define transition elements
transition elements are d block elements that form one or more stable ions with partially filled d subshell
because of this definition, we exclude scandium and zinc from the class of transition elements as they have 0 and 10 electrons in the 3d subshell respectively
(recall from atomic structure) why is the atomic radius of the first row of transition elements relatively invariant?
- across the first row of TM, number of protons increase so nuclear charge increases
- electrons are added to the inner 3d subshell, contributing to shielding effect
- increasing shielding effect nullifies, to a considerable extent, the influence of each additional proton in the nucleus, so effective nuclear charge remains almost constant, hence atomic radius is relatively invariant
(recall from atomic structure) why is the first ionisation energies of the first row of transition elements relatively invariant?
- across the first row of TM, number of protons increase so nuclear charge increases
- electrons are added to the inner 3d subshell, contributing to shielding effect
- increasing shielding effect nullifies, to a considerable extent, the influence of each additional proton in the nucleus, energy required to remove the outermost electron of each succeeding element is relatively invariant
why is the melting point of TM higher than s-block elements?
both 3d & 4s electrons are involved in metallic bonding of TM -> more energy is required to overcome stronger metallic bonding compared to in s-block elements where only 4s electrons are involved
why are TM denser than s-block elements?
the atomic radii of TM are smaller and have larger atomic masses than s-block elements, so they have higher density than s-block elements
chemical properties of TM?
- tendency to have variable oxidation states
- catalytic properties
- forms complex ions
- forms coloured ions
why do TM have variable oxidation states?
- it is due to the close similarity in energy of the 4s & 3d electrons
- once the 4s electrons are removed, some or all of the 3d electrons may also be removed without requiring much more energy
lower oxidation state typically found in ionic compounds, and higher oxidation state typically exist as covalent bonds
number of available oxidation states of TM increases from Ti to Mn before decreasing from Mn to Cu. this is bc after Mn (5 electrons in 3d subshell), pairing of d-electrons occurs so there is a fall in number of electrons available for bond formation
what is a catalyst and why are TM suitable to be used as catalysts?
a catalyst increases rate of reaction by providing an alternative reaction pathway of lower activation energy than the uncatalysed reaction
- homogeneous catalysts operate in the same phase as reactants. TM are suitable to act as homogeneous catalysts due to the variable oxidation states of the TM in its ions
- heterogeneous catalysts operate in a different phase as reactants. TM are suitable to act as heterogeneous catalysts due to their partially filled 3d orbitals which accept or donate electron pairs to reactant molecules for adsorption of reactants onto catalyst
heteroogeneous catalyst - finely divided Fe (s) used in Haber process
homogeneous catalyst - oxidation of iodide ion by peroxodisulfate ion catalysed by Fe (III) (aq) ions
what is a complex?
a complex is a molecule or ion formed by a central metal atom or ion dative bonded to one or more surrounding molecules/ions called ligands
- to maintain charge neutrality, a complex ion is typically associated counter ions. when dissolved, counter ions separate from the complex ion, with the ligands remaining attached to the central metal ion
- the net charge on the complex is the sum of the oxidation number of the central metal ion and the total charges of the ligands that surround it
- TM tend to form complex ions because they are relatively small & highly charged, resulting in high charge density and hence high polarising power, hence there is a high tendency towards covalent bond formation. the vacant 3d subshells can also accommodate the lone pair of electrons from the ligands, resulting in dative bond formation
what is a ligand?
a ligand is an ion or molecule with one or more lone pairs of electrons available to be donated into the vacant orbitals of a central metal atom or ion
classified according to the number of dative bonds that it forms. monodentate (1 dative bond), bidentate (2 dative bonds), hexadentate (6 dative bonds)
idk if this is impt but not in LOs
why are some aqua complexes acidic?
- aqua complexes containing high charge density transition metal cations undergo hydrolysis in water to give a mildly acidic solution
- high charge density TM cations include Cr³⁺ and Fe³⁺
- [Fe(H₂O)₆]³⁺ (aq) + H₂O (l) ⇌ [Fe(H₂O)₅OH]²⁺ (aq) + H₃O⁺ (aq)
- [Cr(H₂O)₆]³⁺ (aq) + H₂O (l) ⇌ [Cr(H₂O)₅OH]²⁺ (aq) + H₃O⁺ (aq)
idk if this is impt but not in LOs
chromium complexes & oxoanions
when NaOH (aq) or NH₃ (aq) is added to a Cr³⁺ (aq), a green grey ppt of Cr(OH)₃ is formed
- Cr³⁺ (aq) + 3OH⁻ (aq) → Cr(OH)₃ (s) OR
- [Cr(H₂O)₆]³⁺ (aq) + 3OH⁻ (aq) → Cr(H₂O)₃(OH)₃ (s) + 3H₂O (l)
when NaOH (aq) is added in excess, a dark green solution of [Cr(OH)₆]³⁻ is formed
- Cr(H₂O)₃(OH)₃ (s) + 3OH⁻ (aq) → [Cr(OH)₆]³⁻ (aq) + 3H₂O (l)
when acid is added to a yellow solution of CrO₄²⁻, an orange solution of Cr₂O₇²⁻ is formed. oxidation state of Cr ion remains the same
- 2CrO₄²⁻ (aq) + 2H⁺ (aq) ⇌ Cr₂O₇²⁻ (aq) + H₂O (l)
in LOs!
ligand exchange reactions involving copper complexes
add NH₃ (aq) dropwise, and then in excess, to a solution of Cu²⁺ (aq). a pale blue ppt of Cu(OH)₂ is formed, which dissolves in excess NH₃ (aq) to give a dark blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺
- Cu(OH)₂ is first precipitated when NH₃ (aq) is added dropwise
- Cu²⁺ (aq) + 2OH⁻ (aq) → Cu(OH)₂ (s)
- when excess NH₃ (aq) is added, ligand exchange reaction takes place, with NH₃ ligands replacing H₂O ligands to form a dark blue solution. [Cu(H₂O)₆]²⁺ (aq) same as Cu²⁺ (aq)
- [Cu(H₂O)₆]²⁺ (aq) + 4NH₃ (aq) → [Cu(NH₃)₄(H₂O)₂]²⁺ (aq) + 4H₂O (l)
- this causes conc of Cu²⁺ (aq) to fall, so POE of the 1st equation shifts left, causing ppt of Cu(OH)₂ to dissolve to give a dark blue solution
- Cu(OH)₂ (s) ⇌ Cu²⁺ (aq) + 2OH⁻ (aq)
add conc. HCl when dropwse to Cu²⁺ (aq). the solution turns from pale blue to green, then to yellow
- when conc. HCl is added, ligand exchange reaction takes place, with Cl⁻ ligands replacing H₂O ligands
- [Cu(H₂O)₆]²⁺ (aq) + 4Cl⁻ (aq) → [CuCl₄]²⁻ (aq) + 6H₂O (l)
- the [Cu(H₂O)₆]²⁺ is pale blue in colour and [CuCl₄]²⁻ is yellow in colour, so the green colour is observed as both the blue aqua complex & yellow chloro complex are present
describe the ligand exchange reaction in haemoglobin
- a haemoglobin molecule in the lungs picks up an O₂ molecule, which forms a dative bond with the iron in haemoglobin to form a oxyhaemoglobin
- CO can undergo a ligand exchange reaction and replace O₂ as the ligand attached to the iron (II) ion, forming carboxyhaemoglobin. CO forms a strong dative bond with Fe2+
- the binding between CO and the iron (II) is irreversible, making carboxyhaemoglobin a very stable compound
- a relatively small quantity of CO can inactivate a substantial fraction of haemoglobin for oxygen transport
- haemoglobin is unable to transport oxygen and the victim dies as the body is deprived of oxygen
LO (splitting of d orbitals)
describe the splitting of degenerate d orbitals into two energy levels in octahedral complexes
splitting of degenerate d orbitals
- in a free transition metal ion, all 5 orbitals are degenerate. ligands approach the metal ion along the mutually perpendicular x, y & z axis
- as ligands approach, their electron pairs repel electrons in the five d orbitals of metal ions, causing all five d orbitals to increase in energy
- d electrons are repelled unequally due to the different orientations of d orbitals. ligands directly approach the lobes of d(x2-y2) and dz2 orbitals but between the lobes of dxy, dyz and dxz orbitals, so electrons in the d(x2-y2) and dz2 orbitals experience stronger repulsion and have higher energy
- the five d orbitals are no longer degenerate, but are split into 2 groups with an energy gap between them
d-d transition
- when white light shines on the solution, the electron at the lower energy level will absorb energy from the light and jump to one of the higher energy orbitals
- the energy gap between the 2 levels corresponds to the energy of photons with a range of wavelength in the visible spectrum
MRS LEE SAY MEMORISE
explain, in terms of d orbital splitting & d-d transition, why TM complexes are usually coloured
- in the presence of ligands, the partially filled 3d orbitals of a TM ion are split into 2 different energy levels with a small energy gap ΔE between them
- there are vacancies in the higher energy d orbitals. the promotion of an electron from the lower to the higher of these d orbitals requires the absorption of radiation in the visible spectrum with energy corresponding to the energy gap
- such d-d transition is responsible for the colour of the complex ion
- the colour observed is the complement of the colours absorbed
what factors affect the colour of complexes
oxidation state of the metal
- different number of electrons in d orbitals of TM ion will result in different interaction with the electrons from the ligands, giving rise to different energy gap
- hence wavelengths of light absorbed will be different
- absence of colour in Sc³⁺, Ti⁴⁺, Cu⁺, Zn²⁺ is due to them having no d electrons or fully filled d orbitals, so no d-d electronic transition is possible
nature of ligand
- different ligands have different effects on the splitting of the d orbital
- difference in magnitude of ΔE will result in different colours observed
why are some compounds with transition elements colourless?
for TM with fully filled 3d subshell/empty 3d subshell, d-d transition is not possible. absorption of radiation in the visible light region cannot occur, so all colours are transmitted, resulting in the white colour
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atomic structure18
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chem bonding46
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gaseous state16
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energetics24
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kinetics19
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chem equilibria14
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intro to organic chem16
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isomerism15
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alkanes21
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alkenes16
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arenes25
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solubility equilibria15
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acid base equilibria18
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halogen derivatives29
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hydroxy compounds39
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carbonyl compounds18
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carboxylic acid & derivatives32
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nitrogen compounds23
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electrochemistry13
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periodicity21
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transition elements18
