CAPITULO 2: MARCO TEÓRICO
2.2 Seguridad en el trabajo
2.2.2 El accidente de trabajo
Properties of transition metals
Complex ion formation
Bonding in complex ions. The simplest view is that the ligands form dative covalent bonds by donating a lone pair of electrons into empty orbitals of the transition metal ion. These could be empty 3d, 4s, 4p or 4d orbitals in the ion (e.g. see the arrangement of electrons in boxes in Figure 5.3).
3d 4s 4p 4d
OJ]
Fe2+ electrons
\
Ligand electrons: six pairsT R A N S I T I O N M E TA L S , Q U A N T I TAT I V E K I N E T I C S A N D A P P L I E D O R G A N I C C H E M I S T R Y
[Cu(Hz0)6f+ is turquoise blue, [Cu(NH3)4(H20)2]2+ is deep blue, and [CuC14t is yellow.
Hydrated cations of charge 4+ or more do not exist because the sum of all the ionisation energ ies is too large to be compensated for by the hydration energy.
Copper forms Cu+ and Cu2+, and chromium Cr2+ and Cr3+.
Typical ligands are:
neutral molecules: H20 (aqua), NH3 (ammine) anions: F- (fluoro), cr (chloro), (CNt (cyano).
Shapes of complex ions (see pages 1 4-1 5). If the complex ion has 6 pairs of electrons donated by ligands, the complex will be octahedral. This is so that the electron pairs in the ligands will be as far apart from each other as possible.
Coloured complex ions
If the ion has partially filled d orbitals, it will be coloured. Sc3+ and Ti4+
have d0 structures, and Cu+ and Zn2+ have d to and so are not coloured.
d orbitals point in different directions in space, and so interact to different extents with the electrons in the ligands.
This causes a splitting of the d orbitals into two of higher energy and three of lower energy.
When white light is shone into the substance, a d electron is moved from the lower energy to the higher energy level.
The frequency of the light that causes this jump is in the visible range, so that colour at this frequency is removed from the white light.
The colour depends on the size of the energy gap which varies with the metal ion and with the type of ligand.
Variable oxidation state
Successive ionisation energies increase steadily until all the 4s and the 3d electrons have been removed, after which there is a large jump in the value.
Formation of cations. The increase between successive ionisation energies is compensated for by a similar increase in hydration energies.
Thus cations in different oxidation states are energetically favourable for all transition metals.
Formation of covalent bonds. Transition metals can make available a variable number of d electrons for covalent bonding. The energy required for the promotion of an electron from a 3d to a higher energy orbital is compensated for by the bond energy released.
Formation of oxoions. In ions such as Mn04-, V03-, VO/ and V02+, the oxygen is covalently bonded to the transition metal which uses a varying number of d electrons.
Manganese exists:
in the +2 state as Md+
in the +4 state as Mn02 in the +6 state as MnOt in the +7 state as Mn04-.
Catalytic activity
Transition metals and their compounds are often good catalysts.
Vanadium(V) oxide is used in the oxidation of S02 to S03 in the contact process for the manufacture of sulphuric acid.
Iron is used in the Haber process for the manufacture of ammonia.
Nickel is used in the addition of hydrogen to alkenes (hardening of vegetable oils).
Ion a Colour
Cr3+ Green Mn2+ Pale pink
Reactions of transition metal compounds
Reactions with sodium hydroxide and ammonia solutions
The table gives a summary of these reactions.
Addition of Excess NaOH(aq) Addition of Excess NHl aq)
NaOH(aq) NH3(aq)
Green ppt Green solution Green ppt Ppt remains
Sandy pptb Ppt remains Sandy pptb Ppt remains
Fe2+ Pale green Dirty green pp( Ppt remains Dirty green pp( Ppt remains
Fe3+ Brown/yellow Foxy red ppt Ppt remains Foxy red ppt Ppt remains
Ni2+ Green Green ppt Ppt remains Green ppt Blue soln
Cu2+ Pale blue Blue pptct Ppt remains Pale blue ppt Deep blue soln Zn2+ None White ppt Colourless solution White ppt Colourless soln a The correct formula for the ions should be the hexa-aqua ion, except for zinc, which forms a tetraqua ion.
b The precipitate of Mn(OH)2 goes brown as it is oxidised b air.
c The precipitate of Fe(OH)2 goes brown on the surface a it is o idi ed by the o.x.·y en in the air.
The precipitate of Cu(OH)� goes black as it lo e '\.Vater to form u .
Amphoteric hydroxides 'redissolve' in excess strong alkali, e.g.
Cr(OH)J(s) + 30W(aq) � [Cr(OH)st (aq)
and Zn(OH)2(s) + 20W(aq) �
[Zn(OH)4t(aq).
This reaction is used as a test for:
a nickel( l l): with aqueous ammonia nickel ( l l ) ions first give a green precipitate which then forms a blue solution with excess ammonia.
b copper( l l ) ions give a pale blue precipitate which forms a deep blue solution with excess ammonia and
c zinc ions g ive a white precipitate which forms a colou rless solution with excess ammonia.
Deprotonation
The aqua ions in solution are partially deprotonated by water. he greater the surface charge density of the ion the greater the e t nt of this reaction, e.g. hexa-aqua iron(III) ions:
[Fe(H20)6]3+(aq) + H20 .:: [Fe(H20)5(0H)]2+(aq) + H30+(aq) This means that solutions of iron(III) ions are acidic (pH < 7).
When an alkali such as sodium hydroxide is added, the equilibrium is driven to the right, the ion is considerably deprotonated to form a neutral molecule which loses water to form a precipitate of the metal hydroxide:
[Fe(H20)6f+(aq) + 30H-(aq) � Fe(OH)3(s) + 6H20 If aqueous ammonia is added, the same precipitate is formed:
[Fe(H20)6f+(aq) +3NH3(aq) � Fe(OH)3(s) + 3NH/(aq) + 3H20
Ligand exchange
When aqueous ammonia is added to aqua complexes of d block
elements such as those of nickel, copper and zinc, ligand exchange takes place and a solution of the ammine complex is formed.
First the hydroxide is precipitated in a deprotonation reaction:
[Cu(H20)6]2+(aq) + 2NH3(aq) � Cu(OH)2(s) + 2NH/(aq) + 4H20 The hydroxide then ligand exchanges to form an ammine complex with excess ammonia:
Cu(OH)2(s) + 4NH3(aq) +2H20 � [Cu(NH3)4(H20)z]2+ + 20H-(aq) The final result is that the NH3 ligand has taken the place of four H20 ligands.
Addition of cyanide ions to iron(II) ions produces a solution of hexacyanoferrate(II):
[Fe(H20)6]2+(aq) + 6CN-(aq) � [Fe(CN)t-(aq) + 6H20 A test for iron(III) ions is to add a solution of potassium thiocyanate,KCNS. Iron(III) ions give a blood red solution:
[Fe(H20)6]3+(aq) + SCN-(aq) � [Fe(SCN)(H20)5]2+(aq) + H20
T R A N S I T I O N M E TA L S , Q U A N T I TAT I V E K I N E T I C S A N D A P P L I E D O R G A N I C C H E M I S T R Y
VO/
Fe3+
Vanadium chemistry
Vanadium compounds exist in four oxidation states:
+5: V03-(aq) which is colourless, but in acid solution reacts to form a yellow solution of VO/(aq) ions:
V03- + 2H+ ..= VO/ + HzO
+4: V02+(aq), which is blue.
+3: V3+(aq) which is green and is mildly reducing.
+2: V2+(aq) which is lavender and is strongly reducing.
Reduction by zinc. If zinc and dilute hydrochloric acid are added to a solution of sodium vanadate, NaV03, the colour changes in distinct sequence, as it is steadily reduced from the +5 to the +2 state:
start: yellow caused by V02+(aq)ions (+5 state)
then: green caused by a mixture of yellow VO/(aq) and blue then:
then:
finally:
V02+(aq)
blue caused by V02+(aq) ions ( +4 state) green caused by V3+(aq) ions ( +3 state) lavender caused by V2 ... (aq) ions ( +2 state)
Redox reactions
Vanadium compounds can be oxidised or reduced by suitable reagents. For a redox reaction to work, the E-eceu value must be positive. A list of E-evalues for the half reactions of vanadium in its various oxidation states and for some oxidising and reducing agents is gi en below. These values show, for in tance, that anadium( ) v\Till be reduced only to vanadium (IV) by Fe2+
ion , \Yherea n2 ion \\ill reduce it first to vanadium (IV) and then to vanadium ( III).
+5 VO/ +5
+ 2H+ + e-..= V02 ... + H20 E-e = + 1 . \' • + e - ..= Fe2+ E-e = +0. 7 7 Fe2+
1/2Cl2 + e- ..= cr E-e = +1 .36 +4 y +4
I
Cl2Fe3+
voz ... + 2H ... + e-..= V3+ + H20 E-e = +0.34 V
Sn4+ + 2e- ..= Sn2+ E-e = +0. 1 5 V Sn2 ...
Fe3 ... + e - ..= Fe2 ... E-e = +0.77 V y
+3 v3+ + e - ..= vz ... E-e = -0.26 V
Zn2+ + 2e- ..= Zn E-e = -0.76 V Zn/H+
H+ + e- ..,: 1/zHz E-e = o.oo v y
+2 +2
E-eceu for the reduction of: E-ecell for the oxidation of:
voz+ by Fe2+ = + 1 .0 - (+0. 77) = +0.23 v
V02+ by Sn2 ... = +0.34 - (+0. 1 5) = +0. 1 9 v
V3+ by Zn = -0.26 - (-0. 76) = +0.50 v
rg! Checklist
vz+ by H+ = o.o - (-0.26) = +0.26 v
V3 ... by Fe3+ = +0. 7 7 - (+0.34) = +0.43 v
voz ... by c12 = + 1 .36 - (+ LO) = +0.36 v
Before attempting the questions on this topic, check that you:
Can define d block and transition elements.
Can write the electronic structure of the d block elements and their ions.
[Cu(H20)6t turquoise-blue.
Dative covalent (ligand to ion) and covalent (within the water molecule) Octahedral
Deprotonation
Ligand exchange
t5, VO/, yellow. t4, V02·, blue.
+3, V3·, green. +2, V2+, lavender.
I ron in the Haber process to manufactu re ammonia
Vanadium(V) oxide in the Contact process to manufacture sulphuric acid
The answers to the numbered questions are on pages 1 37-1 38.
Can recall the characteristic properties of the transition elements.
Understand the nature of the bonding in complex ions, the shape of the ions and their colour.
Can recall the action of aqueou odium hydroxide and ammonia on solutions of their aqua ions.
Understand deprotonation and li and e reactions.
Can recall the oxidation state of vanadium, and the colours of its ions.
Know reactions to interconvert th oxid tion tate of vanadium.
Can recall examples of anadium, ir n nd ni kel and their compounds as catalysts .
.
Testin g yo u
rk1 1o1\ I d and und
rt ndin g
For the ing t of que tion , CO\ er th mar o th p for ) ou answer, then check to see if you are orr t.
Write down the electronic structure of a anadium atom and \ ion.
State the formula and colour of the aqua complex of copper(II).
State the formula and colour of the ammine complex of copper(I I . Name the types of bonding in the aqua complex of iron(II).
What is the shape of the iron(II) aqua complex ion?
State the type of reaction that occurs when aqueous sodium hydroxide is added to a solution of the aqua complex of copper(I I).
State the type of reaction that occurs when excess aqueous ammonia is added to a solution of the aqua complex of copper(II).
What are the oxidation states of vanadium? Give the formula and the colour of the cation for each oxidation state.
Give an example of the use of iron as an industrial catalyst.
Give an industrial use of a vanadium compound as a catalyst.
1 Explain why the [Cu(H20)6]2• ion is coloured.
2 Give the equations for the reactions caused by small additions of sodium hydroxide solution, followed by excess to:
a a solution of the aqua complex of chromium(III) b a solution of the aqua complex of iron(II) c a solution of the aqua complex of zinc(II).
3 Give the equations for the reactions caused by small additions of ammonia solution, followed by excess to:
a a solution of the aqua complex of iron(III) b a solution of the aqua complex of copper(II).
4 Write the half equations for the following changes in oxidation state:
a vanadium(V) to vanadium(II) b vanadium(V) to vanadium (IV) c vanadium(III) to vanadium(IV)
T R A N S I T I O N M E TA L S , Q U A N T I TA T I V E K I N E T I C S A N D A P P L I E D O R G A N I C C H E M I S T R Y