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d-Block: Standard Reduction Potential Trends And Stability Of Higher Oxidation States

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d-Block: Standard Reduction Potential Trends And Stability Of Higher Oxidation States - Lesson Summary

The plots of the experimental and calculated values of the reduction potentials shows that the experimental and calculated values are in close agreement with each other.

The M+2/ M reduction potentials have enthalpic contributions from the terms in the equation


The equation indicates that the magnitude of the reduction potential is governed by the values of three relatively large terms -

The irregularity in the variation of electrode potentials is due to the irregular variation of the ionisation enthalpies and also the hydration energies of the divalent ions of these elements.

All those elements with negative reduction potentials act as strong reducing agents and liberate hydrogen from dilute acids.

Copper does not liberate hydrogen from dilute acids because of its positive electrode potential.

The values of the reduction potentials for Mn, Zn &Ni are more negative than expected. The electrode potential values of manganese and zinc can be explained on the basis of the stability of the half-filled d sub-shell in Mn+2, and the completely filled d10 configuration in Zn+2.

Ni is related to the highest negative hydration enthalpy corresponding to its smaller radius.

Trends in the standard electrode potentials of M+3/ M+2ion:

The observed electrode potentials for these elements are shown in the table

Element (M) Δ aH - (M) I 1 I 2 I 3 Δ hydH - (M 2+) E -/V obs (M3+)/M 2+ Ti +469 661 1310 2657 -1816 -0.37 V +514 651 1414 2833 -1980 -0.26 Cr +397 653 1591 2990 -1945 -0.41 Mn +281 717 1509 3260 -1888 +1.57 Fe +416 762 1562 2962 -1988 +0.77 Co +425 760 1648 3243 -2051 +1.97 Ni +430 737 1753 3403 -2121 - Cu +339 745 1960 3556 -2121 - Zn +130 908 1730 3829 -2059 -

The lower value for vanadium is due to the stability of V2+ as it has a half-filled t2g level.

Manganese has a higher electrode potential value than Cr and Fe because of its very high third ionisation energy, which is due to the stability of the half-filled d5 configuration.

The comparatively low value for iron shows that the reduction of ferric ion to ferrous ion is less favourable, since ferric ion is extra stable due the half-filled d5 configuration.

Trends in stability of the higher oxidation states:

List of the stable halides of the 3d series of the transition elements:

number Ti V Cr Mn Fe Co Ni Cu zn            +6   CrF 6            +5   VF 5   CrF 5            +4   TiX 4   VX I 4   CrX 4   MnF 4            +3   TiX 3   VX 3   CrX 3   MnF 3   FeX I 3     CoF 2            +2   TiX 2 III   VX 2   CrX 2   MnF 2   FeX 2     CoX 2   NiX 2   CuX 2 II  ZnX 2            +1   CuX 2 III

From the table, TiX4, VF5 and CrF6 have the highest oxidation numbers.

The highest oxidation state +7, for manganese is not seen in simple halides, but MnO3F is known.

VF5 is stable, while the other halides undergo hydrolysis to give oxohalides of the type VOX3.

Fluorine stabilises higher oxidation states either because of its higher lattice energy or higher bond enthalpy.

Fluorides are unstable in their lower oxidation states, and, therefore, chlorides, bromides and iodides exist in +2 oxidation state, while fluorides do not.

Copper in +2 oxidation state forms all the halides, except iodides, because cupric ion oxidises iodide to iodine.

The stability of Cu +2ions rather than Cu+ ions is due to the higher negative hydration enthalpy of cupric ion than cuprous ion, which more than compensates for the second ionisation enthalpy of copper.

List of the oxides of the 3d series of elements:

Number Groups 3 4 5 6 7 8 9 10 11 12                 +7   Mn 2O 7                 +6   CrO 3                 +5   V 2O 5                 +4   TiO 2 V 2O 4 CrO 2 MnO 2                 +3   Sc 2O 3   Ti 2O 3   V 2O 3   Cr 2O 3   Mn 2O 3   Fe 2O 3   Mn 3O 4   Fe 3O 4   Co 3O 4                 +2   TiO   VO   (CrO)   MnO   FeO CoO  NiO  CuO  ZnO                 +1   Cu 2O

The highest oxidation number in an oxide coincides with the group number, No higher oxides are seen beyond manganese.

Oxygen also stabilises higher oxidation states in the form of oxocations.

EX: V (V) is stabilised as dioxovanadium (V) ion, vanadium (IV)as Oxo- vanadium (IV) ion

Oxygen exceeds fluorine in its ability to stabilise higher oxidation states. Thus, the highest manganese fluoride isMnF4, while the highest oxide is MnO7.

The ability of oxygen to form multiple bonds with metal atoms is responsible for its superiority over fluorine in stabilising higher oxidation states.


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