Variable Oxidation States - Transition Metals
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Why do transition metals show variable oxidation states? (3d/4s obrtls. cls. enrgy. - e- cn. rmvd. frm. bth.)
The 3d and 4s orbitals are close in energy, so electrons can be removed from both
What happens when oxidation states change?
A redox reaction occurs
Fill in the table stating the oxidation states and colours of vanadium ions
V²⁺ → violet V³⁺ → green VO²⁺ → blue VO₂⁺ → yellow
How can vanadium be reduced?
Using zinc in acidic solution
What happens in the first vanadium reduction step?
VO₂⁺ is reduced to VO²⁺, with a colour change of yellow to blue
What happens in the second step of vanadium reduction?
VO²⁺ is reduced to V³⁺ with a colour change of blue to green
What happens in the third step of vanadium reduction?
V³⁺ is reduced to V²⁺, with a colour change of green to violet
What is happening to zinc in these reactions?
Zinc is oxidised to Zn²⁺.
What do electrode potentials measure?
The tendency of a species to be reduced
What does a more positive electrode potential indicate? (grtr. tndncy. rdce - lss. stble.)
Greater tendency to be reduced, being less stable
How do ligands affect electrode potentials? (lgnds. frm. cmplxs. w/ mtl. ions. chngng. hw. esly. rdcd. + thrfre. altrng. E vls.)
Ligands form complexes with metal ions, changing how easily they are reduced and therefore altering the E° values
How do ligands affect oxidation state stability? (hghly. elctrneg. lgns. stbls. hghr. ox. sts. - whrs. ls. elcrneg. lgnds. stbls. lwr. ox. sts.)
Highly electronegative ligands stabilise higher oxidation states, whereas less electronegative ligands stabilise lower oxidation states
Why do different oxidation states have different colours? (diff. nmbrs. - d elctrns. chnge. enrgy. gap - diff wvelngths. absrbd.)
Different numbers of d - electrons change the energy gap, so different wavelengths are absorbed
How does pH affect electrode potential? (>acdc. cndnts. - inc. E - mkng. rdctn. >lkly)
More acidic conditions (higher [H⁺]) increase E°, making reduction more likely
How do you calculate overall E°?
E°cell = E°(reduction) − E°(oxidation)
How do E° values relate to oxidising/reducing ability?
The more positive E°, the better the oxidising agent. The more negative E°, the better reducing agent
What is Tollens’ reagent?
A solution containing [Ag(NH₃)₂]⁺ complex ions
How is Tollens’ reagent made?
By adding aqueous ammonia to silver nitrate
What does Tollens’ reagent test for, and what does a positive result show?
Tests for aldehydes, with a positive result showing silver mirror formation
In MnO₄⁻ titration, where is each solution placed and what is the endpoint in MnO₄⁻ titration?
Endpoint: permanent pale pink colour Fe²⁺ is placed in the conical flask and MnO₄⁻ in burette