Transition metals Flashcards
(25 cards)
MnO4^- (+7)
Purple
MnO2 (+4)
Black
Mn^2+
Pale pink
Cr2O7 2- (+6)
Orange
CrO4 2- (+6)
Yellow
Cr 3+
Green
Fe 2+
Pale green
Fe 3+
rust orange or red/brown
Co 2+
pink
Co 3+
blue
Cu 2+
sky blue
Cr(aq) 3+
dark green
Cu(aq) 2+
blue
[Cu(NH3)4(H2O)] 2+
royal blue
CoCl4 2-
blue
CuCl4 2-
yellow-green (olive green if yellow is mixed with blue)
Explain why some transition element compounds are coloured?
When ligands form dative covalent bonds with a metal ion, d orbitals split to two sets, three of lower energy and two of higher energy.
Electrons tend to occupy the lower and if an electron in a 3d orbital gain sufficient energy, it can move from a lower energy set to a higher energy set.
The energy needed to promote is a certain frequency of light, this corresponds to the energy gap between the orbitals (E=hf).
When light of a particular colour is absorbed, its complementary colour is reflected, this is the colour observed.
What properties affect the colour or the energy difference between the split 3d orbitals
-Oxidation state of the metal
-Nature and number of ligands
-Atomic number of the transition element.
Why are different colours observed?
Different ligands cause different splitting of orbitals, so different frequencies are absorbed giving a different ∆E and different colours are produced.
Why do copper(I) and scandium(III) complexes appear colourless?
Cu+ complexes have an electronic config with a full d sub-shell, while Sc3+ ions have an empty d sub-shell, meaning that electrons cannot move from lower to higher orbitals.
Describe the reaction of Zn 2+ with NaOH
On addition with OH- , white precipitate forms (Zn(OH)2) , which dissolves in excess NaOH to give a colourless solution. This indicates that Zn(OH)2 is amphoteric.
Describe the reaction of Cr 3+ with NaOH
On addition with OH- grey-green precipitate forms Cr(OH)3 which dissolves in excess sodium hydroxide to give a pale green solution. This shows that Cr(OH)3 is amphoteric
Describe the reaction of Fe2+ with NaOH
On addition with OH-, a dark muddy green precipitate forms which does not redissolve. Precipitate darkens on standing OH- and turns orange on top.
Describe the reaction of Fe3+ with OH-
yellow orange solution turns rust orange precipitate
