Edexcel International A Level Chemistry

Revision Notes

International A LevelChemistryEdexcelRevision Notes6. Practical Skills in Chemistry II6.2 Advanced Inorganic & Organic Chemistry Core Practicals6.2.3 Transition Metal Complexes

6.2.3 Transition Metal Complexes

Test Yourself

Core Practical 12: Preparing a Transition Metal Complex

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

bucher-funnel

Buchner filtration set-up to filter crystals

Exam Tip

You may be asked about some of the steps in more detail.

The solution produced in step 6 is a dark blue solution due to the formation of the [Cu(NH3)4(H2O)2]2+ complex ion.

Adding ethanol in step 7 causes the precipitation of dark blue Cu(NH3)4SO4•H2O crystals. This is because ethanol is less polar than water and any ionic compounds will be less soluble in it than they are in water. As ethanol is added, the solubility of Cu(NH3)4SO4•H2O, decreases and crystals are formed.

In step 8, ethanol is added to remove any impurities. Cold ethanol is used as Cu(NH3)4SO4•H2O is less soluble in cold ethanol - if hot ethanol were used, Cu(NH3)4SO4•H2O would dissolve.

Worked example

A student carried out the method mentioned above and measured the final mass of Cu(NH3)SO4·H2O after drying the crystals of 1.2 g.

  1. Write an equation for this reaction
  2. Calculate the relative formula masses of CuSO4·5H2O and Cu(NH3)SO4·H2O
  3. Calculate the number of moles of copper(II) sulfate used in the reaction (assume 1.5 g was used)
  4. Use your answer to calculate the theoretical yield of tetraaminecopper(II) sulfate-1-water your reaction should have produced
  5. Calculate the percentage yield obtained in this reaction

Answers

   1. CuSO4•5H2O + 4NH3 → Cu(NH3)4SO4•H2O +4H2O  

   2. Mr of CuSO4•5H2O: = 63.5 + 32.1 + (4 x 16.0) + 5 x ((2 x 1.0) + 16.0)
                                                  = 249.6

              Mr of Cu(NH3)4SO4•H2O = 63.5 + (4 x (14.0 + 3 x 1.0) + 32.1 + (4 x 16.0) + (2 x 1.0) + 16.0
                                                          = 245.6

   3. Moles of copper(II) sulfate used (based on 1.5 g of CuSO4•5H2O):

moles = begin mathsize 14px style fraction numerator 1.5 over denominator 249.6 end fraction end style = 0.00601

   4. Theoretical yield of Cu(NH3)4SO4•H2O:

As there is a molar ratio of 1:1 we know we will have 0.00601 moles of Cu(NH3)4SO4•H2O
  
The theoretical mass that we would expect to produce can then be calculated

mass = 0.00601 x 245.6 = 1.48 g

   5. Percentage yield = begin mathsize 14px style fraction numerator 1.2 over denominator 1.48 end fraction end style x 100 = 81%

Exam Tip

The percentage yield in this preparation is often calculated to be greater than 100%. This is usually because the crystals have not been completely dried. This causes the final mass to be greater than the actual value as it also includes the mass of the water remaining.

Safety

acute-toxicity corrosive hazardous-to-the-environment flammable
TOXIC CORROSIVE ENVIRONMENTAL HAZARD FLAMMABLE

The hazard symbols associated with this experiment

  • 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. Ammonia gas can also be evolved from the solution, which is toxic.
  • 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. 

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