8.2.2 Transition Metal Complexes

Method

1. Weigh between 1.4 g and 1.6 g of copper(II) sulfate.
   • To do this you should weigh a test tube and record its mass.
   • Then add the copper(II) sulfate to the test tube, reweigh and record the mass.
   • The mass of the copper(II) sulfate is the difference between the two masses.
2.  Add 4 cm³ of water to the test tube using a graduated pipette.
3. Prepare a water bath using hot water from a kettle in a 100 cm³ beaker and stand the test tube in the water bath.
4. Stir gently to dissolve the copper(II) sulfate.
5.  Remove the test tube containing copper(II) sulfate solution from the water bath.
6.  Perform this step in the fume cupboard wearing gloves. Add, while stirring, 2 cm³ of concentrated ammonia solution to the copper(II) sulfate solution.
7.  Pour the contents of the test tube into 6 cm³ of ethanol that has been pipetted into a beaker – mix well and then cool the mixture in an ice bath.
8.  Using a Bϋchner funnel and flask, filter the crystals. Wash your test tube with cold ethanol and add the washings to the Bϋchner funnel. Finally, rinse the crystals with cold ethanol.
9.  Carefully scrape the crystals off the filter paper onto a fresh piece of filter paper. Cover the crystals with a second piece of filter paper, Carefully pat the paper to dry the crystals. Note that to get the crystals completely dry, you may need to repeatedly move the crystals to dry parts of the filter paper.
10.  Once dry, measure and record the mass of crystals

Buchner filtration set-up to filter crystals

Worked Example

Answers

   1. CuSO₄ + 4NH₃ → Cu(NH₃)₄SO₄•H₂O +4H₂O

Remember that as we have dissolved the copper sulfate in solution this is actually hydrated copper(II) sulfate CuSO₄•5H₂O = 249.5

   2. CuSO₄•5H₂O = 249.5
              Cu(NH₃)₄SO₄•H₂O = 245.5

   3. 0.00601 moles (based on 1.5 g of CuSO₄•5H₂O)

   4. Theoretical yield of Cu(NH₃)₄SO₄•H₂O = 1.48 g

As there is a molar ratio of 1:1 we know we will have 0.00481 moles of Cu(NH₃)₄SO₄•H₂O

The theoretical mass that we would expect to produce can then be calculated

   5. Percentage yield =

Safety

• Wear a lab coat and use eye protection.
• Tie long hair back.
• Concentrated ammonia solution is corrosive and dangerous to the environment. It must be kept 
  in the fume cupboard and handled wearing gloves.
• Copper salts are harmful and dangerous to the environment.
• Ethanol is flammable and must be kept away from flames.

Errors

• Losses could be from the reaction not going to completion and product staying in solution (i.e. not crystallising out)
• Gains could be from impure or wet crystals.