Transition Chemistry
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What happens during a redox titration?
A reducing agent is titrated against an oxidising agent. The former loses electrons which the latter gains simultaneously.
Give the main equation for a manganate titration.
MnO4- + 5Fe2+ + 8H+ -> Mn2+ + 4H2O + 5Fe3+
What is the use of a manganate titration?
Determination of the concentration of an unknown reducing agent directly.
Why do we need an excess of sulphuric acid for a manganate titration?
The reaction must be acidified so that the manganate ions act as good oxidising agents; H2SO4 is a strong acid so provides sufficient [H+]. Sulphate ions aren’t oxidised under these conditions whereas others e.g. Cl- might.
Why is a manganate titration considered self-indicating?
MnO4- has a violet colour. When is is reduced to Mn2+ at the endpoint, it instead produces a near-colourless, pale pink colour.
Give the equation for an iodine-thiosulfate titration.
2S2O3 2- + I2 -> S4O6 2- + 2I-
What is the use of a thiosulfate titration?
It is used after an initial redox reaction (e.g. reduction of Cu2+ with I- to produce CuI and I2). It allows for the indirect determination of the concentration of an unknown compound by determining the amount of I2 it reacts to produce.
Describe the colour change during an iodine-thiosulfate titration, and why this may pose a challenge practically.
I2 is a brown solution, and I- is colourless. However, the end-point can be hard to judge since the colour of the solution changes very slowly.
What extra solution do we add towards the endpoint of an iodine-thiosulfate titration, and why?
We add starch when the solution turns to a pale straw colour. Starch has a much stronger blue-black in the presence of iodine - so we can determine exactly when all the iodine has been used up, when this colour disappears.
What is a transition element defined as?
A d-block element which forms stable ions with partially-filled d-orbitals.
Why does the 4s sub shell fill and empty before the 3d subshell?
As energy levels increase, the subshells move closer together in terms of energy, to the point at which they overlap.
What common exceptions do we need to know when determining the electronic configurations of transition elements?
Chromium: 3d5 4s1, because half-filled d-orbitals are more energetically favourable Copper: 3d10 4s1, because a full d-orbital is preferable to a partially-filled one
Why aren’t scandium and zinc transition elements?
They do not form partially-filled d-subshells.
What are the major properties of transition elements?
• They form coloured compounds and solutions • They form complexes • They exhibit variable oxidation states • They act as good catalysts (think nickel for reduction, or iron in the Haber process)
Describe the trend in number of achievable oxidation states across the period.
• Initially the number of possible oxidation states increases as the number of electrons in the d-subshell increases • Beyond manganese the increased nuclear charge makes it energetically unfeasible to remove extra electrons • Number of oxidation states falls
What chemical quantity helps us to understand the feasibility of certain oxidation states?
Successive ionisation energies - if they are too high then it becomes thermodynamically unfeasible to remove electrons to form differently-charged ions
What is a ligand?
A molecule or ion which can donate a lone pair of electrons to a central metal atom.
What is a complex?
A metal ion which has formed co-ordinate bonds with ligands
What is the co-ordination number?
Describes the number of covalent bonds formed by a metal ion in a complex.
What phenomenon prevents large ligands from forming octahedral complexes?
Steric hindrance - large ligands can’t ‘fit’ around the central metal atom in large numbers.
How many co-ordinate bonds do the following ligands form with a complex? - :Cl- - :OH-
• x4 • x6
What shape of complex do Pt, Ni form?
Square planar, the rest form tetrahedral complexes.
What is a mono dentate ligand?
A ligand which donates 1 pair of electrons to a metal ion
What is a bidentate ligand?
A ligand donating 2 pairs of electrons to a central metal ion.