Download 14. Reactions of transition elements

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14. Reactions of transition elements
Topic
Transition elements – complex formation.
Level
Post-16.
Timing
20 min.
Apparatus (per group)
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One student worksheet
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One clear plastic sheet (eg ohp sheet).
Chemicals (per group)
Solutions contained in plastic pipettes, see p. 2
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Copper(II) sulphate
0.5 mol dm–3
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Cobalt nitrate
0.5 mol dm–3
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Nickel nitrate
0.5 mol dm–3
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Magnesium nitrate
0.5 mol dm–3
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Ammonia solution
3 mol dm–3
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Sodium hydroxide
1 mol dm–3.
Observations
The pink, green and blue colours of the cobalt, nickel and copper solutions contrast
strongly with the colourless magnesium solution.
Reactions with ammonia
Cobalt
The addition of one drop of ammonia gives a deep green precipitate. Adding more
ammonia gives a green or brown solution.
For cobalt, the Co2+ ion is much more stable than the strongly oxidising Co3+ ion.
However, when Co2+ is reacted with ligands such as ammonia the hexaamminocobalt(II) formed is oxidised to the stable hexa-amminocobalt(III) complex
(this is the cause of any green-brown colour change). (Thus the presence of certain
complexing ligands around Co3+ can greatly reduce its oxidising power.)
Nickel
The addition of one drop of ammonia slowly gives a light green precipitate –
nickel(II) hydroxide. Adding more ammonia dissolves the precipitate to give a blue
solution consisting of the hexa-amminonickel(II) complex.
Copper
The addition of one drop of ammonia with stirring gives a light blue precipitate of
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copper(II) hydroxide. Adding more ammonia dissolves the precipitate to give the
deep blue tetra-amminocopper(II) ion.
Magnesium
The addition of ammonia gives a white precipitate of magnesium hydroxide. Adding
more ammonia produces no noticeable effect since the magnesium ion does not form
complexes with ammonia.
The reactions of the transition element ions with ammonia show that not only are
coloured products formed but that the colour and identity of the products depend on
the proportion of ammonia added.
Reactions with sodium hydroxide
Precipitates of metal hydroxides are seen in each case. The transition metal
hydroxides are coloured.
References
N. N. Greenwood and A. Earnshaw, Chemistry of the elements. Oxford: Pergamon,
1984.
Safety
Students must wear eye protection.
It is the responsibility of the teacher to carry out a risk assessment.
Microscale Chemistry
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14. Reactions of transition elements
The purpose of this experiment is to observe and interpret some of the chemistry of
three first row transition elements and to compare them with a typical s-block
element. Follow the instructions carefully recording all your observations.
Instructions
1.
Cover the worksheet with a clear plastic sheet.
2.
Place two drops of each of the solutions of cobalt, nickel, copper and
magnesium ions in the appropriate boxes in the column headed Metal ion
solution.
Metal ion solution
Ammonia solution
Sodium hydroxide
solution
Solution of
cobalt ions
Solution of
nickel ions
Solution of
copper ions
Solution of
magnesium ions
Reactions with ammonia
1.
In the column headed ammonia solution put four drops of each element
solution in two separate lots either side of the dashed line.
2.
For cobalt, add one drop of ammonia solution. Add three further drops of
ammonia solution to the right hand side of the dashed line only.
3.
For nickel, add one drop of ammonia solution. After one minute add five
further drops of ammonia to the right hand side of the dashed line.
4.
For copper, add one drop of ammonia solution and stir with the tip of a
pipette. Add three further drops of ammonia solution to the right hand side of
the dashed line.
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5.
For magnesium add two drops of ammonia solution. Add two further drops of
ammonia solution to the right hand side of the dashed line.
Reactions with sodium hydroxide
1.
Put two drops of each metal ion solution in the boxes in the column headed
sodium hydroxide solution.
2.
Add two drops of sodium hydroxide solution to each.
1.
How do the reactions of the transition metal ions differ from those of the
s-block metal ions?
Question