Important Compound of Transition Elements

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Important Compound of Transition Elements

Oxides and Oxoanions of Metals

Generally, transition metal oxides are formed by the reaction of transition metals with molecular oxygen at high temperatures. Except the first member of 3d series, Scandium, all other transition elements form ionic metal oxides. The oxidation number of metal in metal oxides ranges from +2 to +7. As the oxidation number of a metal increases, ionic character decreases, for example, Mn₂O₇ is covalent.

Mostly higher oxides are acidic in nature, Mn₂O₇ dissolves in water to give permanganic acid (HMnO₄), similarly CrO₃ gives chromic acid (H₂CrO₄) and dichromic acid (H₂Cr₂O₇). Generally lower oxides may be amphoteric or basic, for example, Chromium (III) oxide – Cr₂O₃, is amphoteric and Chromium(II) oxide, CrO, is basic in nature.

Potassium Dichromate K₂Cr₂O₇

Preparation:

Potassium dichromate is prepared from chromate ore. The ore is concentrated by gravity separation. It is then mixed with excess sodium carbonate and lime and roasted in a reverbratory furnace.

The roasted mass is treated with water to separate soluble sodium chromate from insoluble iron oxide. The yellow solution of sodium chromate is treated with concentrated sulphuric acid which converts sodium chromate into sodium dichromate.

The above solution is concentrated to remove less soluble sodium sulphate. The resulting solution is filtered and further concentrated. It is cooled to get the crystals of Na₂SO₄.2H₂O.

The saturated solution of sodium dichromate in water is mixed with KCl and then concentrated to get crystals of NaCl. It is filtered while hot and the filtrate is cooled to obtain K₂Cr₂O₇ crystals.

Physical Properties:

Potassium dichromate is an orange red crystalline solid which melts at 671K and it is moderately soluble in cold water, but very much soluble in hot water. On heating it decomposes and forms Cr₂O₃ and molecular oxygen. As it emits toxic chromium fumes upon heating, it is mainly replaced by sodium dichromate.

Structure of Dichromate Ion:

Both chromate and dichromate ion are oxo anions of chromium and they are moderately strong oxidizing agents. In these ions chromium is in +6 oxidation state. In an aqueous solution, chromate and dichromate ions can be interconvertible, and in an alkaline solution chromate ion is predominant, whereas dichromate ion becomes predominant in acidic solutions. Structures of these ions are shown in the figure.

Chemical Properties:

1. Oxidation

Potassium dichromate is a powerful oxidising agent in acidic medium. Its oxidising action in the presence of H⁺ ions is shown below. You can note that the change in the oxidation state of chromium from Cr⁶⁺ to Cr³⁺. Its oxidising action is shown below.

Cr₂O₇^2- + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O

The oxidising nature of potassium dichromate (dichromate ion) is illustrated in the following examples.

(i) It oxidises ferrous salts to ferric salts.

Cr₂O₇^2- + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

(ii) It oxidises iodide ions to iodine

Cr₂O₇^2- + 6I^– + 14H⁺ → 2Cr³⁺ + 3I₂ + 7H₂O

(iii) It oxidises sulphide ion to sulphur

Cr₂O₇^2- + 3S^2- + 14H⁺ → 2Cr³⁺ + 3S + 7H₂O

(iv) It oxidises sulphur dioxide to sulphate ion

Cr₂O₇^2- + 3SO₂ + 2H⁺ → 2Cr³⁺ + 3SO^2-₄ + H₂O

(v) It oxidises stannous salts to stannic salt

Cr₂O₇^2- + 3Sn²⁺ + 14H⁺ → 2Cr³⁺ + 3Sn⁴⁺ + 7H₂O

(vi) It oxidises alcohols to acids.

2K₂Cr₂O₇ + 8H₂SO₄ + 3CH₃CH₂OH →
2K₂SO₄ + 2Cr₂(SO₄)₃ + 3CH₃COOH + 11H₂O

Chromyl Chloride Test:

When potassium dichromate is heated with any chloride salt in the presence of Conc H₂SO₄, orange red vapours of chromyl chloride (CrO₂Cl₂) is evolved. This reaction is used to confirm the presence of chloride ion in inorganic qualitative analysis.

The chromyl chloride vapours are dissolved in sodium hydroxide solution and then acidified with acetic acid and treated with lead acetate. A yellow precipitate of lead chromate is obtained.

CrO₂Cl₂ + 4NaOH → Na₂CrO₄ + 2NaCl + 2H₂O

Uses of Potassium Dichromate:

Some important uses of potassium dichromate are listed below.

1. It is used as a strong oxidizing agent.
2. It is used in dyeing and printing.
3. It used in leather tanneries for chrome tanning.
4. It is used in quantitative analysis for the estimation of iron compounds and iodides.

Potassium Permanganate – KMnO₄

Preparation:
Potassium permanganate is prepared from pyrolusite (MnO₂) ore. The preparation involves the following steps.

(i) Conversion of MnO₂ to potassium manganate:

Powdered ore is fused with KOH in the presence of air or oxidising agents like KNO₃ or KClO₃. A green coloured potassium manganate is formed.

(ii) Oxidation of potassium manganate to potassium permanganate:

Potassium manganate thus obtained can be oxidised in two ways, either by chemical oxidation or electrolytic oxidation.

Chemical Oxidation:

In this method potassium manganate is treated with ozone (O₃) or chlorine to get potassium permanganate.

2MnO₄^2- + O₃ + H₂O → 2MnO₄^– + 2OH^– + O₂
2MnO₄^2- + Cl₂ → 2MnO₄^– + 2Cl^–

Electrolytic Oxidation

In this method aqueous solution of potassium manganate is electrolyzed in the presence of little alkali.

K₂MnO₄ ⇄ 2K⁺ + MnO₄^2-
H₂O ⇄ H⁺ + OH^–

Manganate ions are converted into permanganate ions at anode.

H₂ is liberated at the cathode.

2H⁺ + 2e^– → H₂↑

The purple coloured solution is concentrated by evaporation and forms crystals of potassium permanganate on cooling.

Physical Properties:

Potassium permanganate exists in the form of dark purple crystals which melts at 513 K. It is sparingly soluble in cold water but, fairly soluble in hot water.

Structure of Permanganate ion

Permanganate ion has tetrahedral geometry in which the central Mn⁷⁺ is sp³ hybridised.

Chemical Properties:

1. Action of Heat:

When heated, potassium permanganate decomposes to form potassium manganate and manganese dioxide.

2KMnO₄ → 2K₂MnO₄ + MnO₂ + O₂

2. Action of conc H₂SO₄

On treating with cold conc H₂SO₄, it decomposes to form manganese heptoxide, which subsequently decomposes explosively.

But with hot conc H₂SO₄, Potassium permanganate give MnSO_4. 

3. Oxidising Property:

Potassium permanganate is a strong oxidising agent, its oxidising action differs in different reaction medium.

(a) In neutral medium:

In neutral medium, it is reduced to MnO₂

MnO₄^– + 2H₂O + 3e^– → MnO₂ + 4OH^–

(i) It oxidises H₂S to sulphur

2MnO₄^– + 2H₂O + 3e^– → MnO₂ + 4OH^–

(ii) It oxidises thiosulphate into sulphate

8MnO₄^– + 3S₂O₃^2- + H₂O → 6SO₄^2- + 8MnO₂ + 2OH^–

(b) In alkaline medium:

In the presence of alkali metal hydroxides, the permanganate ion is converted into manganate.

MnO₄^– + e^– → MnO₄^2-

This manganate is further reduced to MnO2 by some reducing agents.

MnO₄^2- + H₂O → MnO₂ + 2OH^– + [O]

So the overall reaction can be written as follows.

MnO₄^– + 2H₂O + 3e^– → MnO₂ + 4OH^–

This reaction is similar as that for neutral medium.

Bayer’s Reagent:

Cold dilute alkaline KMnO₄ is known as Bayer’s reagent. It is used to oxidise alkenes into diols. For example, ethylene can be converted into ethylene glycol and this reaction is used as a test for unsaturation.

(c) In acid medium:

In the presence of dilute sulphuric acid, potassium permanganate acts as a very strong oxidising agent. Permanganate ion is converted into Mn²⁺ ion.

MnO₄^– + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O

The oxidising nature of potassium permanganate (permanganate ion) in acid medium is illustrated in the following examples.

(i) It oxidises ferrous salts to ferric salts.

2MnO₄^– + 10Fe₂₊ + 16H⁺ → 2Mn²⁺ + 10Fe³⁺ + 8H₂O

(ii) It oxidises iodide ions to iodine

2MnO₄^– + 10 I^– + 16H⁺ → 2Mn²⁺ + 5I₂ + 8H₂O

(iii) It oxidises oxalic acid to CO₂

2MnO₄^– + 5(COO)^2-₂ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O

(iv) It oxidises sulphide ion to sulphur

2MnO^–₄ + 5 S^2- + 16H⁺ → 2Mn²⁺ + 5 S + 8H₂O

(v) It oxidises nitrites to nitrates

2MnO₄^– + 5 NO₂^– + 6H⁺ → 2Mn²⁺ + 5NO^–₃ + 3H₂O

(vi) It oxidises alcohols to aldehydes.

2KMnO₄ + 3H₂SO₄ + 5CH₃CH₂OH → K₂SO₄ + 2MnSO₄ + 5CH₃CHO + 8H₂O

(vii) It oxidises sulphite to sulphate

2MnO₄^– + 5SO₃^2- + 6H⁺ → 2Mn²⁺ + 5SO₄^2- + 3H₂O

Uses of Potassium Permanganate:

Some important uses of potassium permanganate are listed below.

1. It is used as a strong oxidizing agent.
2. It is used for the treatment of various skin infections and fungal infections of the foot.
3. It used in water treatment industries to remove iron and hydrogen sulphide from well water.
4. It is used as Bayer’s reagent for detecting unsaturation in an organic compound.
5. It is used in quantitative analysis for the estimation of ferrous salts, oxalates, hydrogen peroxide and iodides.

F-Block Elements – Inner Transition Elements

In the inner transition elements there are two series of elements.

1. Lanthanoids (previously called lanthanides)
2. Actinoids (previously called actinides)

Lanthanoid series consists of fourteen elements from Cerium (₅₈Ce) to Lutetium (₇₁Lu) following Lanthanum (₅₇La). These elements are characterised by the preferential filling of 4f orbitals, Similarly actinoids consists of 14 elements from Thrium (₉₀Th) to Lawrencium (₁₀₃Lr) following Actinium (₈₉Ac). These elements are characterised by the preferential filling of 5f orbital.

The position of Lanthanoids in the periodic table

The actual position of Lanthanoids in the periodic table is at group number 3 and period number 6. However, in the sixth period after lanthanum, the electrons are preferentially filled in inner 4f sub shell and these fourteen elements following lanthanum show similar chemical properties. Therefore these elements are grouped together and placed at the bottom of the periodic table. This position can be justified as follows.

1. Lanthanoids have general electronic configuration [Xe] 4f^1-14 5d^10-1 6s²
2. The common oxidation state of lanthanoides is +3
3. All these elements have similar physical and chemical properties.

Similarly the fourteen elements following actinium resemble in their physical and chemical properties. If we place these elements after Lanthanum in the periodic table below 4d series, the properties of the elements belongs to a group would be different and it would affect the proper structure of the periodic table. Hence a separate position is provided to the inner transition elements as shown in the figure.

Electronic Configuration of Lanthanoids:

We know that the electrons are filled in different orbitals in the order of their increasing energy in accordance with Aufbau principle. As per this rule after filling 5s, 5p and 6s and 4f level begin to fill from lanthanum, and hence the expected electronic configuration of Lanthanum(La) is [Xe] 4f¹ 5d° 6s² but the actual electronic configuration of Lanthanum is [Xe] 4f° 5d¹ 6s² and it belongs to d block.

Filling of 4f orbital starts from Cerium (Ce) and its electronic configuration is [Xe] 4f¹ 5d¹ 6s². As we move from Cerium to other elements the additional electrons are progressively filled in 4f orbitals as shown in the table.

Table: Electronic Configuration of Lanthanum and Lanthanoids

In Gadolinium (Gd) and Lutetium (Lu) the 4f orbitals, are half-filled and completely filled, and one electron enters 5d orbitals. Hence the general electronic configuration of 4f series of elements can be written as [Xe] 4f^1-14 5d^0-1 6s²

Oxidation State of Lanthanoids:

The common oxidation state of lanthanoids is +3. In addition to that some of the lanthanoids also show either +2 or +4 oxidation states. Gd³⁺ and Lu³⁺ ions have extra stability, it is due to the fact that they have exactly half filled and completely filled f-orbitals respectively their electronic configurations are

Gd³⁺: [Xe]4f⁷
Lu³⁺: [Xe]4f¹⁴

Similarly Cerium and terbium attain 4f° and 4f⁷ configurations respectively in the +4 oxidation states. Eu²⁺
and Yb²⁺ ions have exactly half filled and completely filled f orbitals respectively.

The stability of different oxidation states has an impact on the properties of these elements the following table shows the different oxidation states of lanthanoids.

Atomic and Ionic Radii:

As we move across 4f series, the atomic and ionic radii of lanthanoids show gradual decrease with increse in atomic number. This decrease in ionic size is called lanthanoid contraction.

Cause of Lanthanoid Contraction:

As we move from one element to another in 4f series (Ce to Lu) the nuclear charge increases by one unit and an additional electron is added into the same inner 4f sub shell. We know that 4f sub shell have a diffused shapes and therefore the shielding effect of 4f elelctrons relatively poor hence, with increase of nuclear charge, the valence shell is pulled slightly towards nucleus. As a result, the effective nuclear charge experienced by the 4f elelctorns increases and the size of Ln³⁺ ions decreases. Lanthanoid contraction of various lanthanoids is shown in the graph.

Consequences of Lanthanoid Contraction:

1. Basicity Differences

As we from Ce³⁺ to Lu³⁺, the basic character of Ln³⁺ ions decrease. Due to the decrease in the size of Ln³⁺ ions, the ionic character of Ln – OH bond decreases (covalent character increases) which results in the decrease in the basicity.

2. Similarities Among Lanthanoids:

In the complete f – series only 10 pm decrease in atomic radii and 20 pm decrease in ionic radii is observed because of this very small change in radii of lanthanoids, their chemical properties are quite similar.

The elements of the second and third transition series resemble each other more closely than the elements of the first and second transition series. For example

Series	Element	Atomic Radius
3d Series	Ti	132 pm
4d Series	Zr	145 pm
5d Series	Hf	144 pm

Actinoids:

The fourteen elements following actinium, i.e., from thorium (Th to lawrentium (Lr) are called actinoids. Unlike the lanthanoids, all the actinoids are radioactive and most of them have short half lives. Only thorium and uranium (U) occur in significant amount in nature and a trace amounts of Plutonium (Pu) is also found in Uranium ores. Neptunium (Np) and successive heavier elements are produced synthetically by the artificial transformation of naturally occuring elements by nuclear reactions. Similar to lanthanoids, they are placed at the bottom of the periodic table.

Electronic Configuration:

The electronic configuration of actinoids is not definite. The general valence shell electronic configuration of 5f elements is represented as [Rn]5f^1-146d^0-27s². The following table show the electronic configuration of actinoids.

Table: Electronic configuration of actinoids

Oxidation State of Actinoids:

Like lanthanoids, the most common state of actinoids is +3. In addition to that actinoids show variable oxidation states such as +2 , +3 , +4 ,+5,+6 and +7. The elements Americium(Am) and Thrium (Th show +2 oxidation state in some compounds, for example thorium iodide (ThI₂). The elements T, Pa, U, Np, Pu and Am show +5 oxidation states. Np and Pu exhibit +7 oxidation state.

Differences Between Lanthanoids and Actinoids:

Lanthanoids	Actinoids
1. Differentiating electron enters in 4f orbital	1. Differentiating electron enters in 5f orbital
2. Binding energy of 4f orbitals are higher	2. Binding energy of 5f orbitals are lower
3. They show less tendency to form complexes	3. They show greater tendency to form complexes
4. Most of the lanthanoids are colourless	4. Most of the actinoids are coloured. For example. U3+(red), U4+(green), UO22+(yellow)
5. They do not form oxo cations	5. They do form oxo cations such as UO22+, NPO22+ etc
6. Besides +3 oxidation states lanthanoids show +2 and +4 oxidation states in few cases	6. Besides +3 oxidation states actinoids show higher oxidation states such as +4, +5, +6 and +7