Quick answer: At chemical equilibrium the forward and reverse reaction rates are equal, so concentrations stay constant. This AP Chemistry lab on chemical equilibrium gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can the equilibrium constant stay the same when the starting concentrations change? You mix iron(III) and thiocyanate ions, which form the blood-red complex FeSCN²⁺, and measure how much of it forms. Then you disturb the equilibrium and explain each colour change with Q and K.
Level: AP Chemistry · Curriculum: AP Chem Unit 7
Aim
Determine Kc for Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) from four mixtures, and observe how the equilibrium responds to disturbances.
Equipment
- Fe(NO₃)₃ solutions in dilute nitric acid, KSCN solution, 0.1 M NaOH
- Test tubes and rack, graduated pipettes, colorimeter with cuvettes
- Hot-water bath (about 60 °C) and ice bath
Safety
Wear eye protection and gloves. Iron(III) nitrate in nitric acid is acidic and stains; potassium thiocyanate is harmful if swallowed and must not be mixed with strong acid. Wipe spills and wash hands. Dispose of solutions as directed by your teacher.
Method
- Prepare four tubes with 1.00 × 10⁻³ M SCN⁻ (after mixing) and Fe³⁺ of 1.00, 2.00, 3.00 and 4.00 × 10⁻³ M (after mixing).
- Read the colorimeter and use the calibration to find [FeSCN²⁺] at equilibrium.
- To a fresh tube of the equilibrium mixture, add one drop of Fe³⁺, then SCN⁻, then 0.1 M NaOH (removes Fe³⁺ as brown Fe(OH)₃); note the colour each time.
- Warm one tube in the hot bath and cool another in ice.
Results
| Tube | [Fe³⁺]₀ (M) | [SCN⁻]₀ (M) | [FeSCN²⁺] at equilibrium (M) |
|---|---|---|---|
| 1 | 1.00 × 10⁻³ | 1.00 × 10⁻³ | 1.8 × 10⁻⁴ |
| 2 | 2.00 × 10⁻³ | 1.00 × 10⁻³ | 3.1 × 10⁻⁴ |
| 3 | 3.00 × 10⁻³ | 1.00 × 10⁻³ | 4.1 × 10⁻⁴ |
| 4 | 4.00 × 10⁻³ | 1.00 × 10⁻³ | 4.8 × 10⁻⁴ |
| Disturbance | Observation |
|---|---|
| Add Fe³⁺ | Deeper red |
| Add SCN⁻ | Deeper red |
| Add NaOH | Paler, brown solid forms |
| Warm | Paler red |
| Cool | Deeper red |
Analysis questions
Question 1 K expression
Write the expression for Kc and give its units.Question 2 Tube 1
Complete an ICE table for tube 1 and calculate Kc.Question 3 Other tubes
Calculate Kc for tubes 2, 3 and 4.Question 4 Is K constant?
Calculate the mean and the range of Kc and comment on whether it is constant.Question 5 Explaining the disturbances
Explain the effect of adding Fe³⁺ and NaOH using Le Chatelier’s principle and Q.Question 6 Temperature
Use the colour change on warming to deduce the sign of ΔH for the forward reaction and the effect on K.Question 7 Dilution
A tube at equilibrium is diluted with an equal volume of water. Predict the shift using Q.
