Quick answer: Transition metals are d-block elements that form ions with a partially filled d subshell. This Chemistry 11-12 lab on transition metals 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.
What colours do copper(II) complexes show when the ligands change, and how can a colorimeter turn colour into a concentration? You will make four complex ions, explain the colour changes, and use a calibration graph to find the concentration of an unknown ammonia complex solution.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.2.5
Aim
To make different copper(II) complex ions by ligand substitution, record their colours and use a colorimeter to find the concentration of an unknown solution of tetraamminecopper(II).
Equipment
- 0.10 mol dm⁻³ copper(II) sulfate solution
- Dilute and 1 mol dm⁻³ ammonia solution, concentrated hydrochloric acid (teacher dispensed)
- Test tubes and rack, droppers, 10 cm³ measuring cylinder, volumetric flask or graduated pipettes
- Colorimeter or phone light sensor with orange filter and cuvettes
Safety
Wear goggles. Concentrated hydrochloric acid and ammonia vapours irritate the lungs: use only in a ventilated area or fume cupboard. Copper(II) compounds are harmful, so wash hands and dispose of waste in the labelled container.
Method
- Put 2 cm depth of copper(II) sulfate in tubes 1 to 5. Tube 1 stays as the control.
- To tube 2 add 5 drops of ammonia, record the colour; add excess ammonia to tube 3 and record.
- To tube 4 add 2 cm of concentrated hydrochloric acid and record the colour; pour half of it into tube 5, add water drop by drop and record the colour.
- Prepare standards of [Cu(NH₃)₄(H₂O)₂]²⁺ by diluting a stock solution to 0.020, 0.040, 0.060 and 0.080 mol dm⁻³ and measure the absorbance of each at 600 nm. Measure the unknown, which was diluted 5.0 cm³ to 25.0 cm³.
Results
| Tube | Reagent added | Observation |
|---|---|---|
| 1 | none | pale blue solution |
| 2 | few drops ammonia | pale blue precipitate |
| 3 | excess ammonia | deep blue solution |
| 4 | conc. HCl | yellow-green solution |
| 5 | half of tube 4, then water | turns blue again |
| Concentration (mol dm⁻³) | 0.020 | 0.040 | 0.060 | 0.080 | unknown |
|---|---|---|---|---|---|
| Absorbance | 0.15 | 0.31 | 0.44 | 0.60 | 0.36 |
Analysis questions
Question 1 Colour changes
Write the formula of the copper complex responsible for the colour in tubes 1, 3 and 4.Question 2 Equations
Write ionic equations for the reactions in tubes 2 and 3.Question 3 Explain the change in tube 4
Explain why the coordination number changes in tube 4 and why adding water in tube 5 reverses the colour.Question 4 Calibration graph
Plot absorbance against concentration for the standards and describe the graph.Question 5 Molar absorptivity
Calculate the slope of the line through the origin for these data and say what the slope tells you about the complex.Question 6 Unknown solution
Use the graph to find the concentration of the diluted unknown and then of the original solution.Question 7 Why a filter
Explain why an orange filter (about 600 nm) is chosen for the blue complex.Question 8 Improvements
Suggest two improvements to make the concentration more precise.
