A Level Chemistry Transition Metal REDOX PDF

A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS VARIABLE OXIDATION STATES We learned earlier that one of the features of transition metals is that they have multiple stable oxidation states. This means that they feature in many...

A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS VARIABLE OXIDATION STATES We learned earlier that one of the features of transition metals is that they have multiple stable oxidation states. This means that they feature in many REDOX reactions. How their oxidation state changes in a REDOX reaction depends on a couple of factors… 1. pH Transition metals in higher oxidation states tend to be reduced in acidic conditions. We’ve already seen an example of this in year 12 organic chemistry. Potassium manganate (VII) (KMnO4) and sodium dichromate (VI) (Na2Cr2O7) are used as oxidising agents, so they are therefore reduced themselves. Acidic conditions are needed for this reaction to occur. MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l) Cr2O72-(aq) + 14H+(aq) + 6e- → 2Cr3+(aq) + 7H2O(l) Transition metals in lower oxidation states tend to be oxidised in alkaline conditions. No further details needed here! 2. Ligand Type The types of ligands that are coordinately bonded to the central transition metal ions in a complex are also a factor in how easily it is oxidised or reduced. The bond enthalpy of the coordinate bonds is the major factor here. The stronger the bond enthalpy of the ligand-ion coordinate bond, the more difficult it is for the ion to be oxidised / reduced. You do not need to know examples here. Just know that there's a link between these two factors. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS VANDIUM + ZINC Vanadium has several stable oxidation states, including +V, +IV, +III and +II. Vanadium (V) is reduced down through these various oxidations states by reaction with zinc in acidic conditions. We can use the half-equations and E𝚹 values to show this: ELECTRODE POTENTIAL E𝚹 (V) HALF-EQUATION V2+(aq) + e- ⇌ V(s) -1.20 Zn2+(aq) + 2e- ⇌ Zn(s) -0.76 V3+(aq) + e- ⇌ V2+(aq) -0.26 VO2+(aq) + 2H+(aq) + e- ⇌ V3+(aq) + H2O(l) 0.34 VO2+(aq) + 2H+(aq) + e- ⇌ VO2+(aq) + H2O(l) 1.00 Zn is a powerful enough reducing agent to reduce vanadium from: 1. +V to +IV 2. +IV to +III 3. +III to +II However, it is not strong enough to reduce vanadium from +II to 0. Make sure you know the colours of the different oxidation states for vanadium so you can give the observations as these consecutive REDOX reactions occur: O.S. Ion Complex in (aq) Colour +V VO2+ [VO2(H2O)4]+ Yellow +IV VO2+ [VO(H2O)5]2+ Blue +III V3+ [V(H2O)6]3+ Green +II V2+ [V(H2O)6]2+ Purple AQA www.chemistrycoach.co.uk © scidekick ltd 2024 A LEVEL CHEMISTRY 3.2.5 TRANSITION METALS TOLLEN’S REAGENT Tollen’s reagent (ammoniacal silver nitrate) [Ag(NH3)2]+ is used in organic chemistry as a test to identify carbonyls (aldehydes and ketones). REDOX is the underlying principle behind the test. Aldehydes can be oxidised further to carboxylic acids. Ag+ is able to cause this to occur and is therefore reduced to Ag(s) itself, giving the positive result of a “silver mirror” being deposited. Ketones cannot be further oxidised. As such, the Ag+ is not reduced, so a silver mirror is not formed. MANGANATE (VII) (MnO4-) TITRATIONS The colour changes of complex ions as they change oxidation states is put to use in REDOX titrations. For example, acidified MnO4- can be used to find the concentrations of Fe2+ and C2O42- ions in solution. As the MnO4- ions oxidise the other species, the Mn is reduced itself. This causes a colour change: MnO4-(aq) + 8H+(aq) + 5e- → Mn2+(aq) + 4H2O(l) PURPLE COLOURLESS These reactions are said to be “self indicating” as the colour change only occurs so long as there are in the conical flask to react with. As soon as the solution in the conical flask stays purple, this means that there are no more Fe2+ / C2O42- ions to be oxidised and the reaction is complete. Reaction with Fe2+ (1:5 ratio) MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) Reaction with C2O42- (2:5 ratio) 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l) More on REDOX titrations in the A Level Practical Book. AQA www.chemistrycoach.co.uk © scidekick ltd 2024