Transition metals Flashcards
ligand
molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons.
complex
central metal atom or ion surrounded by ligands.
co-ordination number
number is number of co-ordinate bonds to the central metal atom or ion.
transition metal
incomplete d sub-level in atoms or ions
monodentate
(e.g. H2O, NH3 and Cl- ) which can form one coordinate bond per ligand
bidentate
(e.g. NH2CH2CH2NH2 and ethanedioate ion C2O4 2- ) which have two atoms with lone pairs and can form two coordinate bonds per ligand
multidentate
(e.g. EDTA4- which can form six coordinate bonds per ligand).
[Co(H2O)6]2+(aq) + 6NH3 (aq)
[Co(H2O)6]2+(aq) + 6NH3 (aq) -> [Co(NH3)6]2+(aq) + 6H2O (l)
[Cu(H2O)6]2+(aq) + 4NH3 (aq)
[Cu(H2O)6]2+(aq) + 4NH3 (aq) -> [Cu(NH3)4(H2O)2]2+ (aq) + 4H2O (l)
Cl-
The Cl- ligand is larger than the uncharged H2O and NH3 ligands so therefore ligand exchange can involve a change of co-ordination number.
[CuCl4]2-
yellow/green solution
[CoCl4]2-
blue solution
[Cu(H2O)6]2+ +2C2O42-
[Cu(C2O4)2(H2O)2]2- +4H2O
Chelate effect
substitution of monodentate ligand with a bidentate or a multidentate ligand leads to a more stable complex.
increased entropy
Free energy ΔG will be negative as ΔS is positive and ΔH is small
Tollens
[Ag(NH3)2]+
colour changes arise from
- oxidation state,
- co-ordination number
- ligand
how does colour change arise
Colour arises from electronic transitions from the ground state to excited states: between different d orbitals.
A portion of visible light is absorbed to promote d electrons to higher energy levels. The light that is not absorbed is transmitted to give the substance colour.
ΔE=hv=hc/λ
v = frequency of light absorbed (unit s-1 or Hz) H = Planck’s constant 6.63 × 10–34 (J s) E = energy difference between split orbitals (J) c = speed of light 3.00 x 108 (m s–1) λ = wavelength of light absorbed (m)
Sc
Scandium is a member of the d block. Its ion (Sc3+) hasn’t got any d electrons left to move around. So there is not an energy transfer equal to that of visible light.
spectroscopy
The amount of light absorbed is proportional to the concentration of the absorbing species (and to the distance travelled through the solution).
Some complexes have only pale colours and do not absorb light strongly. In these cases a suitable ligand is added to intensify the colour.
spectroscopy method
- Add an appropriate ligand to intensify colour
- Make up solutions of known concentration
- Measure absorption or transmission
- Plot graph of absorption vs concentration
- Measure absorption of unknown and compare
coloured filter
The colour of the filter is chosen to allow the wavelengths of light through that would be most strongly absorbed by the coloured solution.
trends in oxidation states
- Relative stability of +2 state with respect to +3 state increases across the period
- Compounds with high oxidation states tend to be oxidising agents e.g MnO4-
- Compounds with low oxidation states are often reducing agents e.g V2+ & Fe2+
VO2 +
Oxidation state +5 ( a yellow solution)
VO 2+
Oxidation state + 4 (a blue solution)
V 3+
Oxidation state + 3 (a green solution)
V 2+
Oxidation state + 2 (a violet solution)
Tollens’ reaction
Red
[Ag(NH3)2]+ + e- -> Ag +2NH3
Ox
CH3CHO + H2O->CH3CO2H + 2H+ + 2e-
NH3 and H2O
similar size
uncharged
Catalyst
Catalysts increase reaction rates without getting used up. They do this by providing an alternative route with a lower activation energy
heterogenous catalyst
different phase from the reactants
homogenous catalyst
same phase as the reactants
Strength of adsorption
The strength of adsorption helps to determine the effectiveness of the catalytic activity.
Some metals e.g. W have too strong adsorption and so the products cannot be released.
Some metals e.g. Ag have too weak adsorption, and the reactants do not adsorb in high enough concentration.
Ni and Pt have about the right strength and are most useful as catalysts.
adsorption
Adsorption of reactants at active sites on the surface may lead to catalytic action. The active site is the place where the reactants adsorb on to the surface of the catalyst. This can result in the bonds within the reactant molecules becoming weaker, or the molecules being held in a more reactive configuration. There will also be a higher concentration of reactants at the solid surface, so leading to a higher collision frequency.
steps in heterogeneous catalysis
- Reactants form bonds with atoms at active sites on the surface of the catalyst (adsorbed onto the surface)
- As a result bonds in the reactants are weakened and break
- New bonds form between the reactants held close together on catalyst surface.
- This in turn weakens bonds between product and catalyst and product leaves (desorbs).
V2O5
heterogeneous catalyst in the Contact process.
Overall equation : 2SO2 + O2 -> 2SO3
step1 SO2 +V2O5 ->SO3 + V2O4
step 2 2V2O4 + O2 -> 2V2O5
Fe
Fe is used as a heterogeneous catalyst in the Haber process.
N2 + 3H2 ->2NH3
poisoning of heterogeneous
Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency; this has a cost implication.
e.g. poisoning by sulfur in the Haber process and by lead in catalytic converters in cars means that catalysts lose their efficiency and may need to be replaced.
Cr2O3
hetrogenous catalyst is used in the manufacture of methanol from carbon monoxide and hydrogen.
CO + 2H2 ->CH3OH
Fe2+
homogenous catalyst
reaction is slow as both are negative
S2O8 2-(aq) + 2I-(aq) -> I2(aq) + 2SO4 2-(aq)
Step 1
S2O8 2- + 2Fe2+ -> 2Fe3+ + 2SO4 2-
Step 2
2Fe3+ + 2I- -> I- + 2Fe2+
Autocatalysis
When a product catalyses the reaction
overall
2 MnO4- + 5 C2O42- + 16 H+ ->2Mn2+ + 10 CO2 + 8 H2O
Step 1
4Mn2+ + MnO4- + 8 H+ -> 5Mn3+ + 4 H2O
Step 2
2Mn3+ + C2O42- -> 2Mn2+ + 2 CO2
Autocatalysis reaction
The initial uncatalysed reaction is slow because the reaction is a collision between two negative ions which repel each other leading to a high activation energy.
The Mn2+ ions produced act as an autocatalyst and therefore the reaction starts to speed up because they bring about the alternative reaction route with lower activation energy. ThereactioneventuallyslowsastheMnO4- concentrationdrops.
Silver
Silver behaves like the transition metals in that it can form complexes and can show catalytic behaviour (although it adsorbs too weakly for many examples).
Silver is unlike the transition metals in that it does not form coloured compounds and does not have variable oxidation states.
Silver complexes all have a +1 oxidation state with a full 4d subshell (4d10). As it is 4d10 in both its atom and ion, it does not have a partially filled d subshell and so is not a transition metal by definition. It is not therefore able to do electron transitions between d orbitals that enable coloured compounds to occur.
[Ag(NH3)2]+ + NaBr
AgBr
cream ppt
AgBr + Conc NH3
[Ag(NH3)2]+
colourless solution
AgCl + Dilute NH3
[Ag(NH3)2]+
[Ag(H2O)2]+ (colourless) + NaCl
AgCl
White ppt
[Ag(H2O)2]+ + Dilute NH3
[Ag(NH3)2]+
Ag + Conc HNO 3
[Ag(H2O)2]+
[Ag(NH3)2]+ + aldehyde
Ag
AgCl dissolving
AgCl(s) + 2NH3(aq) -> [Ag(NH3)2]+ (aq) + Cl- (aq)
Tollens reactions
Red : [Ag(NH3)2]+ + e- -> Ag +2NH3
Ox : CH3CHO + H2O -> CH3CO2H + 2H+ + 2e-
Cr3+
green/violet
Cr2O7 2-
Orange
Mn2+
pale pink
MnO4 -
Purple
Fe 2+
pale green
Fe 3+
yellow
Co 2+
pink
Ni 2+
green
Cu 2+
blue
