Post on 14-Aug-2018
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Lecture 29-30: • Calculations involving Ksp and Q (sat solutions of sparingly soluble electrolytes) • Common Ion, T, pH, solvent effects • Enthalpy-entropy interplay • Solubility curves for electrolyte solutions • Liquid-liquid solubility equilibria • Gas-liquid solubility equilibria and solubility curves
Lecture 31: • Lewis Acids and Bases
• Transition Metals • Properties of Transition Metals
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At the same time that Brønsted and Lowry were working out their theory of acids and bases (According to Brønsted–Lowry definition, an acid is a compound that can donate a proton, and a base is a compound that can receive a proton. An acid–base reaction is, thus, the removal of a hydrogen ion from the acid and its addition to the base.), Lewis was considering a more general property of molecules.
1923
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What role do lone pairs and empty orbitals (incomplete octet) play in chemistry?
A base (referred to as a Lewis base) to be a compound that can donate an electron pair, and an acid (a Lewis acid) to be a compound that can receive this electron pair.
Thus an acid does not exchange atoms with a base, but combines with it.
1923 But not fully elaborated until
1938
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AlCl3 Al2Cl6 GaH3 Ga2H6
AlH3 (AlH3)n Polymerised
B compounds- Diborane (a 3-centre-2-electron bond).
A characteristic reaction is found with molecules which have a lone pair. 6
The lone pair on N interacts with the empty p-orbital on B:
Cl
B
ClCl
H
NHH
H
NHH
Cl
BClCl
+ - +
The two molecules react to form an adduct (complex) 7
• These reactions are closely related to those of acids and bases.
• In general, we can write:
A: + B A: B+ -
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• This leads to a more general definition of acids and bases.
• Brønsted: acid = proton donor base = proton acceptor
• Lewis:
Generally:
• a Lewis acid has an empty orbital.
• a Lewis base has a lone pair
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• This definition also includes Brønsted acids and bases:
H+ + :NH3 NH4+
Brønsted Brønsted acid base
Lewis Lewis acid base
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Why are Transition Metals important?
Applications include: -Used as colours: in paints, glass-stained windows, … -Ti (metal used in construction of aircraft frames; TiO2 used in paper, sunscreen, toothpaste, cosmetics), Cr (alloys: contained in stainless steel; 60:40 Ni:Cr used for heating wires in toasters & hairdryers), Cu, Ag, Au (known for 1000s years, coinage), Cu alloys (bronze and brass for tools, cookware, weapons, art), Zn (cosmetics, sunscreen (ZnO), batteries,…), Hg (component of street lamps, fluorescent lights, thermometers, barometers…)
-Biological importance:
Metal compounds consist of a central metal ion or atom surrounded by a set of ions or molecules which, when bonded to the metal, are called ligands.
Some Ls are neutral (H2O:, :NH3, CO, some are ions (Cl-, OH-).
:
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• The formation of metal complexes is an essential feature of the chemistry of the transition metals.
• Both the electrons forming the M-L covalent bond are formally contributed by the Ligand. Dative (or coordinate) bonds coordination compounds coordination chemistry.
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A note about Ligands (Ls):
-Can have neutral Ls: H2O, NH3,… -Can also have anionic Ls: Cl-, NH2
-, NO2
-,…
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Transition element (IUPAC definition): element for which an atom has an incomplete d-subshell or which readily gives rise to cations (i.e. it loses electrons) having an incomplete d-subshell.
Thus elements in group 12 - Zn, Cd and Hg – are not transition metals and their properties are those of the representative group metals (s-block and p-block). However they are included for comparison purposes and also because when they give rise to cations, they behave as transition metals.
First Second and Third Transition Series
• In “hydrogen-like” atoms, the 3d orbitals are higher in energy than the 4s orbitals, so that the latter are filled to give the elements K ([Ar]4s1) and Ca ([Ar]4s2).
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• The early metals are more difficult to extract from their ores than those on the right side of the series, i.e.
• Sc, Ti, V, Cr are much harder to extract than Co, Ni, and Cu.
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• The development of civilisation parallels this.
• Bronze Age (Cu)
• Iron Age (Fe)
• Steel Age (Fe; increasing use of Mn, Cr, and V),
• Most recently, Ti has been used in aircraft and for other light-weight construction.
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• The d orbitals are
• This results in a steady decrease of atomic radius and increase in ionisation potential as nuclear charge increases across the series.
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D-block element
Sc Ti V Cr Mn Fe Co Ni Cu Zn
χ 1.36 1.54 1.63 1.66 1.55 1.83 1.88 1.91 1.90 1.65
IE1 (Kj/mol)
631 656 650 653 717 762 758 737 745 906
Atomic radius (pm)
162 147 134 130 135 126 125 124 128 137
Density of
metal (g/cm3)
2.99 4.55 6.11 7.19 7.47 7.87 8.80 8.91 8.93 7.10
M.p. (oC)
1541 1660 1917 1857 1244 1537 1494 1455 1084 420
• All elements are electropositive (but < Groups 1 & 2).
• The first series transition metals are commonly found with oxidation numbers +2 and +3 in their compounds.
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• +3 is more stable for elements on the l.h.s. of the Periodic Table [Ti, V, Cr].
• +2 is more stable for elements on the r.h.s. of the Periodic Table [Mn, Fe, Co, Ni].
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• In these elements, the orbital energy sequence is reversed, with the highest energy electrons being the 4s, so that these electrons are lost first when the atom is ionised:
Fe Fe2+ Fe3+
[Ar]3d64s2 [Ar]3d6 [Ar]3d5
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• The compounds often have paramagnetic properties, which results from the presence of unpaired electrons in the d orbitals.
• This reaches a maximum with
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