Transition metals Flashcards
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1
Q
D-block elements
A
- ‘True’ Transition metals are elements with incomplete d sub-shells in their atoms or common ions
- Chromium takes an electron from its 4s shell so that it has 1 electron in every orbital in 3d, as this is just more stable
- Copper takes an electron from 4s as well so that it can fill its 3d orbital
- Zinc and Scandium aren’t ‘true’ transition metals as they have complete d sub-shells
- When transition metal ions are formed, electrons are removed from 4s shell first. To find electronic structure first write the structure of the atom, then remove electrons to find ion
2
Q
Physical and chemical properties
A
- transition metals have variable oxidation states, as 4s and 3d sub-shells have very similar energies and electrons can be removed from either with similar amounts of energy
- they can act as catalysts
- they form coloured compounds
- they form complex ions
3
Q
Complex ions
A
- a metal ion with ligands co-ordinately bonded to it
- a ligand is a molecule with one or more lone pairs of electrons to donate to a metal ion
- the co-ordination number of an ion is the number of bonds it can form with ligands
- monodentate ligand= can form 1 bond (e.g H2O, NH3, Cl-), bidentate ligand= can form 2 (e.g. H2NCH2CH2NH2, C2O2 2-), polydentate = can form many (e.g EDTA 4-)
4
Q
Complex ion shapes
A
- shape depends on the co-ordinate number
- 2 ligands = linear shape, 180 degrees
- 4 ligands = square planar, 90 degrees
- (or) 4 ligands = tetrahedral, 109.5 degrees
- 6 ligands = octahedral (90 degrees)
5
Q
Haemoglobin
A
- Haemoglobin is a complex ion around an Fe2+ ion, with a multidentate ligand
- Oxygen can bond to it as a ligand, allowing it to be transported around the bloodstream
- CO or CN can also bond as a ligand, and they bond more strongly, poisoning the haemoglobin so it can no longer transport oxygen through the blood stream
6
Q
Isomerism in complex ions
A
- optical isomerism happens in octahedral complexes that have 3 bidentate ligands, as two mirror-image enantiomers can be produced
- cis/trans isomerism happens in square planar complexes with two pairs of monodentate ligands
- it could also occur in octahedral complexes with four ligands on one type and 2 of another
- cisplatin is the cis isomer
- Tollens reagent is a liner complex [Ag(NH3)2]+
7
Q
Ligand substitution
A
- Ligands can replace each other in a complex
- when the two sets of ligands are of similar size there is no change in co-ordination number
- e.g [Co(H2O)6]2+ + 6NH3 -> [Co(NH3)6]2+ +6H2O
- sometimes there is only a partial substitution, eg [Cu(H2O)6]2+ + 4NH3 -> [Cu(NH3)4(H2O)2]2+ + 4H2O
- if a small ligand is replaced by a large one, the co-ordination number decreases because fewer large ions can fit around the metal (e.g H2O or NH3 being substituted by Cl-)
8
Q
Chelate effect
A
- when ligands are substituted by ligands that can form more co-ordinate bonds, there are more molecules of product than of reactant so entropy increases
- as there is little enthalpy change, this means the Gibbs free energy is negative, so the reaction is spontaneously feasible
- the ligands cannot be substituted back because the reaction isn’t feasible in that direction
- EDTA4- is a good chelating agent, and it can be used to ‘neutralise, complexes
9
Q
Coloured ions
A
- energy of a photon = h*frequency
- the incomplete d-orbital splits when it bonds to ligands, and the electrons are excited up the energy gap between the two levels by certain certain frequencies of light according to e=hf
- the colour of the compound is a mixture of all the remaining frequencies in the visible spectrum, and can identify a transition metal complex as they are mostly unique
- the colour is changes by oxidation, co-ordination number or the type of ligand and metal ion in the complex
10
Q
Spectroscopy
A
- more concentrated solutions have more intense colour, and this can be measured with a colorimeter
- sometimes a ligand can be added to intensify the colour of a complex like SCN-
- in analysis, light is shone through a filter to only let through the colour of light absorbed by the sample. The frequency of light that is absorbed the most is the frequency corresponding to the energy gap in the complex
- a graph of intensity/concentration can be plotted using a known complex, and this can be used to find the concentration of a solution of the same complex
11
Q
Variable oxidation states
A
- the oxidation state of a transition metal often changes in reactions, and how easily this occurs depends on the pH of the solution and the ligands it bonds to
- oxidising occurs more easily in alkaline solutions, reduction in acidic conditions
- redox potentials show how easy it is to reduce an ion, these are equivalent to electrode potentials (when in standard conditions)
- a higher redox potential means the oxidation state is more likely to be reduced
12
Q
Vanadium chemistry
A
- Vanadium can be reduced in acidic conditions with a zinc catalyst
- Vandium (v) is a yellow solution
- Vanadium (iv) is a blue solution (although initially you see a blue solution as the yellow and green mix)
- Vanadium (iii) is a green solution
- Vanadium is (ii) is a purple solution
- adding concentrated ammonia can re-oxidise it back
13
Q
Redox titrations
A
- MnO4- +8H+ + 5e- -> Mn2+ +4H2O
- KMnO4 is a common oxidising agent, when used in redox titration with a transition metal the endpoint is clear as the solution goes from purple to colourless (or light pink)
- example, Fe2+ to Fe3+ with KMO4:
5Fe2+ + MnO4- + 8H+ -> 5Fe3+ + Mn2+ + 4H2O - The Fe2+ can either be reduced from Fe3+ by zinc, or oxidised from Fe by H2SO4
14
Q
Autocatalysis
A
- When ethandioate is oxidised by KMnO4 to CO2, it undergoes auotcatalysis as the Mn2+ ion product is also a catalyst
- the redox reaction is:
5C2O42- + 2MnO4- +16H+ -> 10CO2 + 2Mn2+ + 8H2O - initially it is slow because two negative ions are reacting together, but as Mn2+ ions are produced the reaction speeds up as it catalyses it
- 4Mn2+ + MnO4- + 8H+ -> 5Mn3+ + 4H2O
- 2Mn3+ + C2O4 2- -> 2Mn2+ + 2CO2
- towards the end of the reaction the rate slows down again as the MnO4- is used up
15
Q
Catalysts
A
- a catalyst is a substance that increases the rate of a chemical reaction, without being used up, by providing an alternate reaction pathway with a lower Ea
- at least one reactant is adsorbed onto the surface of the catalyst at an active site. This can weaken the bonds of the molecule, position the molecule more favourably or bring the reactants closer together
- solid catalysts can be poisoned when a molecule is permanently adsorbed onto the surface
- maximising the available surface area of the catalyst will increase efficiency
