notes of d-block elements : chemistry for 12th

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What are called d-block elements?

Those elements in which d-orbitals are progressively filled by electrons are called d-block elements. Elements of this block are also known as transition elements. As d- subshell has maximum10 electrons hence, there are 10 groups in this block from 3 to 12.

Transition elements: Notes of d-block elements

As we know that d-block elements are known as transition elements. If strictly speaking, a transition element is defined as the one which has incompletely filled d-orbitals in its ground state or in any one of the ground states. Zinc, cadmium and mercury of group 12 are not regarded as transition elements because they have completely filled d-orbitals in their ground state as well as in their common oxidation states.

Classification of transition elements:

There are mainly four series of the transition elements
(1) 3d series of transition elements or 1st series of transition elements. In this series of elements, 3d orbitals are progressively filled. The elements of this series are from Sc(21) to Zn(30).

(2)4d series of transition elements or 2nd series of transition elements In this series of elements, 4d orbitals are progressively filled. The elements of this series are from Y(39) to Cd(48).

(3) 5d series of transition elements or 3rd series of transition elements. In this series of elements, 5d orbitals are progressively filled. The elements of this series are from La(57), Hf(72) to Hg(80).(4) 6d series or fourth transition series in which 6d orbitals are progressively filled with electrons starts with Ac(89), Ku (104) to Uub (112).

Important physical and chemical properties of transition elements:

Electronic Configuration of d-block elements

The general electronic Configuration of d-block elements are (n-1)d^1-10 ns^0-1. Exceptional electronic Configuration of some elements of this block are followings –
Cr(24)- 3d⁵4s¹,  Cu(29)- 3d¹⁰4s¹
Nb(41)- 4d⁴5s¹, Mo(42)- 4d⁵5s¹
Tc(43)- 4d⁶5s¹,  Ru(44)- 4d⁷5s¹
Rh(45)- 4d⁸5s¹,  Pd(46)- 4d¹⁰5s⁰
Ag(47)- 4d¹⁰5s¹,  Pt(78)- 5d⁹6s¹
Au(79)- 5d¹⁰6s¹, Uum(111)-6d¹⁰7s¹.

The following reasons are behind exceptional configurations. In case of Cr, Cu, Ag and Au are due to the gain of additional stability by the atom by either having half filled Configuration or completely filled Configuration. While in other cases are due to the significant role of the nuclear – electron and electron – electron forces.

Atomic and Ionic radii:

• The atomic radius of transition metals lie in between those of s- and p- block elements.
•  Generally the atomic radii of d- block elements in a series decreases with increase in atomic number but the decrease in atomic size is small after midway due to increase in screening effects.
• .At the end of the period, there is a slight increase in the atomic radii. Because repulsion between electrons are greater than the attractive forces of nucleus.
• The atomic radii increase down the group but the atomic radii of the second and third transition series are almost the same due to lanthanoid contraction . The trend of Ionic radii is same as followed by atomic radius.

Metallic character:

Except for mercury which is a liquid, all the transition elements have typical metallic structure as hcp, ccp or bcc. They exhibit all the characteristics of metals like, they are hard, lustrous, malleable and ductile. They have high m.p. and b.p., high thermal and electrical conductivity and high tensile strength.

Melting and boiling point:

The transition metals have very high melting and boiling points. The melting points of the transition metals rise to a maximum and then fall as the atomic number increases. Manganese and technetium have abnormally low melting points.
Note that tungsten (W) has the highest melting point among the d-block elements.

Density:

As we move along a transition series from left to right, the atomic radii decrease due to increase in nuclear charge. Hence, the atomic Volume decreases. At the same time, atomic mass increases. Hence, density increases. For the first transition series, the last element zinc is an exception, having large atomic Volume and hence lower density. Remember that among the d-block elements, indium has the highest density and scandium has the lowest density. Osmium and indium have almost same density

Ionisation enthalpy:

(1) The first Ionisation enthalpy of d-block elements lie between s- block and p-block elements. They are higher than those of s- block elements and are lesser than those of p-block elements. The Ionisation enthalpy gradually increases with increase in atomic number along a given transition series though some irregularities are observed.

(2) In a given series, the difference in the Ionisation enthalpies between any two successive d-block elements is very much less than the difference in case of successive s- block or p-block elements.

(3) The first Ionisation enthalpy of Zn, Cd and Hg are, however, very high because of the fully filled (n-1)d¹⁰ns² Configuration.

(4) Although second and third Ionisation enthalpies also, in general, increase along a period, but the magnitude of increase for the successive elements is much higher.

(5) The first Ionisation enthalpies of 5d elements are higher as compared to those of 3d and 4d elements. This is because the weak shielding of nucleus by 4f electrons in 5d elements result in greater effective nuclear charge acting on the outer valence electrons

Standard electrode potentials (E^o) and chemical reactivity:

In solution, the stability of the compounds depends upon electrode potentials rather than ionisation enthalpy. Electrode potential values depend upon factors such as enthalpy of sublimation of the metals, the Ionisation enthalpy and the hydration enthalpy. The lower the electrode potential, more stable is the oxidation state of the transition metal in the aqueous solution.

The transition metals vary very widely in their chemical reactivity. Some of them are highly electropositive and dissolve in mineral acids whereas a few of them are noble.
Some results of chemical reactivity of transition elements as related to their Eo values are followings
(1)The metals of the first transition series except Cu are relatively more reactive than the other series.

(2)Less negative E^o values for Mn²⁺ /Mn along the series indicate a decreasing tendency to form divalent cations.

(3)More negative E^o values than expected for Mn, Ni and Zn show greater stability for Mn²⁺, Ni²⁺ and Zn²⁺.

(4)E^o values for the redox couple M³⁺/M²⁺ indicate that Mn³⁺ and Co³⁺ are the strongest oxidising agent in aqueous solution whereas Ti²⁺,V²⁺ and Cr²⁺ are strongest reducing agent and can liberate hydrogen from a dilute acid.
2Cr²⁺(aq) + 2H⁺(aq) → 2Cr³⁺(aq) + H²(g)

Oxidation state:

1. All transition elements except the first and last member of the series, exhibit a number of oxidation state.
2. The most common oxidation state of the first row transition metals is +2 except in the case of scandium which has +3.
3. Mostly Ionic bonds are formed in +2 and +3 oxidation states. In compounds of higher oxidation states, the bonds formed are mostly covalent by sharing d- electrons.
4. The maximum oxidation states of reasonable stability in the first transition series is equal to the sum of s and d electrons upto Mn followed by an abrupt decrease in the stability of higher oxidation states.
5. The variability of oxidation states of transition elements arises in such a way that successively differ by unity.
   In a group of d-block elements, the higher oxidation states are more stable for heavier elements. For example in group 6, Mo(Vl) and W(Vl) are more stable than Cr(Vl).

Catalytic properties:

Transition elements and their compounds act as catalyst due to the reasons that they have unpaired electrons in their incomplete d-orbitals and showing variable oxidation states .

Coloured ions:

Most of the transition metal compounds are coloured both in solid state and in aqueous solution in contrast to the compounds of s and p-block elements. Colour is due to the presence of incomplete d-subshell. These unpaired electrons undergo d-d transition at the approach of ligands. The required amount of energy to do this is obtained by absorption of light of a particular wave length in the region of visible light and appear coloured due to emission of the remainder as coloured light.

Magnetic properties:

On the basis of behaviour in magnetic field, substances are classified as paramagnetic, diamagnetic and ferromagnetic. Those substances which are attracted by the applied magnetic field are called paramagnetic. Those which are repelled by the applied magnetic field are called diamagnetic and which are strongly attracted by the applied field are called ferromagnetic.

Paramagnetism is a property due to the presence of unpaired electrons. In case of transition metals, as they contain unpaired electrons in the (n-1)d orbitals. Most of the transition metal ions and their compounds are paramagnetic. As the number of unpaired electrons increases from one to five, the magnetic moment and hence paramagnetic character also increases. Those transition elements which have paired electrons are diamagnetic. Magnetic moment is calculated from spin formula μ =√n(n+2) B.M.

Complex formation:

Transition metal ions form a large number of complex compounds. Ex:- [Fe(CN)₆]^3- , [Ni(CN)₄]^2- etc. The transition elements form complexes due to the following reasons. Comparatively smaller size of metal ions, high ionic charge and availability of vacant d-orbitals.

Interstitial compounds:

The transition metals form a large number of interstitial compounds in which small atoms such as hydrogen, carbon, boron and nitrogen occupy the empty spaces in their lattices. They are non- stoichiometric compounds, e.g.,TiH_1.7, VH_0.56. They are very hard and rigid. They have high m.p than those of pure metals. They acquire chemical inertness. They show conductivity like that of pure metals.

Alloy formation:

Alloys are homogeneous solid solutions of two or more metals obtained by melting the components and cooling the melt. These are formed by metals whose atomic radii differ by not more than 15% so that the atom of one metal can easily take up the position in the crystal lattice of the other. As the transition metals have similar atomic radii and other characters, hence, they form alloys very readily. The metals Chromium, vanadium, molybdenum and manganese are used in the formation of alloys steels and stainless steels.

Important compounds of d-block elements:

1.Potassium dichromate (K₂Cr₂O₇):

Preparation of K₂Cr₂O₇:

(1) preparation of Sodium chromate The ores of Potassium dichromate called chromite FeO.Cr₂O₃ is finely powdered, mixed with sodium carbonate and quick lime and then roasted in a reverberatory furnace with free exposure to air. When Sodium chromate is formed (yellow in Colour) and CO₂ is evolved. Quick lime keeps the mass porous and thus facilitates oxidation.
4FeO.Cr₂O₃ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂

After the reaction, the roasted mass is extracted with water when Sodium chromate is completely dissolved while ferric oxide is left behind and separated out by filtration.

(2) Conversion of Sodium chromate into Sodium dichromate: The filtrate containing sodium chromate solution is treated with conc.H₂SO₄ and Sodium chromate is converted into Sodium dichromate.
2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O
Sodium sulphate being less soluble crystalised out as decahydrate.

(3) Conversion of Sodium dichromate into potassium dichromate: Hot concentrated solution of Sodium dichromate is treated with calculated mass of potassium chloride when potassium dichromate, being less soluble crystalizes out on cooling as orange crystals.
Na₂Cr₂O₇ + 2KCL→ K₂Cr₂O₇ + 2NaCl.

Properties of Potassium dichromate:

(1) Colour and melting point: It forms orange crystals which melt at 669K.
(2) Solubility: It is moderately soluble in cold water but freely soluble in hot water.
(3) Action of heat: On heating till white, it decomposes with the evolution of oxygen.
4 K₂Cr₂O₇ → 4K₂CrO₄ + 2Cr₂O₃ + 3O₂

(4) Action of alkalies: when an alkali is added to an orange red solution of dichromate, a yellow solution results due to the formation of chromate.
K₂Cr₂O₇ + 2KOH → 2K₂CrO₄ + H₂O
On acidifying, the Colour again changes to orange red due to the formation of dichromate.
2k₂CrO₄ + H₂SO₄ → K₂Cr₂O₇ + K₂SO₄ + H₂O

(5) Action of concentrated H₂SO₄: In cold, red crystals of chromic anhydride are formed
K₂Cr₂O₇ + 2H₂SO₄ → 2CrO₃ + 2KHSO₄ + H₂O
On heating the mixture, Oxygen is evolved.
2K₂Cr₂O₇ + 8H₂SO₄ → 2Cr₂(SO₄)₃ + 8H₂O + 3O₂

(6) Oxidising properties: It is a powerful oxidising agent. In the presence of dilute Sulphuric acid, one molecule of potassium dichromate furnishes 3 atoms of oxygen atoms and oxidises ferrous salt into ferric salt.
K₂Cr₂O₇ + 7H₂SO₄ + 6FeSO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 3Fe₂(SO₄)₃ + 7H₂O
Similarly KI into I₂, H₂S into S, sulphite to sulphate, thiosulphate to sulphate and sulphur, nitrites to nitrates, halogen acids to halogens, SO₂ to Sulphuric acid and ethyl alcohol to acetaldehyde and acetic acid.

(7) Chromyl chloride test: when Potassium dichromate is heated with conc. HCl or with a chloride and strong Sulphuric acid, reddish brown vapours of chromyl chloride are obtained. This reaction is used in the detection of chloride ions in qualitative analysis.
K₂Cr₂O₇ + 4KCl + 6H₂SO₄ → 2CrO₂Cl₂ + 6KHSO₄ + 3H₂O

(8) Reaction with hydrogen peroxide: Acidified K₂Cr₂O₇ solution reacts with H₂O₂ to give a deep blue solution of [CrO(O₂)₂].
Cr₂O₇^2- + 2H⁺ + 4H₂O₂ → 2CrO₅ + 5H₂O
The blue Colour fades away gradually due to the decomposition of CrO₅ into Cr³⁺ ions and oxygen.

Structure of chromate and dichromate ions:

(2)Potassium permagnate (KMnO4)

Preparation of Potassium permagnate:
On a large scale, it is prepared from the mineral, pyrolusite, MnO₂. The preparation involves the following steps:
(1) Conversion of MnO₂ into Potassium manganate: The finely powdered pyrolusite mineral is fused with potassium hydroxide or potassium carbonate in the presence of air or oxidising agent such as potassium nitrate or Potassium chlorate when green coloured Potassium manganate is formed.
2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
2MnO₂ + 2K₂CO₃ + O₂ → 2K₂MnO₄ + 2CO₂

(2) Oxidation of K₂MnO₄ to KMnO₄:
The alkaline manganate solution is electrolysed between iron electrodes separated by diaphragm. The reactions take place in the following way:

Thus manganate ions are oxidised to permanganate at the anode and hydrogen gas is liberated at the cathode. After the oxidation is complete, the solution is filtered and evaporated under controlled conditions to obtain the crystal of Potassium permanganate.

Properties of KMnO₄:

(1) Colour: KMnO₄ exists as deep purple black prisms with a greenish lustre which become dull in air due to superficial reduction.
(2) Solubility: It is moderately soluble in water at room temperature and it is more soluble in hot water.

(3) Action of heat: When heated to 513K, it readily decomposes giving oxygen.
2KMNO₄ → K₂MnO₄ + MnO₂ + O₂
At red heat, Potassium manganate formed decomposes into Potassium manganite and oxygen.
2K₂MnO₄ → 2K₂MnO₃ + O₂

 

(4) Action of conc. H₂SO₄: With well cooled conc. H₂SO₄, it gives Mn₂O₇ which on warming decomposes to MnO2
2KMnO₄ + 2H₂SO₄ → Mn₂O₇ + 2KHSO₄ + H₂O
2Mn₂O₇ → 4MnO₂ + 3O₂
With warm conc. H₂SO₄, O₂ is given out.
4KMnO₄ + 6H₂SO₄ → 2K₂SO₄ + 4MnSO₄ + 6H₂O + 5O₂

(5) Action of heat in current of hydrogen: When heated in a current of H₂, solid KMnO₄ gives KOH, MnO and water vapours.
2KMnO₄ + 5H₂ → 2KOH + 2MnO + 4H₂O

(6) Oxidising properties: Potassium permanganate is a powerful oxidising agent. The actual course of reaction depends on the use of the permanganate in neutral, acidic or alkaline solution.
In neutral solution : In this medium, two moles of permanganate gives three atoms of oxygen and during the course of reaction, the alkali generated renders the medium alkaline even when we start with neutral solution.

Eq.wt. of KMnO₄ in neutral or weakly alkaline medium is mol.wt./3 or 158/3.
In this medium, it oxidises hot MnSO₄ into MnO₂.
2KMnO₄ + 3MnSO₄ + 2H₂O → K₂SO₄ + 2H₂SO₄ + 5MnO₂
It oxidises Sodium thiosulphate to sodium sulphite
8KMnO₄ + 8MnO₂ + H₂O → 3K₂SO₄ + 8MnO₂ + 3Na₂SO₄ + 2KOH
It oxidises hydrogen sulphide to sulphur.
2KMnO₄ + 4H₂S → 2MnS + S + K₂SO₄ + 4H₂O

In alkaline solution, one atom of oxygen is produced, hence Eq.wt. of KMnO₄ is mol.wt./1=158
In alkaline medium, it oxidises potassium iodide to potassium iodate.
2KMnO₄ + H₂O + KI → 2MnO₂ + 2KOH + KIO₃

In acidic medium, KMnO4 provides five atoms of oxygen, hence its equivalent weight is mol.wt./5=31.6
In acidic medium, it oxidises H₂S to S, SO₂ to Sulphuric acid, nitrite to nitrate, oxalate or Oxalic acid to CO₂, ferrous sulphate to ferric sulphate, HX to X₂, KI to I₂ and ethyl alcohol to acetaldehyde.
2KMnO₄ + 3H₂SO4 + 5H₂S — K₂SO₄ + 2MnSO₄ + 3H₂O + 3S.

Structure of permanganate ion: