What is the difference between a d-block element and a transition element? Clue: Sc and Zn are not transition elements.

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1 TRANSITION METALS What is the difference between a d-block element and a transition element? Clue: Sc and Zn are not transition elements. Write the electronic configurations of the transition elements in period 4.. Sc 1s2 2s2 2p6 3s2 3p6 3d1 4s2 Ti [Ar] 3d2 4s2 Etc Zn [Ar] 3d10 4s2 Why are the configurations of Cu and Cr as they are?

2 These elements are all d-block elements because the last electron that they received went into a d-orbital. However, the definition of a transition element depends on its ability to form one or more stable ions with a partially filled d-orbital. This property has consequences that affect the chemistry of these elements. Write the electronic configurations of the stable ions of scandium and zinc

3 Atomic radii, ionisation energies and reactivity The d-orbital is inside the fourth shell, and so the radii of the atoms vary slightly across the 1st series (see p25). Although there is a slight decrease in radius due to increased nuclear charge, the screening from the inner shells reduces the effects of this (why are there little increases for Mn and Zn?)

4 The first and second ionisation energies vary little, and the differences are not significant. Compare successive IEs of Ca and Fe The effective nuclear charge increases, causing an increase in IE. Note the large jump for Ca. With Fe, the extra ionisation energy is compensated more or less by the extra lattice enthalpy or hydration enthalpy evolved when the 3+ compound is made. The enthalpy of formations of FeCl2 or FeCl3 aren t really that different.

5 The hydration of the ions formed by transition metals and the formation of complex ions and oxoions leads to the variable valency of transition metals. This is because these processes are exothermic and compensate the successive ionisation energies.

6 The chemistry of the elements is less varied than the p-block elements in the same period due to the filling of the inner d-orbitals.

7 Vanadium is a great example of variable valency and coloured ions Oxidation State Ion Colour Redox Equation Eo Values +5 VO3VO2+ YELLO W VO2+(aq) + 2H+(aq) + evo2+(aq) + H2O(l) V +4 VO2+ BLUE VO2+(aq + 2H+(aq) +ev3+(aq) + H2O(l) V +3 V3+ GREEN +2 V2+ VIOLET V3+(aq) + e(aq) V2+ Compounds V The +5 oxidation state is oxidising, and the +2 is reducing (it will even reduce water!!). The +4 state is the most stable in the presence of air, and the +3 in its absence.

8 Write the half equations and the overall equation for the mild reduction of V(+5) to V(+4) using sulphite ions in acidic solution. Write the half equations and the overall equation for the reduction of V (+5) by Zn Expt: Vanadium

9 COMPLEX IONS A central metal ion surrounded by neutral molecules or anions called ligands. Ligands always have a lone pair of electrons that they can donate to the central ion and form dative covalent bonds. Usually, 6 bonds are formed in an octahedral arrangement.

10 Other possibilities

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13 The overall charge on the ion is the sum of the charges of the central ion and the ligands, and the name given to the complex ion includes the type of ligand and the oxidation state of the central transition metal ion. Eg. The Hexaamminecobalt(III) ion

14 Monodentate Ligands H2O CNNH3 OH- aqua cyano ammine hydroxy Ions Fe2+/Fe3+ ferrate Cu2+ cuprate

15 A bidentate ligand Chelation: the formation of multiple dative bonds between a polydentate ligand and the central metal atom/ion Hydrazine cannot do this. Why?

16 EDTA - ethylenediaminetetraacetic acid: A polydentate ligand

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18 Transition metal ions and colour Remember that the 3 d-orbitals are not identical:

19 When the ligands form dative covalent bonds with the central ion, the energy of all of the orbitals increases due to the greater electron density in general. However, the orbitals that coincide with the bonds have their energy increased to a greater extent. This can be represented as energy levels

20 Transition metal ions must have a partially filled d-orbital, so that the absorption of light causes an electron to jump up to the higher energy level (excited state), and then return to the ground state releasing electromagnetic radiation which happens to be in the visible range for transition metals. This explains why Zn and Sc are not transition elements, and do not form coloured compounds (although they do form complexes). Cu+ also forms colourless complexes but Cu2+ no (greens and blues). Task: Go to and write down the possible oxidation states of the first transition series.

21 Hydration, deprotonation and ligand exchange The hydrated ions can now be conceived of as the complex ion [Fe(H2O)6]3+ Aqua ions of transition metals are often acidic [Fe(H2O)6]3+ + H2O [Fe(H2O)5OH]2+ + H3O+ This is a deprotonation reaction. How could we shift this equilibrium to the right? Eqns: The intense and distinct colours of the hydroxides formed can be used to identify the metal ion in question. The same precipitates can be observed when we add aqueous ammonia solution, due to the presence of OH-

22 Some transition metal ions in solution also show further reactions with aqueous alkali and ammonia solutions. In this case, the metal hydroxide formed must be amphoteric, eg. Cr(H2O)3(OH)3 (s) + 3H+ (aq) Cr(H2O)3(OH)3 (s) + 3OH- (aq) We can shift these equilibria in either direction by adding acid or alkali

23 Ammonia is not a strong enough base to cause the metal hydroxides to show their amphoteric properties, but with Co2+, Ni2+ and Cu2+, a soluble ammine complex is formed This is an example of ligand exchange. Think about what complex ions formed would look like.

24 Water molecules are replaced when a more competitive ligand is added to the solution, eg.

25 NB: Cr2+ salts tend to be blue

26 Ligand exchange can be explained in terms of stability. The formation of dative covalent bonds is favoured thermodynamically, and the ligands that can donate their lone pair more efficiently will form complexes of lower energy. Complexes are also formed by Be, Mg and Al due to their high charge density, although they are not coloured. Why? See worksheet on Complex Metal Ion stability Chemguide (VLE)

27 Disproportionation, transition elements and SEPs Use the SEPs to show that the reaction is feasible Does Fe(II) disproportionate?

28 TRANSITION METALS AS CATALYSTS They are affected by the reactions they catalyse, they do change form but they are not used up. An alternative route of lower energy is provided for the reaction. There are two types: HOMOGENEOUS (same state as reactants) and HETEROGENEOUS (solids, usually). Transition metals use their varied oxidation states to catalyse other reactions.

29 Eg. 2Ce4+ (aq) + Tl+ (aq) 2Ce3+ (aq) + Tl3+ (aq) The reaction is slow, but can be catalysed with aqueous Mn2+ ions. How? Catalytic Converters Is this a heterogeneous or homogeneous catalyst in this case?

30 Heterogeneous catalysts work by absorbing the reactants onto their surface, thus weakening the bonds present and favouring the reaction to form the product which is then released from the surface. This process depends on: the size of the lattice of the catalyst. the strength of the bonds formed with the reactants and the products (must be strong and weak respectively) the use of available d-orbitals to form these bonds. Catalysts are very specific for given reactions, for the above reasons.

31 Uses of transition elements as catalyst: The production of ethanoic acid See SS: Catalysts for making Ethanoic Acid NB. The catalysts and the atom economy in the processes described.

32 Transition Metal Chemistry

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