Friday 5 August 2011

Chemistry: d-block elements (Transition elements)

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Transition metals
Features: high m.p., high density, good conductors of electricity, rigid and similar atomic radius.
-          Ability to make alloy: similar atomic radius make them able to stay smoothly on the same metallic layer, so their alloy can be strong.
-          Colored ion: Ti3+ (purple), Ni2+ (green), variable state of V and Mg; this is because their absorbance spectrum appears in the visible light spectrum. White light – absorbed spectrum = color of the ion. For example, Cu2+ absorbs the spectrum 650~900nm, leaving the visible light from 400~600nm which is purple, blue and green. So it appears to be blue.
Variable oxidation state
-          Except Sc and Zn, all transition metals in period 4 exist in more than one oxidation state. Ti (+1~+4), V(+2~+5), Cr(+1~+6), Mn(+1~+7), Fe(+1~+6), Co(+1~+5), Ni(+1~+5), CU(+1~+3), Sc, Ti, V, Cr appears commonly in +3 while the rest also exist commonly in +2.
-          Lower oxidation states are usually found in simple ions or ionic compound, like Cr3+, FeO. Higher oxidation states exist with covalent bonding with highly electronegative atoms like O and Cl, e.g. VO3-, CrO42-.
Catalytic properties of transition metal, e.g., Fe in Haber process, V2O5 in contact process, Pt-Rh (Rhodium) in producing ammonia, Ziegler-Natta Catalyst (Et3Al/TiCl4) in producing PE, Ni in hydrogenation and CuO in oxidation of ethanol to ethanal.
It’s also used as tracers like 60Co (radioactive) and nutrition like Co in Vitamin B12.
Vanadium usually it exist in 4 oxidation state: +5(yellow, dioxovanadium(V) ion VO2+), +4(blue, oxovanadium(IV) ion VO2+), +3(green, vanadium(III) ion V3+) and +2(purple, vanadium(II) V2+), starting from ammonium metavanadate NH4VO3 solution (red solution initially and turn yellow later), VO2+ is produced. Since in ECS the reaction between Zn and Zn2+ is above the reduction of VO2+, VO2+, V3+, so under finely powdered Zn and H2SO4 as catalyst, it reduces VO2+ all the way to V2+. It can’t reduce V2+ to V because this is higher than Zn in the ECS.
Manganese usually exist in 5 oxidation state: +7(MnO4-), +6(permanganate, dark green, in alkaline only, MnO42-), +4(MnO2, black solid), +3(Mn(OH)3, brown solution) and +2(Mn2+). It’s an useful oxidizing agent with dilute H2SO4. In alkaline condition permanganate change to MnO2 by MnO4- + 2H2O + 3e- MnO2 + 4OH-.
Iron usually exists in oxidation state of +2 and +3, Fe2+ act as both oxidizing and reducing agents.
Note: Sc and Zn are usually regarded as non-typical transition metal because their ion is colourless and they only have one oxidation state.

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