Quick answer: Chemical equilibrium is reached when forward and reverse reactions occur at the same rate, so concentrations stay constant. This Chemistry 11-12 lab on chemical equilibrium gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
This lab uses a colorimeter to find the equilibrium constant of the iron(III) thiocyanate reaction. You will turn absorbance readings into concentrations, build ICE tables for four mixtures, compare the four values of Kc and judge the effect of the assumption made for the standard.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R2.3
Aim
Find the equilibrium constant Kc for Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq) by measuring the absorbance of the red complex in four equilibrium mixtures.
Equipment
- 0.0200 mol/dm³ iron(III) nitrate in 0.10 mol/dm³ nitric acid, 0.200 mol/dm³ iron(III) nitrate in 0.10 mol/dm³ nitric acid, 0.00200 mol/dm³ potassium thiocyanate, 0.10 mol/dm³ nitric acid
- colorimeter with a blue-green filter and matched cuvettes
- two 10 cm³ graduated pipettes or syringes, five test tubes, labels, safety goggles and gloves
Safety
Wear safety goggles and gloves. Dilute nitric acid and iron(III) nitrate are irritants, and potassium thiocyanate is harmful if swallowed or mixed with strong acid. Wash spills from skin at once with plenty of water. Dispose of all solutions as directed by your teacher.
Method
- Prepare the standard tube: 5.0 cm³ of 0.200 mol/dm³ iron(III) nitrate + 1.0 cm³ of thiocyanate + 4.0 cm³ of nitric acid. Almost all of the thiocyanate becomes the red complex.
- Prepare four test tubes: 5.0, 4.0, 3.0 and 2.0 cm³ of 0.0200 mol/dm³ iron(III) nitrate, each with 2.0 cm³ of thiocyanate, and add nitric acid to make 10.0 cm³.
- Zero the colorimeter with a tube of nitric acid. Measure the absorbance of the standard and of each mixture, at the same room temperature.
- Record the absorbances. They are proportional to the concentration of the red complex.
Results
| Tube | Volume 0.0200 mol/dm³ Fe³⁺ (cm³) | Absorbance |
|---|---|---|
| Standard | (5.0 cm³ of 0.200 mol/dm³) | 0.380 |
| 1 | 5.0 | 0.470 |
| 2 | 4.0 | 0.425 |
| 3 | 3.0 | 0.366 |
| 4 | 2.0 | 0.286 |
Analysis questions
Question 1 Complex concentration
The standard tube has [Fe(SCN)²⁺] = 2.00 × 10⁻⁴ mol/dm³ (all thiocyanate converted). Use absorbance proportional to concentration to calculate [Fe(SCN)²⁺] in tubes 1 to 4.Question 2 Initial concentrations
Calculate the initial [Fe³⁺] and [SCN⁻] in each tube after mixing to 10.0 cm³.Question 3 Equilibrium concentrations
Write an ICE table and calculate the equilibrium [Fe³⁺] and [SCN⁻] for each tube.Question 4 Calculating Kc
Write the Kc expression and calculate Kc for each tube.Question 5 Mean and consistency
Calculate the mean Kc and comment on the consistency of the four values.Question 6 Assumption in the standard
At [Fe³⁺] = 0.100 mol/dm³ in the standard tube, about 93 % of the thiocyanate is converted (not 100 %) when Kc is near 140. Explain the effect on the calculated Kc.Question 7 Prediction with silver ions
A few drops of silver nitrate are added to tube 2 (Ag⁺ + SCN⁻ → AgSCN(s)). Predict the colour change and the effect on Kc.Question 8 Improving the method
Describe two changes to the procedure that would make the final Kc more dependable.
