d- & f- Block Elements Topic Name : General introduction The elements, which are placed between s and p block elements in the long form of periodic table, are called transition elements. The properties of these elements are thus intermediate between those of s and p block elements. These elements are called transition elements since they represent a change of properties from the most electropositive s-block elements to the least electropositive p-block elements. Transition elements are also called d-block elements simply because the incoming electrons are filled in the d-orbital. This filling of electrons follow all the rules as for s and p-block elements, the only difference being that the last incoming electron enters (n - 1) d sublevel, i.e. the d-sublevel of the penultimate shell. The general electronic configuration for transition elements is thus written as (n - 1) d 1 - 10 ns 0 - 2 where n is the outermost shell. The electronic configuration of transition elements, has the following characteristics: i. An inner core of electrons with noble gas configuration. ii. (n – 1) d orbitals are progressively filled-up with electrons. Thus, it can be easily seen that the classification of d-block elements is primarily based on the electronic configuration of their atoms. You must be wondering as to why some elements have only one electron in the ns sublevel. Let us try to understand such exceptions by taking two examples from the first transition series. Chromium (3d 5 4s 1 ) and copper (3d 10 4s 1 ) show such electronic configuration. It is important to see that the 3d orbital is half-filled and completely filled for Cr and Cu respectively. You know from your previous chapters that half-filled and completely filled orbitals have extra stability owing to their symmetry and exchange energy. Thus, Cr and Cu have exceptional electronic configurations so as to gain extra stability. Definition of transition (d-block) elements Transition elements (d-block) may be defined as those elements, which have partly filled (n – 1) d-subshell in their elementary state. But this definition will exclude elements like Cu, Ag, Au, etc., which have completely filled (n – 1) d-subshell. To make the point more clear let us look at the electronic configuration of copper atom, cuprous ion and cupric ion. Cu (Z = 29) : 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 1 Cu 1+ ion : 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 Cu 2+ ion : 1s 2 2s 2 2p 6 3s 2 3p 6 3d 9 Copper atom or cuprous ion (Cu + ion) cannot be considered as transition element according to the above definition and in fact they do not exhibit the characteristic properties of transition elements. But cupric ion (Cu 2+ ion), having a d 9 configuration has incomplete d-subshell and hence is a transition ion. Transition element is thus defined as the element whose atom in ground state or ion in one of the common oxidation states has incomplete or partially filled (n – 1) d-subshell.
Transcript
d- & f- Block Elements
Topic Name : General introduction
The elements, which are placed between s and p block elements in the long form of periodic table, are called transition elements. The properties of these elements are thus intermediate between those of s and p block elements. These elements are called transition elements since they represent a change of properties from the most electropositive s-block elements to the least electropositive p-block elements.
Transition elements are also called d-block elements simply because the incoming electrons are filled in the d-orbital. This filling of electrons follow all the rules as for s and p-block elements, the only difference being that the last incoming electron enters (n - 1) d sublevel, i.e. the d-sublevel of the penultimate shell. The general electronic configuration for transition elements is thus written as (n - 1) d1 - 10 ns0 - 2 where n is the outermost shell.The electronic configuration of transition elements, has the following characteristics:
i. An inner core of electrons with noble gas configuration.ii. (n – 1) d orbitals are progressively filled-up with electrons.
Thus, it can be easily seen that the classification of d-block elements is primarily based on the electronic configuration of their atoms.
You must be wondering as to why some elements have only one electron in the ns sublevel. Let us try to understand such exceptions by taking two examples from the first transition series. Chromium (3d5 4s1) and copper (3d10 4s1) show such electronic configuration. It is important to see that the 3d orbital is half-filled and completely filled for Cr and Cu respectively. You know from your previous chapters that half-filled and completely filled orbitals have extra stability owing to their symmetry and exchange energy. Thus, Cr and Cu have exceptional electronic configurations so as to gain extra stability.
Definition of transition (d-block) elementsTransition elements (d-block) may be defined as those elements, which have partly filled (n – 1) d-subshell in their elementary state. But this definition will exclude elements like Cu, Ag, Au, etc., which have completely filled (n – 1) d-subshell.
To make the point more clear let us look at the electronic configuration of copper atom, cuprous ion and cupric ion.
Cu (Z = 29) : 1s2 2s2 2p6 3s2 3p6 3d10 4s1
Cu1+ ion : 1s2 2s2 2p6 3s2 3p6 3d10
Cu2+ ion : 1s2 2s2 2p6 3s2 3p6 3d9
Copper atom or cuprous ion (Cu+ ion) cannot be considered as transition element according to the above definition and in fact they do not exhibit the characteristic properties of transition elements. But cupric ion (Cu2+ ion), having a d9 configuration has incomplete d-subshell and hence is a transition ion.
Transition element is thus defined as the element whose atom in ground state or ion in one of the common oxidation states has incomplete or partially filled (n – 1) d-subshell.
This definition again excludes zinc, cadmium and mercury from the transition elements. These elements do not have partly filled d-subshell in their atomic state or their common oxidation state (i.e., Zn2+, Cd2+ and Hg2+). These elements do not show properties of transition elements except for their ability to form complexes. These elements are still classified with d-block elements so as to maintain the logical form of the modern periodic table.
Topic Name : Characteristics of d-block elements
The electronic configuration of d-block elements differ from one another only in the number of electrons present in the (n – 1) d-subshell. This difference in their electronic configuration is reflected in the trends in their physical and chemical properties across a series.Some of the general characteristics of transition elements are discussed below which distinguish them from other metals of s and p-block elements.
Atomic and ionic radiiAtomic radii: The atomic radii of transition metals lie in between those of s and p-block elements. The trends in atomic radii being as follows:
i. The atomic radii of transition elements decrease across a series with increase in atomic number but the decrease is minimal after midway.The initial decrease in the atomic radius is due to the increase in the nuclear charge as the atomic number increases. This increased nuclear charge pulls the electrons more towards the nucleus thus resulting in a decrease in atomic radii. The electrons enter the penultimate shell [(n - 1) d-subshell] and the added d-electrons shield (or screen) the outermost electrons [ns2 electrons] from the nucleus resulting in a decrease in the attractive forces operating between the nucleus and the electrons. This is called screening effect. With increase in the number of d-electrons, the screening effect increases. After middle of the series, this screening effect counter balances the increased nuclear charge due to increase in atomic number. As a result, the decrease in atomic radii is small after midway.
ii. At the end of the series, there is slight increase in the atomic radii.After the middle element in each series, pairing up of d-electrons starts. This pairing introduces repulsions between the electrons of the same orbital which increases as pairing increases towards the end of the series. These inter-electronic repulsions tend to increase the atomic radius while increase in nuclear charge tends to decrease the atomic radius across a series. For elements like Fe which occur in the middle of the series, these two opposing tendencies counterbalance and thus there is no change in size from the previous element (Mn in this case). At the end of the series, the electron-electron repulsions are greater than the attractive force due to the increased nuclear charge and hence there is an increase in the atomic radius.
iii. The atomic radii increases down the group.This increase is attributed to the addition of a new shell as we move down the group. Thus, the electrons are in the energy levels farther away from the nucleus in the second transition series. Hence, the atomic radii of the element of the second transition series are larger than those of the elements of the first transition series. The atomic radii of the elements of the second and third transition series are nearly same due to lanthanide contraction which will be discussed later.
Ionic radiii. The trend followed by ionic radii is the same as that of atomic radii.ii. The ionic radii of transition metals are different in different oxidation states. As the oxidation state increases, the
net positive charge on the nucleus increases. This increase in the effective nuclear charge decreases the ionic radius with increase in oxidation state. Thus, the ionic size of M3+ cations are smaller than those of M2+ cations. However, the ionic radii for bivalent metal cations decreases with increase in atomic number.
iii. The ionic radii of the transition metals are smaller than those of the representative elements belonging to the same period. This is because of the poor shielding (or screening) effect of the d-orbitals. The d-orbitals are more diffused as compared to s and p-orbitals and hence cannot shield the outermost electrons as effectively as s or p-orbitals. This results in a greater attraction for the outermost electrons towards the nucleus and thus decreasing the ionic radii.
Metallic characterTransition elements exhibit all the characteristic properties of metals. They are hard, lustrous, malleable and ductile. Transition elements have high melting and boiling points, high thermal and electrical conductivity, and high tensile strength as metals. The metallic character of transition elements is due to their relatively low ionization energies and large number of vacant orbitals in the outermost shell. The availability of vacant orbitals in the outermost shell gives rise to the possibility of excitation of electrons to these orbitals thus explaining the high thermal and electrical conductivity.
The unpaired electrons present in d-orbitals of the transition elements interact to form metallic bonds. Thus, the strength of the metallic bond and consequently metallic character depends on the number of unpaired d-electrons. If the number of unpaired d-electrons is more, the overlapping of such electrons will be more leading to stronger bonding Cr, Mn and W have maximum number of unpaired d -electrons and thus behave as hard metals while elements like Zn, Cd and Hg are not very hard metals due to absence of unpaired d-electrons.
Melting and boiling pointTransition metals have very high melting and boiling points. As seen above, the presence of unpaired d-electrons in transition elements give rise to metallic bonds. Due to these strong metallic bonds, transition metals have high melting and high boiling points.
The melting points of the transition elements rise to a maximum and then fall as the atomic number increases across a series. This can be explained as follow. As we move across a series, the number of unpaired d-electrons increases up to the middle and then decreases. Consequently, the metallic strength increases up to the middle, (i.e. d5 configuration) and then starts decreasing. Accordingly, the melting points increase up to the middle of the series and decreases thereafter.The elements like Zn, Cd and Hg are soft and have low melting points because of the absence of unpaired d-electrons.
Ionization energiesi. The first ionization energies of d-block elements lie between s-block and p-block elements. The ionization
energy gradually increases with increase in atomic number across a series. The increase in ionization energy is due to the increase in nuclear charge with increasing atomic number. As the nuclear charge progressively increases it becomes more and more difficult to remove the outermost electron from the element and hence ionization energy increases.In a given series, the difference in the ionization energy between any two successive d-block elements is very less than the difference between successive s-block and p-block elements. This is because in transition elements, the electrons are being filled in (n - 1) d-subshell (penultimate) which screen the outermost ns electrons from the nucleus. This screening effect opposes the effect of increased nuclear charge and thus little difference in ionization energy is seen for successive elements.
ii. The first ionization energies of 5d elements are higher as compared to those of 3d and 4d elements. It should be noted that in 5d elements, 4f subshell is filled completely. The electrons in 4f subshell have a very weak shielding effect owing to the highly diffused nature of f-orbital. This shielding results in greater effective nuclear charge and hence a higher value of high ionization energies for elements of 5d-series.The trends in ionization energies of 3d and 4d elements are irregular.
iii. The magnitude of ionization energies of transition metals reflects the thermodynamic stability of their compounds. A small value of ionization energy means that the formation of ion from the metal atom is easy and consequently compound formation is favoured. Thus, smaller the ionization energy of the metal, more stable is its compound. The significance of the value of ionization energies can be seen from the following table:
Element (IE1 + IE2) MJ mol-1 (IE3 + IE4) MJ mol-1 Total IE MJ mol-1
Ni 2.49 8.80 11.29Pt 2.66 6.70 9.36
The sum of the first and second ionization energies (IE1 + IE2) is less for Ni. This means that Ni (II) compounds are easily formed as compared to Pt (II) compounds. Thus, Ni (II) compounds are more stable than Pt (II) compounds. Now, the sum of the third and fourth ionization energies (IE3 + IE4) is less for Pt. Thus, Pt (IV) is more easily attained while more energy would be required for obtaining Ni (IV) ion. Hence, Pt (IV) compounds are more stable than Ni (IV) compounds. For example, K2PtCl6 (Pt in IV oxidation state) is a well-known compound while corresponding nickel salt is not known.
Electrode potentialThe stability of compounds in solution does not depend upon ionization energy solely. It also depends upon factors such as energy of sublimation of the metal and the hydration energy. All these factors can be chemically depicted as follows:
Where, subH is the enthalpy of sublimation.
Where, IE is the ionization energy
Where, hyd H is the enthalpy of hydration Thus, the total enthalpy change ( HT) when solid metal, M, is brought in the aqueous medium in the form of monovalent ion, M+(aq),
will be the sum of the enthalpies of the three reactions involved, i.e. HT = subH + IE + hydHThe electrode potentials for elements are a measure of HT. Thus, the stability of the compounds in solution depends upon the value of electrode potential.Since transition elements can occur in more than one oxidation states, the oxidation state for which the value of HT is the lowest will be the most stable oxidation state for that metal in aqueous solution. In other words, the lower the electrode potential (i.e. more negative the standard reduction potential) of the electrode, more stable is the oxidation state of the transition metal in the aqueous medium.The standard reduction potential of transition metals may be determined by using standard hydrogen electrode as the counter electrode.
values for first row transition metals
Element V Cr Mn Fe Co Ni Cu-1.18 -0.91 -1.18 -0.44 -0.28 -0.25 +0.34
As seen in the table above, there is no regular trend in the values of the reduction potentials as the ionization energies (IE1 + IE 2) and sublimation energies of these metals do not show any regular trend.
Oxidation statesThe valence electrons in transition elements are placed in two sets of orbitals, viz. (n – 1) d-orbital and ns-orbital. The difference between the energies of these two orbitals is very less and hence both energy levels are used for bond formation. In ground state, ns electrons are used to give an oxidation state of +2 while for higher oxidation state, (n –1) d-electrons are also used. As a matter of fact, in excited state the (n – 1) d-electrons become bonding. These features impart the property of having variable oxidation states to transition elementsTable generally shows oxidation states of transition metals (very rare oxidation states are given in parenthesis):
i. The most common oxidation state of the first row transition metals is + 2 except in case of scandium. The oxidation state of + 2 is obtained by losing ns electrons.
ii. Bonding is primarily ionic in compounds having transition elements in low oxidation state like + 2 or + 3, covalent character increases for higher oxidation states since bonds are formed by sharing of electrons in
higher oxidation states. For example, all the bonds between manganese and oxygen in are covalent (Mn in + 7 oxidation state).
iii. The possibility of having a higher oxidation state increases with increase in atomic number within group. For example, the common oxidation state for iron (Fe) are + 2 an + 3 whereas for ruthenium (Ru) and osmium (Os) the oxidation states of + 4, + 6 and + 8 are also observed.
iv. Transition metals also show low oxidation states of + 1 and zero in some of their compounds. The nature of bonding of such compounds is complex. When many bonds are formed with a transition metal, the negative charge on the metal increases. This causes accumulation of charge on the central metal and hence the bonds are not very stable. In such cases, the transition metals donate their d-electrons to the other constituents of the compounds. A good example of such bonding is the compounds of transition metals with sulphur and carbon monoxide which have vacant orbitals to accommodate the metal donated d-electrons. It is important to note that such a situation of back donation can occur only if the metal is in a low oxidation state. If the metal is in high oxidation state the increased nuclear charge will hold the d-electrons firmly to the nucleus and donation of d-electrons will not be favoured.
v. A transition metal in solution will have an oxidation state that is most stabilized by the solvent. Thus, the oxidation state of a metal in solution depends on the nature of the solvent. A metal in a particular oxidation state may oxidize or reduce in solution under appropriate conditions. For example, Cu+ is unstable in water and gets oxidized to Cu2+, while Cr3+ is stable in water. Similarly, Fe2+ is unstable in treated water as it undergoes oxidation in it.
vi. The oxidation state of the transition metal also depends on the nature of combining atoms. The compounds of metals with fluorine and oxygen exhibit the highest oxidation state as fluorine and oxygen have high electronegativities.
Catalytic propertiesMany transition metals and their compounds can be used as effective catalysts in various processes. These metals or their compounds provide catalytic effect simply because they can attain variable oxidation states. This fact is made more clear in the following points.
i. Transition elements have unpaired d-electrons and can have variable oxidation states. This property of showing multiple oxidation state helps them to absorb an electron or lose an electron to attain low and high oxidation states, respectively. Thus, transition metals are capable of absorbing and re-emitting wide range of energies (or electrons) and providing suitable activation energy for a particular reaction.
ii. In some cases, transition elements act as catalysts by forming unstable intermediate compound with one of the reactants. As the reaction proceeds, the intermediate compound decomposes generating the reactant (in another energy state) so that it can react with the other reactant. The transition metal is left unchanged after the process, thus acting as a catalyst.
iii. According to adsorption theory of catalysis, the transition metal provides a large surface area with free valencies where the reactants are adsorbed and facilitates the reaction and a catalytic effect is observed.
The table given below illustrates the use of several transition metals as catalyst in various industrial processes.
Coloured ionsMost of the transition metal compounds are coloured both in the solid state and in aqueous solution. They differ from s and p-block elements in this respect. Colour is always associated with absorption of light of a particular wavelength in visible region. Electrons absorb energy and get promoted to higher energy level. This transition of electrons is responsible for coloured ions. The transition elements have incomplete d-orbitals. Though these d-orbitals are degenerate, i.e. have the same energy, for an isolated ion, but when in compounds these orbitals are affected by the combining atoms or molecules and lose their degeneracy. This loss of degeneracy makes transition of electron from one d-orbital to another d-orbital which is otherwise forbidden. Such an effect on the central metal by incoming ligands is called crystal field effect.The difference in energy for these d-orbitals is very less and the radiation of light corresponding to such small amount of energy comes within visible region of light. Hence, transition metal compounds are coloured.In solution the transition metal ion is not isolated but surrounded by water molecules and thus a similar effect is seen in solution. This process can be expressed schematically as follows:
Sc (III) and Ti (IV) are colourless because they have empty d-orbitals and hence has no electron for promotion to higher level. Cu (I) and Zn (II) are also colourless since they have completely filled d-orbitals leaving no empty orbital for promotion of electrons.
Magnetic propertiesTransition metal ions and their compounds show magnetic behaviour due to the presence of unpaired electrons in (n –1) d-orbitals. A spinning electron has spin as well as orbital motion. Thus, a spinning electron revolving around the nucleus creates a magnetic field amount itself. In other words, an electron behaves as a micro-magnet having a definite value of magnetic moment. When a substance is placed in an external magnetic field, the magnetic moment of the electron is sufficient to overcome the magnetic moment induced by the applied magnetic field. Such a substance experiences attractive influence in a magnetic field and are said to show paramagnetism. The greater the number of unpaired electrons, greater will be the resulting magnetic moment and more will be the paramagnetic character.The magnetic moment is expressed in Bohr magnetons (B.M.)A paramagnetic substance is characterized by its effective magnetic moment ( eff) which is given by the following expression.
B.M
Where n is the number of unpaired electrons.
When pairing up of electrons takes place after middle of the series, the magnetic fields created by two electrons in the same orbital with opposite spins are in opposite direction. Thus, the electron pair will not have any residual magnetic field. Such a compound when placed in an external magnetic field will have induced magnetic movement whose direction will be in opposite direction to the applied magnetic field. Such substances feel a repulsion by the applied magnetic field. This kind of behaviour is called diamagnetic behaviour. Thus, transition elements which have paired electrons are diamagnetic in nature.
Table showing magnetic moments of some of the transition metal ions
Complex formationTransition metal ions form a variety of complexes unlike, s and p-block elements. Complexes are the binary compounds formed by the donation of lone pairs by a negative ion or neutral molecule (called bonds) to the central metal ion. Transition elements form complexes easily because:
¾ They have small size and thus can accommodate a number of ligands around them.¾ They have large effective nuclear charge due to poor shielding effect of d-orbitals.¾ They have vacant d-orbitals of suitable energy so as to accept lone pairs of electrons from ligands.
Interstitial compoundsInterstitial compounds are those compounds in which small atoms (like hydrogen, nitrogen, carbon, etc.) occupy empty spaces (or interstitial sites) within the lattice framework of the compound.
Transition metals form a large number of interstitial compounds. These compounds are hard and rigid. For example, steel and cast iron, the interstitial compound of iron and carbon is hard. Interstitial compounds are less malleable and ductile than metals while their tenacity is higher.
Alloy formationTransition metals have similar atomic sizes and thus atoms of one metal can very well replace the atom of another metal from its position in the crystal lattice. In fact, transition metals give alloys on cooling.Alloys generally have better properties than individual metals. They are more hard than the constituent metals and have high melting points. Alloys are more resistant to corrosion.The metals chromium, vanadium, molybdenum, tungsten and manganese are used in the formation of alloy steel and stainless steel.
Topic Name : Potassium Dichromate
Preparation of potassium dichromatePotassium dichromate is prepared from chromates which in turn are prepared from chrome iron, FeCr2O4. The various steps involved are as follows:
i. Preparation of sodium chromate:The ore, FeCr2O4 is finely powdered, mixed with sodium carbonate and quick lime and then heated to redness in presence of air to evolve carbon dioxide. The reaction involved can be written as follows:
The roasted mass is then extracted with water when sodium chromate is completely dissolved while Fe2O3 is left behind.
ii. Conversion of sodium chromate into sodium dichromate:Sodium chromate solution obtained is filtered and treated with concentrated sulphuric acid to obtain sodium dichromate as shown below:
2Na2CrO4 + H2SO4 Na2Cr2O7 + Na2SO4 + H2OSodium sulphate being less soluble crystallizes and is then filtered off.
iii. Conversion of sodium dichromate into potassium dichromate:Sodium dichromate is more soluble and less stable than potassium dichromate and it is converted to potassium dichromate easily upon treatment with potassium chloride.
Na2Cr2O7 + 2KCl K2Cr2O7 + 2NaClPotassium dichromate being much less soluble than sodium salt, crystallizes out on cooling.
Properties of potassium dichromatePotassium dichromate is an orange red crystalline solid which melts at 669 K. It is moderately soluble in cold water but freely soluble in hot water. Some important properties are as follows:
i. Action of heat: When heated potassium dichromate decomposes with evolution of oxygen.
ii. Action of alkalis: An orange red solution of potassium dichromate turns yellow upon treatment with alkali due to the formation of chromate.K2Cr2O7 + 2 KOH 2K2CrO4 + H2O The reaction is reversed on acidification, i.e.
K2CrO4 + H2SO4 K2Cr2O7 + K2SO4 + H2OThus, the inter conversion of chromate and dichromate is an equilibrium process which is pH dependent.2CrO4
2– + 2H+ 2HCrO4– Cr2O7
2– + H2OChromate(Yellow) HydrogenChromate Dichromate(Orange)In acidic conditions (low pH), dichromate is more stable while under alkaline conditions (high pH) chromate is more stable. Structure of chromate and dichromate ions are shown below:
Chromate ion Dichromate ion
iii. Action of concentrated sulphuric acid: Red crystals of chromic anhydride (chromium trioxide) are formed when potassium dichromate is treated with cold concentrated sulphuric acid.
K2Cr2O7 + 2H2SO4 2CrO3 + 2KHSO4 + H2ORed
On the other hand, heating of potassium dichromate with concentrated sulphuric acid results in evolution of oxygen.
iv. Oxidizing properties: Potassium dichromate is a powerful oxidizing agent. In the presence of dilute sulphuric acid, one mole of potassium dichromate produces three moles of oxygen atoms as indicated by the equation.
K2Cr2O7 + 4H2SO4 K2SO4 + Cr2(SO4)3 + 4H2O + 3O
Cr2O72– + 14H+ + 6e– 2Cr3+ + 7H2O
Where chromium in + 6 oxidation state in Cr2O72– ion is being reduced to Cr3+ (+ 3 oxidation state.)
Let us examine the action of acidified potassium dichromate solution as an oxidizing agent by taking few examples:
8. Chromyl chloride test: When potassium dichromate is treated with a strong (concentrated) sulphuric acid and a chloride, reddish brown vapours of chromyl chloride are formed.
K2Cr2O7 + 2H2SO4 2KHSO4 + 2CrO3 + H2O
KCl + 4HCl KHSO4 + HCl] × 4
2CrO3 + 4HCl 2CrO2Cl2 + 2H2O
K2Cr2O7 +4KCl+6H2SO4 2CrO2Cl2 + 6KHSO4 + 3H2O
Chromyl chloride(Red)This test is used in detection of chloride ions in qualitative analysis.
Topic Name : Potassium PermanganatePreparation of potassium permanganatePotassium permanganate is prepared on a large scale from mineral pyrolusite (MnO2). The preparation involves following steps.
i. Conversion of MnO2 to potassium magnate:Pyrolusite is fused with potassium hydroxide and the molten liquid is stirred well in the presence of air.
2MnO2 + 4KOH + O2 2K2MnO4 + 2H2OOxidizing agents like potassium nitrate can also be used instead of air:
MnO2 + 2KOH + KNO3 K2MnO4 + KNO2 + H2O
ii. Oxidation of potassium manganate to potassium permanganate:Potassium manganate can be chemically oxidized to permanganate by bubbling CO2, Cl2 etc., through the solution of potassium manganate.
3K2MnO4 + 2CO2 2KMnO4 + MnO2 + 2K2CO3
2K2MnO4 + Cl2 2KMnO4 + 2KClThese chemical processes are not very economical at the industry scale and hence electrolytic oxidation is preferred over them.Potassium manganate is oxidized electrochemically to permanganate. The electrode reactions taking place are:
At anode:
2K2MnO4 + H2O + O 2KMnO4 + 2KOH
or
MnO42– MnO4
– + e–
Green Purple
At cathode:
2H+ + 2e– H2
Examples of oxidation by potassium permanganate in neutral solution are discussed below:i. It oxidizes hot manganese sulphate to manganese dioxide.
2KMnO4 + H2O 2KOH + 2MnO2 + 3O
3MnSO4 + 3H2O + 3O 3MnO2 + 3H2SO4
2KOH + H2SO4 K2SO4 + 2H2O
3MnSO4 + 2KMnO4 + 2H2O 5MnO2 + K2SO4 + 2H2SO4
ii. It oxidizes sodium thiosulphate to sodium sulphate.
3Na2S2O3 + 8KMnO4 + H2O 3Na2SO4 + 8MnO2 + 3K2SO4 + 2 KOHiii. It oxidizes hydrogen sulphide to sulphur.
2 KMnO4 + 4H2S 2 MnS + S + K2SO4 + 4H2O
Properties of potassium permanganatePotassium permanganate is a purple crystalline solid melting at 523 K. It is slightly soluble in cold water. The solubility increases in hot water.
i. Action of heat: Potassium permanganate decomposes to oxygen, potassium manganate and manganese dioxide when heated to 746 K.
2KMnO4 K2MnO4 + MnO2 + O2
ii. Action of concentrated sulphuric acid: When treated with cold concentrated sulphuric acid potassium permanganate is converted to Mn2O7 (green oil) which decomposes on warming to MnO2 (it is highly explosive).
2KMnO4 + 2H2SO4 Mn2O7 + 2KHSO4 + H2O
2Mn2O7 4MnO2 + 3O2
iii. Oxidizing properties: Potassium permanganate is a strong oxidizing agent and the reaction is pH dependent.In alkaline solution: In strongly alkaline solution, MnO4
2– ion is produced as shown in the reaction.
2KMnO4 + 2KOH 2K2MnO4 + H2O + O
or
MnO4– + e– MnO4
2–
The MnO42– ion gets further reduced to MnO2,
K2MnO4 + H2O MnO2 + 2KOH + O
or
MnO42– + 2H2O + 2e– MnO2 + 4OH–
Thus, the complete reaction is:
2KMnO4 + H2O 2MnO2 + KOH + 3O
or
MnO4– + 2H2O + 3e– MnO2 + 4OH–
A few examples of oxidation by KMnO4 in alkaline medium are:
a. potassium iodide is oxidized to potassium iodate.
2KMnO4 + H2O + KI 2MnO2 + 2KOH + KIO3
or
I – + 6OH– IO3– + 3H2O + 6e–
In acidic medium: In the presence of dilute sulphuric acid, the following reaction takes place,
2KMnO4 + 3H2SO4 K2SO4 + 2MnSO4 + 3H2O + 5O
or
MnO4– + 8H+ + 5e– Mn2+ + 4H2O
Potassium permanganate acts as a very strong oxidizing agent in acidic media. Few examples of oxidation by acidic potassium permanganate solution are:
¾ oxidation of H2S to S.
2KMnO4 + 3H2SO4 K2SO4 + 2MnSO4 + 3H2O + 5O
H2S + O H2O + S] × 5
2KMnO4 + 3H2SO4 + 5H2S K2SO4 + 2MnSO4 + 8H2O + 5S
or
2MnO4– + 16H+ + 5S2– 2Mn2+ + 8H2O + 5S
¾ oxidation of ferrous sulphate to ferric sulphate.
In neutral medium: In neutral medium, potassium permanganate is weakly oxidizing and the reaction involved is:
2KMnO4 + H2O 2KOH + 2MnO2 + 3O
or
MnO4– + 2H2O + 3e– MnO2 + 4OH–
The alkali (KOH) produced renders the solution basic as the reaction proceeds and the reaction given above is then essentially same as that for alkaline medium.
Structure of permanganate ionThe four oxygen atoms are arranged tetrahedrally around manganese in MnO4
– as manganese is sp3 hybridized.
Structure of permanganate ion
Topic Name : Inner Transition elements (Lanthanides and Actinides)The elements in which the last electron enters (n – 2) f-orbitals are called f-block elements. These elements are also known as inner transition elements. This is because they form a series between the transition metal series.The filling of 4f and 5f-orbitals takes place in f-block elements. This is the basis of classification of f-block elements as lanthanides or actinides.Lanthanides are the elements in which the last electron enters the 4f-orbital. Lanthanides constitutes the first inner transition series. These elements are called lanthanides or lanthanons since the series starts from the element lanthanum.The elements in which the last electron enters the 5f-orbital constitute the second inner transition series. They are also called actinides or actinons because the series starts from the element actinium.The members of two series with their electronic configuration are given in the table below.
Topic Name : General properties of Lanthanides and Actinides
The elements of lanthanide and actinide series are highly dense metals with high boiling points. These metals form alloys with other metals especially with iron. These alloys are used extensively since the presence of rare earth elements is found to improve the workability of steel at high temperatures. Two important alloys of rare earth elements are:
i. Misch metal: Misch metal consists of rare earth elements (94 – 95%), iron (upto 5%) and traces of sulphur, carbon, calcium and aluminium.
ii. Pyrophoric alloys: Pyrophoric alloys consist of following elements:
Cerium 40.5%Lanthanum and neodymium 44%Iron 4.5%Aluminium 0.5%Calcium, silicon and carbon 10.5%
Some characteristic properties of lanthanides
i. The typical oxidation state of lanthanide elements is + 3. Some of the elements also exhibit + 2 and + 4 oxidation states but these oxidation states are not as stable as + 3. Thus, the elements in + 2 and + 4 oxidation states tend to become stable by attaining + 3 oxidation state.Hence, Sm2+, Eu2+ and Yb2+ ions in solution act as good reducing agents since they tend to oxidize to more stable + 3 oxidation state. Similarly, Ce4+ acts as a good oxidizing agent in solution due to its ability to get reduced to Ce3+ easily.
ii. The ionic radii of trivalent lanthanides decrease steadily as the atomic number of the lanthanide element increases. This is because the last incoming electron enters the f-orbital. Now as the atomic number increases along the lanthanide series, the ionic radii decreases as is normally expected. But after the middle of the series, the number of electrons added to (n – 2) f-subshell increases and it is expected that these electrons should shield the outermost electrons from the nuclear charge. Thus, the screening offered by f-electrons would counterbalance the effect of increased nuclear charge. But since f-orbitals are too diffused, shielding of the outermost electrons is not perfect. This imperfect shielding is unable to counterbalance the effect of increased nuclear charge leading to a steady contraction in ionic radii. This regular contraction in size is called lanthanide contraction.
Consequence of lanthanide contraction
i. The ionic radii of lanthanides are very similar and thus separation of lanthanides in pure state is difficult. But, due to lanthanide contraction there is a difference in chemical properties of these elements which enables the separation of individual lanthanide elements by ion-exchange methods.
ii. The effect of lanthanide contraction is also seen in the transition series elements. The ionic radii of Zr(160 pm) and Hf (159 pm) of the second and third transition series are almost same because of the lanthanide contraction.
iii. Lanthanide elements have low ionization energies and thus are highly electropositive in nature.
Some characteristic properties of actinides
i. The dominant oxidation state of these elements is + 3. Actinides also exhibit an oxidation state of + 4. Some actinides such as uranium, neptunium and plutonium also show an oxidation state of + 6.
ii. The actinides show actinide contraction (like lanthanide contraction) due to poor shielding of the nuclear charge by 5f electrons.
iii. All the actinides are radioactive. Actinides are radioactive in nature so the study of their chemistry is difficult in the laboratory. Their chemistry is studied using tracer techniques.
Comparison of lanthanides and actinidesSimilarities:Lanthanides and actinides involve filling of f-orbitals and thus are similar in many respects.
i. The most common oxidation state is +3 for both lanthanides and actinides.ii. Both are electropositive in nature and thus very reactive.iii. Magnetic and spectral properties are exhibited by both lanthanides and actinides.iv. Actinides exhibit actinide contraction just like lanthanides.
Differences:Lanthanides and actinides differ in some of their characteristics as follows:
i. Besides +3, lanthanides also show oxidation states of +2 and +4 while actinides show higher oxidation states of +4, +5, +6 and + 7 as well.
ii. Lanthanide ions are colourless while most of the actinide ions are coloured.iii. Actinides have a greater tendency towards complex formation as compared to lanthanides.iv. Lanthanide compounds are less basic while actinide compounds have appreciable basicity.v. Actinides form few important oxocations such as UO2
2+, PuO22+, etc, while such oxocations are not known
for lanthanides.vi. Almost all actinides are radioactive while lanthanides, except promethium, are non-radioactive.vii. The magnetic properties of actinides can be easily explained while it is difficult to do so in the case
of lanthanides.
Topic Name : Uses of Lanthanides and Actinides
Inner transition elements and their compounds, find applications in variety of fields. Some important applications of these elements and their compounds are listed below:
i. Misch metal, an alloy of rare earth elements is used in the production of different brands of steel like heat resistant, stainless and instrumental steels.
ii. Pyrophoric alloys find their uses in the preparation of ignition devices such as tracer bullet shells and flints for lighters.
iii. Lanthanide oxides can absorb ultraviolet rays. Some such oxides are used as additives in glasses for specialpurposes like:
o Sunglasses (by adding Nd2O3)o Goggles for glass blowing and welding work (by adding Nd2O3 and Pr2O3)o Glasses protecting eyes from neutron radiation (by adding Ce2O3 + Sm2O3)
iv. Lanthanide oxides are also used as abrasives for polishing glasses.v. Lanthanide compounds are used in manufacture of dyes and paints. For example, cerium molybdate is
used in yellow dyes and salts of Nd are used for red colour.vi. Certain compounds of lanthanides are used as catalyst for reactions like hydrogenation, dehydrogenation
and for reactions like hydrogenation, dehydrogenation and oxidation of organic compounds.vii. Lanthanide elements and compounds find uses in nuclear fuel control, shielding and fluxing devices.viii. Actinides like uranium and plutonium are used as a nuclear fuel in nuclear reactors.ix. Aqueous solution of Ce+4 is used as an oxidizing agent while those of Sm2+, Eu2+ and Yb2+ are used
as reducing agents.x. Many radioactive actinides are used in radiotracer techniques for detection of cancer and many
metabolic pathways. Thorium salts are also used in treatment of cancer.
