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Pearson’s context prepares a deep-blue amminecopper(II) sulfate solid from copper(II) sulfate and ammonia. In aqueous solution, ammonia replaces water ligands around copper(II); the metal remains in oxidation state +2. Adding small quantities can first produce hydroxide precipitation through ammonia’s basic behaviour, while sufficient excess ammonia gives the soluble deep-blue ammine complex.
Video by Royal Society of Chemistry. This demonstration explains suction filtration. The written lesson develops the different ligand-substitution and antisolvent chemistry of the copper-complex preparation.
Viewing focus: Explain ligand substitution and crystallisation of a copper complex
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What happens to copper’s oxidation state during the water-to-ammonia ligand substitution?
It remains +2. Ligand substitution need not transfer electrons to or from the metal.
After watching 2: What is ethanol’s main role in the isolation step described?
Reducing the product’s solubility. It acts as an antisolvent in this preparation.
What you'll be able to do
Pearson’s context prepares a deep-blue amminecopper(II) sulfate solid from copper(II) sulfate and ammonia. In aqueous solution, ammonia replaces water ligands around copper(II); the metal remains in oxidation state +2. Adding small quantities can first produce hydroxide precipitation through ammonia’s basic behaviour, while sufficient excess ammonia gives the soluble deep-blue ammine complex.
An aqueous representation is [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O. The isolated solid is represented in the practical’s yield calculation as [Cu(NH₃)₄]SO₄·H₂O. Do not copy the number of coordinated waters in an aqueous complex into the solid’s hydrate formula without evidence. The starting and product formulae determine their different molar masses.
Dissolve the stated copper salt using the approved warming procedure, add the ligand in the laboratory’s ventilation arrangement, then use the prescribed ethanol addition and cooling to reduce product solubility. Ethanol acts as an antisolvent in this context; it is not the ligand responsible for the deep-blue copper complex. Crystal formation separates material from the surrounding solution.
Collect the crystals by suction filtration with apparatus designed for reduced pressure. Wash with a limited amount of suitable cold solvent to remove adhering solution while limiting product dissolution, then dry as directed before weighing. Copper salts need specified waste collection; ammonia is corrosive and volatile, and ethanol is flammable. Follow the approved protection, ventilation and heating procedure.
The copper-containing starting salt and product have a 1:1 amount relationship when ammonia is in sufficient excess. Use the actual hydrate formula of the starting salt. The idealised preparation can be written CuSO₄·5H₂O + 4NH₃ → [Cu(NH₃)₄]SO₄·H₂O + 4H₂O. This equation conserves copper, sulfate, ammonia and water across the stated forms.
A low isolated yield can arise from incomplete conversion, product remaining soluble, transfer or washing losses. A wet or impure solid can give an excessive apparent yield. Deep-blue colour supports the presence of the complex but is not a quantitative purity measurement. Record mass, formula assumptions and drying conditions rather than reporting only an attractive crystal photograph.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: The dry product mass is 1.964 g. Find isolated yield and explain why a wet mass would be misleading.
80.0%; drying is needed for a meaningful isolated mass.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.