* Advanced Inorganic Chemistry Transition metals 10.5 Vanadium Chemistry Doc Brown's

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 Doc Brown's Chemistry  Periodic Table Revision Notes 10.5

Part 10. Transition Metals 3d-block:  10.5 Vanadium Chemistry

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses) GCSE Periodic Table * GCSE notes Transition Metals

INORGANIC Part 10 3d block TRANSITION METALS sub-index: 10.1-10.2 Introduction 3d-block Transition Metals * 10.3 Scandium * 10.4 Titanium * 10.5 Vanadium * 10.6 Chromium * 10.7 Manganese * 10.8 Iron * 10.9  Cobalt * 10.10 Nickel * 10.11 Copper * 10.12 Zinc * 10.13 Other Transition Metals e.g. Ag and Pt * Appendix 1. Hydrated salts, acidity of hexa-aqua ions * Appendix 2. Complexes & ligands * Appendix 3. Complexes and isomerism * Appendix 4. Electron configuration & colour theory * Appendix 5. Redox equations, feasibility, Eø * Appendix 6. Catalysis * Appendix 7. Redox equations * Appendix 8. Stability Constants and entropy changes * Appendix 9. Colorimetric analysis and complex ion formula * Appendix 10 3d block - extended data * Appendix 11 Some 3d-block compounds, complexes, oxidation states & electrode potentials * Appendix 12 Hydroxide complex precipitate 'pictures', formulae and equations

Advanced Level Inorganic Chemistry Periodic Table Index * Part 1 Periodic Table history * Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s-block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p-block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots * All 11 Parts have their own sub-indexes near the top of the pages

10.5. Chemistry of Vanadium V, Z=23, 1s22s22p63s23p63d34s2 

  • V data table 1 summary * extended vanadium data table 2 * Vanadium & electrode potential chart of 3d-block

  • Summary of some complexes-compounds & oxidation states of vanadium compared to other 3d-block elements

  • Vanadium is one of many transition metals alloyed with iron to make specialist steels.

  • Vanadium(V) oxide, V2O5, is used as a catalyst in the 'Contact Process' in the production of sulphur trioxide for the manufacture of sulphuric acid.

    • The catalysing of the conversion of sulphur dioxide into sulphur trioxide is explained via change in oxidation state changes i.e. some classic transition metal chemistry.

    • 2SO2(g) + O2(g) ==> 2SO3(g) 

    • The mechanism, somewhat simplified, goes via the catalytic cycle ...

      • (i) SO2 + V2O5 ==> SO3 + V2O4, then (ii) V2O4 + 1/2O2 ==> V2O5 

      • The vanadium changes oxidation state from +5 to +4 and back to +5 in the catalytic cycle, a classic combination of two characteristics of transition metals - variable oxidation state and catalytic properties.

      • This is an example of heterogeneous catalysis - reactants (g) and catalyst (s) in different phases.

  • Vanadium shows a 'classic' display of variable oxidation states of varying colours when a solution of e.g. ammonium vanadate(V), is reduced by a zinc/dilute sulphuric acid mixture. do picture

    • Acidification changes the vanadate(V) ion into the pale yellow oxo-cation VO2+ (oxovanadium(V) ion)

    • VO43-(aq) + 4H+(aq) rev VO2+(aq) + 2H2O(l) [an acid-base reaction, NOT a redox change]

      • Note: Highly charged cations >3+ rarely exist as the simple 'hydrated' tetra or hexa-aqua ion.

      • The theoretical polarising power of the 'central metal ion' is so strong that they form oxocations (see above) or oxyanions e.g.

      • orange dichromate(VI) Cr2O72-, yellow chromate(VI) CrO42-, purple manganate(VII) MnO4- etc.

      • For transition metals they may be coloured even if electronically the theoretical 'central metal ion'  has a noble gas structure e.g. [Ar] in its maximum oxidation state like V(V), Cr(VI) and Mn(VII).

      • These oxyanions are called charge transfer complexes and the theory is beyond pre-university chemistry.

    • Three successive reduction steps then follow to eventually give V2+ ions, shown as half-cell equations:

    • (i) V(V, +5) ==> V(IV, +4): VO2+(aq) + 2H+(aq) + e- rev VO2+(aq) + H2O(l)

      • EØhalf-cell potential = +1.00V, pale yellow to the blue oxovanadium(IV) ion

    • (ii) V(IV, +4) ==> V(III, +3): VO2+(aq) + 2H+(aq) + e- rev V3+(aq) + H2O(l)

      • EØhalf-cell potential = +0.34V, blue to the green vanadium(III) ion

      • Here the vanadium(III) ion, V3+, is actually the green hexaaquavanadium(III) ion, [V(H2O)6]3+

      • Both V(IV) and V(III) species are slowly oxidised by dissolved oxygen back to the V(V) compound in acid solution (see electrode potential comments later).

    • (iii) V(III, +3) ==> V(II, +2): V3+(aq) + e- rev V2+(aq)

      • EØhalf-cell potential = -0.26V, green to the purple-violet vanadium(II) ion.

      • V2+(aq) is powerful reducing agent and is unstable in the presence of air. Any dissolved oxygen will oxidise V2+(aq) back to the vanadium(III) cation.

      • V2+ is actually the purple-violet hexaaquavanadium(II) ion, [V(H2O)6]2+

    • Note

      1. The standard electrode potential EØZn(s)/Zn2+(aq) is -0.76V, so the reducing power of zinc is sufficient to effect any of the three vanadium oxidation state reduction changes described above.

      2. The reduction occurs on the surface of the zinc metal i.e. the site of electron transfer and you can write the above reductions as a fully balanced complete equations ...

        • (i) 2VO2+(aq) + 4H+(aq) + Zn(s) ==> 2VO2+(aq) + 2H2O(l) + Zn2+(aq)

          • EØreaction = EØreduction - EØoxidation = +1.00 - (-0.76) = +1.76V

          • The half-cell reaction of the reduction will have the most +ve EØpotential.

        • (ii) 2VO2+(aq) + 4H+(aq) + Zn(s) ==> 2V3+(aq) + 2H2O(l) + Zn2+(aq)

          • EØreaction = +0.34 -(-0.76) = +1.10V

        • (iii) 2V3+(aq) + Zn(s ==> 2V2+(aq) + Zn2+(aq)

          • EØreaction = -0.26 - (-0.76) = +0.50V

          • BUT the vanadium(II) cation is unstable in the presence of dissolve oxygen in air.

          • 1/2O2(g) + 2H+(aq) + 2e-   H2O(l) has a standard electrode potential of +1.23V,

          • so, for the vanadium(II) oxidation reaction ...

          • 1/2O2(g) + 2H+(aq) + 2V2+(aq) ==> 2V3+(aq) + H2O(l)

          • EØreaction = EØreduction - EØoxidation = +1.23 - (-0.26) = +1.49V

          • hence the if left standing open to air, the violet V2+(aq) solution will gradually change to a green V3+(aq) solution and in turn V3+(aq) will revert back to VO2+(aq) in the presence of air because of oxidation by dissolve oxygen unless protected by an inert atmosphere. (see Redox Electrode Potential Chart, V2+/V3+ and V3+/VO2+ potentials are less positive (below) that for O2/H2O/H+ potentials).

      3. You will see hydrogen formed simultaneously from the unavoidable metal-acid reaction.

        • Zn(s) + 2H+(aq) ==> Zn2+(aq) + H2(g)

  • Does vanadium chemistry show an example of disproportionation?

    • This is just a little academic exercise using standard electrode potential data.

    • A disproportionation reaction is where a species in one oxidation state spontaneously and simultaneously changes into two species of different oxidation states - one higher and one lower in oxidation number.

    • Examples: disproportionation in manganese(VI) chemistry and disproportionation in copper(I) chemistry

    • Question: In terms of aqueous ions, is the disproportionation of vanadium(III) into vanadium(II) and vanadium (IV) feasible?

      • (i) VO2+(aq) + 2H+(aq) + 2e- rev V3+(aq) + H2O(l)   (EØVO2+/V3+ = +0.34V)

      • (ii) V3+(aq) + e- rev V2+(aq)   (EØV3+/V2+ = -0.26V)

      • The disproportionation equation would be (iii) 2V3+(aq) + H2O(l) rev V2+(aq) + VO2+(aq) + 2H+(aq)

      • For equation (iii), (ii) will be the reduction half-cell equation and (i) reversed will be the oxidation half-cell reaction.

      • EØreaction = EØreduction - EØoxidation = = EØV3+/V2+ - EØVO2+/V3+ = (-0.26) - (+0.34) = -0.60V

      • showing the disproportionation is thermodynamically NOT feasible i.e. EØreaction is less than zero.

      • In fact what can actually happen is if you mix salt solutions of vanadium(IV) and vanadium(II) on an equimolar basis, you end up with a solution of vanadium(III) salts, a sort of 'anti-disproportionation' reaction!

Scandium * Titanium * Vanadium * Chromium * Manganese * Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum

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