Record the precipitate colour and the solution colour change. All cobalt + silver waste → labelled waste beakers.

Procedure: Part B · K_eq of FeSCN²⁺ (quantitative) CED 7.1 / 7.6 · Beer–Lambert

B1 · Make two iron solutions

Solution A: rinse the 25 mL cylinder with a little 0.200 M Fe(NO₃)₃, then measure exactly 6.0 mL. Dilute to the 25.0 mL mark with distilled water; pour into a clean dry beaker. Calculate [Fe³⁺] in A.
Solution B: rinse and measure exactly 2.5 mL of 0.200 M Fe(NO₃)₃; dilute to 25.0 mL; pour into a second clean dry beaker. Calculate [Fe³⁺] in B. (Keep A and B separate!)
Measure 5.0 mL of solution A into test tube 1, and 5.0 mL of solution B into test tube 2.
Rinse the 10 mL cylinder with a little 0.0020 M KSCN, then add exactly 5.0 mL of KSCN to each tube. The amber FeSCN²⁺ colour appears.

B2 · Measure colour

On the spectrophotometer (already zeroed on distilled water), read the absorbance of tube 1, tube 2, and the 0.0010 M FeSCN²⁺ standard.
Record all three absorbances. Iron/thiocyanate waste → Part-1 waste bottle.

Data Tables fill in as you go

Part A, qualitative observations

System / stress applied	Before	After	# drops / notes
A1 indicator + 0.1 M HCl
A1 then + 0.1 M NaOH
A2 cobalt control (tube 1)
A2 cobalt + 6.0 M HCl (tubes 2/3)
A3 tube 2 → hot bath
A3 tube 2 → ice bath
A3 tube 3 (hot) + AgNO₃

Part B, quantitative

Quantity	Tube 1 (sol. A)	Tube 2 (sol. B)
[Fe³⁺] in diluted solution (before mixing)
[Fe³⁺]_initial after mixing with KSCN
[SCN⁻]_initial after mixing
Absorbance (sample)
Absorbance of 0.0010 M standard
[FeSCN²⁺]_eq
K_eq

ICE tables (mol·L⁻¹; ignore sig figs):

Tube 1 Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺	Fe³⁺	SCN⁻	FeSCN²⁺
I
C
E

Tube 2 Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺	Fe³⁺	SCN⁻	FeSCN²⁺
I
C
E

Analysis Questions answer in complete sentences

Part A: Le Châtelier reasoning

When HCl was added to the bromothymol blue, it turned yellow. Explain the shift using Le Châtelier's principle, name what stress was applied and which way HIn + H₂O ⇌ In⁻ + H₃O⁺ shifted.
Then NaOH turned it blue. Explain this using the forward and reverse rates: what did OH⁻ do to [H₃O⁺], and how did that change the rates so the system shifted toward In⁻?
Looking only at the cobalt control (no added HCl), it is pink, meaning Co(H₂O)₆²⁺ dominates. Is K_eq for Co(H₂O)₆²⁺ + 4Cl⁻ ⇌ CoCl₄²⁻ + 6H₂O greater than 1, less than 1, or ≈ 1? Give your reasoning.
Adding 6.0 M HCl turned tubes 2/3 purple/blue. Explain the shift with Le Châtelier's principle (name the stressed species).
From heating (blue) and cooling (pink), is the forward reaction endothermic or exothermic? Treat heat as a reactant or product and justify from your observations.
Adding AgNO₃ to the hot blue solution gave a precipitate and the solution snapped back to pink. Name the precipitate and explain how forming it caused the colour change.

Part B: calculations (show work)

Show how you found [Fe³⁺] and [SCN⁻] initial in each tube, accounting for every dilution (the 6.0 mL or 2.5 mL → 25 mL dilution, then the 5.0 mL + 5.0 mL mixing).
Use Beer–Lambert (absorbance is proportional to concentration, same cuvette/wavelength) to find [FeSCN²⁺]_eq = (A_sample / A_standard) × 0.0010 M for each tube. Show the ratio.
Fill the ICE tables and calculate K_eq = [FeSCN²⁺]/([Fe³⁺][SCN⁻]) for each tube.
Should the two K_eq values match, even though the initial concentrations differed? Explain what that says about an equilibrium constant.
Your two values probably differ. Name the two biggest sources of error in this measurement and state which way each would push K.

Le Châtelier & Equilibrium Shifts