Chem 1102 – Semester 1, 2011

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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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•  In molecules such as BCl3 or AlCl3, there are too few electrons to complete the octet.

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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 special properties of the transition metals depend on the five d orbitals.

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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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Remember from Lecture 25: First IEs

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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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Unpaired electrons confer the property of being attracted to a magnet, a property retained when compounds are formed.

Eg. “Alnico” magnets are an alloy containing Al, Ni and Co (strong permanent magnets). 29