Chemistry General Properties of the Transition Elements (d-Block)-3
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General Properties of the Transition Elements (d-Block)-3

Topics Covered :

● Chemical Reactivity and `E^⊖` Values
● Magnetic Properties
● Formation of Coloured Ions

Chemical Reactivity and `E^⊖` Values :

`=>` Transition metals vary widely in their chemical reactivity. Many of them are sufficiently electropositive to dissolve in mineral acids, although a few are ‘noble’—that is, they are unaffected by simple acids.

`=>` The metals of the first series with the exception of copper are relatively more reactive and are oxidised by `color{red}(1M)` `color{red}(H^+))`, though the actual rate at which these metals react with oxidising agents like hydrogen ion (`color{red}(H^+)`) is sometimes slow.

● For example, titanium and vanadium, in practice, are passive to dilute non oxidising acids at room temperature.

`=>` The `color{red}(E^⊖)` values for `color{red}(M^(2+)|M)` (Table 8.2) indicate a decreasing tendency to form divalent cations across the series. This general trend towards less negative `color{red}(E^⊖)` values is related to the increase in the sum of the first and second ionisation enthalpies.

`color{red}(text(Note ))` : (i) The `color{red}(E^⊖)` values for `color{red}(Mn)`, `color{red}(Ni)` and `color{red}(Zn)` are more negative than expected from the general trend.

(ii) Whereas the stabilities of half-filled `color{red}(d)` subshell (`color{red}(d^5)`) in `color{red}(Mn^(2+))` and completely filled `color{red}(d)` subshell (`color{red}(d^10)`) in zinc are related to their `color{red}(E^⊖)` values; for nickel, `color{red}(E^⊖)` value is related to the highest negative enthalpy of hydration.

`=>` An examination of the `color{red}(E^⊖)` values for the redox couple `color{red}(M^(3+)|M^(2+))` (Table 8.2) shows that `color{red}(Mn^(3+))` and `color{red}(Co^(3+))` ions are the strongest oxidising agents in aqueous solutions.

`=>` The ions `color{red}(Ti^(2+))`, `color{red}(V^(2+))` and `color{red}(Cr^(2+))` are strong reducing agents and will liberate hydrogen from a dilute acid, e.g.,

`color{red}(2 Cr^(2+) (aq) + 2 H^(+) (aq) → 2 Cr^(3+) (aq) + H_2 (g))`
Q 3081201127

For the first row transition metals the `E^(⊖)` values are:

`tt ((E^(⊖) , V , Cr , Mn , Fe , Co , Ni , Cu) , ( (M^(2+)//M) , -1.18 , -0.91 , -1.18 , -0.44 , -0.28 , -0.25 , +0.34))`

Explain the irregularity in the above values.

Solution:

The `E^(⊖) (M^(2+)//M)` values are not regular which can be explained from the irregular variation of ionisation enthalpies `( Δ_1H_1 +Δ_1H_2 )` and also the sublimation enthalpies which are relatively much less for manganese and vanadium.
Q 3011401320

Why is the `E^(⊖)` value for the `Mn^(3+)//Mn^(2+)` couple much more positive than that for `Cr^(3+//Cr^(2+)` or `Fe^(3+)//Fe^(2+)?` Explain.

Solution:

Much larger third ionisation energy of `Mn` (where the required change is `d^5` to `d^4`) is mainly responsible for this. This also explains why the +3 state of `Mn` is of little importance.

Magnetic Properties :

`=>` When a magnetic field is applied to substances, mainly two types of magnetic behaviour are observed :

(i) diamagnetism and

(ii) paramagnetism

`color{green}(text(Diamagnetic Substances ))`: These substances are repelled by the applied magnetic field.

`color{green}(text(Paramagnetic Substances ))` : These substances are attracted by the applied magnetic field.

`color{green}(text(Ferromagnetic Substances ))` : Substances which are attracted very strongly are said to be ferromagnetic. In fact, ferromagnetism is an extreme form of paramagnetism.

`=>` Many of the transition metal ions are paramagnetic.

`=>` Paramagnetism arises from the presence of unpaired electrons, each such electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum.

● For the compounds of the first series of transition metals, the contribution of the orbital angular momentum is effectively quenched and hence is of no significance.

● For these, the magnetic moment is determined by the number of unpaired electrons and is calculated by using the ‘spin-only’ formula, i.e.,

`color{red}(μ = sqrt(n(n + 2)))`

where `color{red}(n)` is the number of unpaired electrons and `color{red}(μ)` is the magnetic moment in units of Bohr magneton (`color{red}(BM)`).

● A single unpaired electron has a magnetic moment of `1.73` Bohr magnetons (`color{red}(BM)`).

`=>` The magnetic moment increases with the increasing number of unpaired electrons.

● Therefore, the observed magnetic moment gives a useful indication about the number of unpaired electrons present in the atom, molecule or ion.

`=>` The magnetic moments calculated from the ‘spin-only’ formula and those derived experimentally for some ions of the first row transition elements are given in Table 8.7. The experimental data are mainly for hydrated ions in solution or in the solid state.

Q 3071401326

Calculate the magnetic moment of a divalent ion in aqueous solution if its atomic number is 25.

Solution:

With atomic number 25, the divalent ion in aqueous solution will have `d^5` configuration (five unpaired electrons). The magnetic moment, μ is

`mu = sqrt(5(5+2)) = 5.92 BM`

Formation of Coloured Ions :

`=>` When an electron from a lower energy `color{red}(d)` orbital is excited to a higher energy `color{red}(d)` orbital, the energy of excitation corresponds to the frequency of light absorbed. This frequency generally lies in the visible region.

`=>` The colour observed corresponds to the complementary colour of the light absorbed.

`=>` The frequency of the light absorbed is determined by the nature of the ligand.

`=>` In aqueous solutions where water molecules are the ligands, the colours of the ions observed are listed in Table 8.8.

`=>` A few coloured solutions of `d`-block elements are illustrated in Fig. 8.5.
 
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