Standard Electrode Potentials
The magnitude of ionization enthalpy gives the amount of energy required to remove electrons to form a particular oxidation state of the metal in a compound. Thus, the value of ionisation enthalpies gives information regarding the thermodynamic stability of the transition metal compounds in different oxidation states. Smaller the ionisation enthalpy of the metal, the stabler is its compound. The sum of first two ionization enthalpies is less for nickel than for platinum.
Ni → Ni2+ + +2e– I.E. = 2.49 x 103 kJ mol-1
Pt → Pt2+ +2e– I.E. = 2.66 x 103 kJ mol-1
Ionization of nickel to Ni2+ is energetically favourable as compared to that of platinum. Thus, the nickel (II) compounds are thermodynamically more stable than platinum (II) compounds. The sum of first four ionisation enthalpies is less for platinum than for nickel as
Ni → Ni4+ + 4e– I.E. = 11.29 x 103 kJ mol-1
Pt → Pt4+ + 4e– I.E. = 9.36 x 103 kJ mol-1
Thus, the platinum (IV) compounds are relatively more stable than nickel (IV) compounds. Therefore, K2PtCl6 [having Pt (IV)] is a well-known compound whereas the corresponding nickel compound is not known. However, in solutions the stability of the compounds depends upon electrode potentials.
In addition to ionisation enthalpy, the other factors such as enthalpy of Sublimation, hydration enthalpy, etc. determine the stability of a particular oxidation state in solution. This can be explained in terms of their electrode potential values. The oxidation potential of a metal involves the followings process:
This process may be considered in terms of the following cycle:This process actually takes place in the following three steps :(1) In the first step, the atoms get isolated from one another and become independent in the gaseous state. This converts solid metal to the gaseous state. The energy needed for this step is known as enthalpy of sublimation.
M(s) → M(g) Enthalpy of sublimation , ΔsubH
M(g) → M+ (g) +e– lonisation enthalpy, IE
M+(g) + nH2O → M+(aq) Enthalpy of hydration, ΔhydH
The oxidation potential which gives the tendency of the overall change to occur, depends upon the net effect of these three steps. The overall energy change is
ΔH = ΔsubH + IE + ΔhydH
Thus, ΔH gives the enthalpy change required to bring the solid metal, M to the monovalent ion in aqueous medium, M+(aq). The ionization enthalpy may be replaced by electron gain enthalpy when the gaseous atom goes to gaseous anion. ΔH helps to predict the stability of a particular oxidation state. The smaller the value of total energy change for a particular oxidation state in aqueous solution, greater will be the stability of that oxidation state. The electrode potential are measure of total energy change.
Stability of the transition metal ions in different oxidation states can be determined on the basis of electrode potential data. The lower the electrode potential i.e., more negative the standard reduction potential of the electrode, the more stable is the oxidation state of the transition metal in the aqueous solution.
The electrode potentials of different metals can also be measured by forming the cell with standard hydrogen electrode. For the measurement of electrode potential of M2+ | 1 M, the e.m.f. of the cell in which the following reaction occurs is measured:
2H+ (aq) + M(s) ↔ M2+ (aq) + H2 (g)
Knowing the potential of 2H+ (aq) | H2(g), it is possible to determine the potential of M2+(aq) | M.
a) The electrode potentials of transition metals are low in comparison to elements of group 2 (e.g., Ca = – 2.87 V). Compared to group 2 elements, the transition elements have fairly large ionisation enthalpies and very large enthalpies of atomisation. These reduce their electrode potentials though their hydration enthalpies are large.
b) Zinc has low enthalpy of atomisation and fairly large hydration energy. But it has also low electrode potential (-0.76 V) because of its very high ionisation enthalpy (IE1+ IE2).
c) Copper has positive reduction potential, (0.34V) and this shows that copper is least reactive metal out of the first transition series. This unique behaviour (+ve E value of copper) also accounts for its inability to liberate H2, from acids. It has been observed that only oxidizing acid (such as nitric acid and hot concentrated sulphuric acid) react with copper in which the acids are reduced. The high energy required to convert Cu (s) to Cu2+ (aq) is not balanced by its hydration enthalpy.
d) The value becomes, less negative across the series. This is due to increase in the sum of first and second ionisation enthalpies. The values of E° of Mn, Ni and Zn are more negative than expected from the general trend. The relatively more negative values of Eø for Mn and Zn are due to stability of half filled d-subshell in Mn2+ (3d5) and the completely filled (3d10) configuration in Zn2+. The exceptionally high E value of Ni from regular trend is related to the highest negative enthalpy of hydration of Ni2+ ion.
Trends in the M3+| M2+ Standard Electrode Potentials
All other elements of first transition series show +3 oxidation states also to form M3+ ions in aqueous solutions. The standard reduction potentials for M3+ | M2+ redox couple are given below:
(i) The low value of scandium reflects the stability of Sc3+ which has a noble gas configuration.
(ii) The comparatively high value for Mn shows that Mn2+ (d5 configuration) is particularly stable. On the other hand, comparatively low value for Fe shows the extra stability of Fe3+ (d5 configuration).
(iii) The comparatively low value of V is related to the stability of V2+ (due to half filled t2g3 energy level of 3d orbitals in octahedral crystal field splitting)
Formation of Coloured lons
Most of the compounds of transition metals are coloured in the solid form or solution form.
Explanation: The colour of the compounds of transition metals may be attributed to the presence of incomplete (n-1) d-subshell. In the case of compounds of transition metals, the energies of the five d-orbitals in the same subshell do not remain equal. Under the influence of approaching ions towards the central metal ion, the d-orbitals of the central metal split into different energy levels. This phenomenon is called crystal field splitting. For example: When the six ions or molecules approach the metal ion (called octahedral field), the five d-orbitals split up into two sets : one set consisting of two d-orbitals (dx2-y2 , dz2) of higher energy and the other set consisting of three d-orbitals ( dxy ,dyz , dzx) of lower energy.
In the case of the transition metal ions, the electrons can be easily promoted from one energy level to another in the same d-subshell. These are called d-d transition. The amount of energy required to excite some of the electrons to higher energy states within the same d-subshell corresponds to energy of certain colours of visible light. Therefore, when white light falls on a transition metal compound, some of its energy corresponding to a certain colour, is absorbed and the electron gets raised from lower energy set of orbitals to higher energy set of orbitals.
The excess of other colours constituting white light are transmitted and the compound appears coloured. The observed colour of a substance is always complementary colour of the colour which is absorbed by the substance.
Example: Ti3+ compounds contain one electron in d-subshell (d1). It absorbs green and yellow portions from the white light and blue and red portions are emitted.
Therefore, Ti3+ ions appear purple. Similarly, hydrated cupric compounds absorb radiations corresponding to red light and the transmitted colour is greenish blue (which is complementary colour to red colour). Thus, cupric compounds have greenish-blue colour.
The colours of some common hydrated transition metal ions are :
|Ion||Outer configuration||Colour of the ion|
|Sc3+ , Ti4+||3d0||Colourless|
|V2+ , Cr3+||3d3||Violet|