Transition Metals 3 (redox titrations, variable oxidation states and coloured ions) Flashcards
Describe redox reactions involving transition metals
An oxidising agent is titrated against a reducing agent (reducing agent in burette (known concentration), oxidising agent in flask).
Transition metals have variable oxidation states associated with colours so titrations are self-indicating (no indicator needs to be added)
What are the variable oxidation states of vanadium and their associated colours?
5+ (VO_2)^+ Yellow
4+ VO^(2+) Blue
3+ V^3+ Green
2+ V^2+ Violet
How can you see the variable oxidation states of vanadium?
NH4VO3 mist be dissolved in sulfuric acid (it is insoluble in water) so forms a yellow solution containing VO2+. Ammonium vanadate (v) is a white solid (often pale yellow due to impurities) which contains V with oxidation state 5+.
How can V be reduced?
Using zinc metal in acidic conditions (reduction of V is successive, from +5 to +2). Also exclude air (use stopper)
What do redox potentials give an indication of?
An atom or ion’s ability to be oxidised or reduced. The more positive the redox potential, the less stable the ion will be and more likely to be reduced to a lower oxidation state.
Why are redox potentials of an ion not always the same as standard electrode potentials?
Varies according to environment (not under standard conditions)
What factors affect redox potentials?
Ligands
pH
How do ligands affect redox potentials?
In a different environment, metal ion can be surrounded by different ligands, which can have a stronger or weaker bond than the H2O ligand. This affects the ability for the ion to gain or lose electrons and hence redox potential.
How does pH affect redox potentials?
Some metal ions need H+ ions to be reduced and some release OH- ions when reduced. Redox potentials are generally more positive in acidic conditions.
Why are redox potentials generally more positive in acidic conditions?
Affects equilibrium; adding more H+ shifts equilibrium to the right to oppose the change hence increasing the ion’s ability to accept electrons and therefore increasing its ability to be reduced.
What transition metal properties does Ag not follow?
Does not form coloured compounds
Does not have variable oxidation states
Why does Ag not follow some transition metal properties?
Ag and Ag+ have full d-orbitals, so electron are unable to transition between d-orbitals as they cannot absorb light.
What transition metal properties does Ag follow?
Forms complex ions e.g. [Ag(NH3)2]+ - Tollen’s
Acts as a catalyst ( with weak adsorption in many cases)
What is the electrode potential of Ag+?
Large and positive, so easily reduced to Ag.
How is Tollen’s reagent formed?
NaOH is added dropwise to silver nitrate solution until there is no further reaction. Light brown precipitate of Ag2O forms.
Dilute ammonia solution is added dropwise to Ag2O precipitate until it redissolves. [Ag(NH3)2]+ complex is formed.
