topic 13 - transition metals Flashcards
define a transition metal
one which forms one or more stable ions which have incompletely filled d orbitals
define a transition metal
one which forms one or more stable ions which have incompletely filled d orbitals
what does this definition mean for zinc?
distinguishes zinc from d-block elements zinc Zn only forms the ion Zn2+, which has a configuration [Ar]3d10
when d sub shell is empty, 4s is at ——— than 3d
lower energy
when d sub shell is occupied, 4s is at ———- than 3d
higher energy
so in the case of ionisation, where are electrons removed from first - 4s or 3d?
4s
in the case of electron sub-shell filling, where are electrons added first - 4s or 3d?
4s
describe copper
[Ar]3d104s1
- can lose 4s electrons to form Cu(I)
- compounds or lose one 4s and one 3d electron to become Cu(II) compounds
describe scandium
[Ar]3d14s2
- normally forms Sc(III) ions
- therefore, it will have no electrons in the D orbital [Ar]3d04s0
- hence, scandium does not show typical properties of transition metals
state the 6 common properties of period 4
- generally hard metals
- exhibit magnetism
- have variable oxidation states
- show catalytic activity
- form complex ions
- ions are coloured
explain the hardness and high melting points of transition metals
- result from the strong metallic bonds in the elements
- due to their ability to release electrons from both outer and inner shells for bonding (eg metals in the first series use both 3d and 4s electrons for bonding)
describe the two forms of magnetism concerned with transition metals
paramagnetism - caused by unpaired electrons and are attracted by a magnetic field
diamagnetism - caused by paired electrons and repelled slightly by a magnetic field
why will all substances exhibit some diamagnetism?
as they have some paired electrons
why does paramagnetism dominate over diamagnetism?
the effect of the diamagnetism is much smaller than the paramagnetic effect and so in substances with an unpaired electron, paramagnetism dominates
the more unpaired electrons,
the greater the paramagnetism (magnetic moment)
Fe2+
[Ar]3d6
paramagnetism
Cr3+
[Ar]3d3
paramagnetism
Cu+
[Ar]3d10
diamagnetism
when a metal forms an ionic compound, what does the formula of the compound produced depend on?
the energetics of the process:
- the compound formed is the one in which most energy is released
- the more energy released, the more stable the compound
what are the two energy terms that must be considered in terms of which oxidation state will be favoured?
- the amount of energy needed to ionise the metal (the sum of the various ionisation energies)
- the amount of energy released when the compound forms (lattice enthalpy for solids, or hydration enthalpies of the ions for solutions)
give an example of a typical non-transition metal and its non-variable oxidation states
calcium chloride
- making Ca2+ ions instead of Ca+ requires greater ionisation energy, but more energy is released as lattice energy (as there is a greater attraction between chloride ions and Ca2+ ions than Ca+ ions)
- third electron comes from 3p shell which is much more difficult to remove; so although there will be a gain in lattice enthalpy, it isn’t anything to compensate for the extra ionisation energy
give an example of a typical transition metal and its variable oxidation states
iron
- the 4s orbital and 3d orbitals have very similar energy
- extra ionisation energy of increasing oxidation states is compensated by the extra lattice enthalpy/hydration enthalpy evolved
(VO2)+
- yellow
- 5+
- oxovanadium (V)
(VO)2+
- blue
- 4+
- oxovanadium (IV)
V3+
- green
- 3+
- vanadium (III)
V2+
- violet
- 2+
- vanadium (II)
what is the catalytic activity of transition elements and their compounds associated with? how does this work?
their variable oxidation states; transition metal ions can change their oxidation number by loss or gain of electrons. they then bind to reactants, forming intermediates as part of a chemical pathway of lower activation energy
describe how transition metal catalysts function
- the first transition series elements have partially filled 3d-orbitals that can accept electron density from the adsorbed molecules)
- reactant molecules are adsorbed onto the surface of the metal and held in place while a reaction occurs
- after the reaction, the products are desorbed and the metal remains unchanged
give 2 examples of reactions that are catalysed by transition metals or their compounds
- metallic iron, Fe, is the catalyst in the Haber process for the synthesis of ammonia (N2 + 3H2 -> 2NH3)
- vanadium (V) oxide, V2O5, is used in the contact process for making sulphuric acid (2SO2 + O2 -> SO3)
how does vanadium (V) oxide function in the contact process?
the sulfur dioxide is oxidised by vanadium (v) oxide, which itself is reduced to vanadium (iv) oxide. the latter is then deoxidised by oxygen to vanadium (V) oxide
define a ligand
an atom, molecule, radical, or ion that forms a complex by donating a pair of electrons into vacant orbitals on the central metal atom (coordinate/dative bond)
when M^n+ ions are dissolved in water,
they become hydrated by aqueous ions
define complex ions
the transition metal ions with ligands attached
why is the whole complex an ion?
it carries a charge
the bonds within the complex are
coordinate bonds
define the coordination number
the number of ligands associated with the central metal ion
give 5 common ligands
water (‘aqua’ when part of a complex), ammonia (ammine), hydroxide ion (hydroxo), chloride ion (chloro) and cyanide ion (cyano)
define a unidentate ligand
a ligand that donates 1 pair of electrons to a complex (eg H2O, NH3, Cl-)
define a bidentate ligand
a ligand that donates 2 pairs of electrons to a complex (eg ethanedioate ion, where a lone pair of electrons on each of the oxygen atoms can for coordinate bonds)
define a multidentate ligand
can form several bonds with the central metal ion as it has a number of atoms with lone pairs of electrons available for bonding (eg EDTA4- which has 6 donor atoms with lone pairs, 2 on N atoms and 4 on oxygen atoms)
what are multi dentate ligands also sometimes called?
chelating ligands
why are the complexes formed by multi dentate ligands very stable?
because the central metal ion is held by many co-ordinates.
state 4 typical complex ion shapes
- octahedral: complexes with six smaller ligands like H2O and NH3 (eg [Co(H2O)6]2+)
- tetrahedral: complexes with four larger ligands like Cl- (eg [CuCl4]2-)
- square planar: complexes with two small and two large ligands (eg Ni(NH3)2Cl2]
- linear: two ligands (eg [Ag(NH3)2]+
how does absorption produce colour?
if an electro in the d sub shell absorbs wavelengths of visible light, it will reflect the colour (which we see) made up of all the other visible wavelengths minus the ones it absorbs
if an ion or a compound has atoms with no d orbitals, or if they have a full d configuration
the ion/compound will absorb outside the visible region and appear colourless/white
quantum theory
dE=hf
- transition: when an electron moves to a higher energy level it absorbs an amount of energy equal to the energy difference between the two allowed energy levels
- the energy absorbed can cover a range of frequencies; if the frequencies are in the visible region, coloured will be observed
if the transition is larger, the radiation may fall into the
UV region
if the transition is smaller, the radiation may occur in the
IR region
the colour of a transition metal is associated with
- incompletely filled d orbitals
- the nature of the ligands surrounding the central cation
how does the ligand affect the d orbitals in a free metal ion?
- the degeneracy of the 3d orbitals is destroyed
- an electron that is close to a ligand will be repelled and hence the energy of such orbitals is raised relative to the others
- in a complex ion the 3d orbitals are split into two of higher energy and three of lower energy
- there is an energy difference between the two sets, dE
- on the absorption of light energy, a d electron is moved from the lower energy level to the higher energy level (d-d transition)
- the frequency of the light that causes this transition is in the visible region, 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 cation and type of ligand
explain why aqueous solutions of copper (II) ions are blue in colour
- aqueous solutions of copper (II) ions contain [Cu(H2O6)2+] aqueous complex ions
- the electronic configuration of a Cu2+ ion is [Ar]3d9
- splitting of 3d orbitals by the ligands occurs in a complex ion
- when white light is shone onto the solution, an electron is promoted from lower to higher level (the energy gap is dE), called a d-d transition.
- the frequency of the light absorbed corresponds to that of red light (dE=hf)
- the solution appears blue as white light minus red light gives blue
why do metal ions with empty (3d0) d orbitals form colourless ions?
there are no d-electrons to promote
why do metal ions with full (3d10) d orbitals form colourless ions?
there are no vacant orbitals to which an electron can be promoted
the light that is reflected or transmitted to the observer’s eye is not a complete spectrum of the wavelengths that make up white light, and therefore
it appears to have a colour complementary to that of the absorbed light
how does nuclear charge affect dE?
a larger nuclear charge will have a stronger force of attraction so more energy will be needed to move the electron from 1d orbital to another
factors affecting dE
- type and size of ligand
- strength of metal-ligand bond in the complex
- shape of the complex
- coordination number
- oxidation state
why does the colour of a transition metal ion depend on the nature of the ligands bonded to the central metal ion?
- some ligands have strong electrical fields which cause a large energy gap when the d orbitals split into 2 groups
- the greater the splitting, the more energy needed to promote an electron from the lower group of orbitals to the higher ones
- greater energy = shorter wavelengths of light absorbed
as splitting increases, the light absorbed will tend to
shift away from the red end of the spectrum towards orange, yellow, and so on
how does changing the oxidation state impact the colour of a ligand?
- changes the number of electrons in the d levels, altering dE
- change colour of light absorbed
how is cobalt chloride used to detect the presence of water?
when water is present, cobalt forms [Co(H2O)6]2+] which is pink as it is a different ligand and coordination number to [Co(Cl4)]2- which is blue
splitting is greater if the ion is —— than ——-
octahedral; tetrahedral
