DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87

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1 Topic Name : General introduction d- & f- Block Elements (Class-12 Chemistry) The elements, which are placed between s and p block elements in the long fm 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-bital. This filling of electrons follow all the rules as f 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 f 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 ce of electrons with noble gas configuration. ii. (n 1) d bitals 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 imptant to see that the 3d bital is half-filled and completely filled f Cr and Cu respectively. You know from your previous chapters that half-filled and completely filled bitals 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 me 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 cuprous ion (Cu + ion) cannot be considered as transition element accding 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 ion in one of the common oxidation states has incomplete 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 their common oxidation state (i.e., Zn 2+, Cd 2+ and Hg 2+ ). These elements do not show properties of transition elements except f their ability to fm complexes. These elements are still classified with d-block elements so as to maintain the logical fm 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.

2 Atomic and ionic radii Atomic 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 me 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 ( screen) the outermost electrons [ns 2 electrons] from the nucleus resulting in a decrease in the attractive fces 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. iii. 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 bital 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. F 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 fce due to the increased nuclear charge and hence there is an increase in the atomic radius. 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 radii i. 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 M 3+ cations are smaller than those of M 2+ cations. However, the ionic radii f 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 po shielding ( screening) effect of the d-bitals. The d-bitals are me diffused as compared to s and p-bitals and hence cannot shield the outermost electrons as effectively as s p-bitals. This results in a greater attraction f the outermost electrons towards the nucleus and thus decreasing the ionic radii. Metallic character Transition 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 bitals in the outermost shell. The availability of vacant bitals in the outermost shell gives rise to the possibility of excitation of electrons to these bitals thus explaining the high thermal and electrical conductivity. The unpaired electrons present in d-bitals of the transition elements interact to fm 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 me, the overlapping of such electrons will be me 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 point Transition 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.

3 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. d 5 configuration) and then starts decreasing. Accdingly, 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 energies i. 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 me and me 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 f 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-bital. This shielding results in greater effective nuclear charge and hence a higher value of high ionization energies f 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 fmation of ion from the metal atom is easy and consequently compound fmation is favoured. Thus, smaller the ionization energy of the metal, me stable is its compound. The significance of the value of ionization energies can be seen from the following table: Element (IE 1 + IE 2 ) MJ mol -1 (IE 3 + IE 4 ) MJ mol -1 Total IE MJ mol -1 Ni Pt The sum of the first and second ionization energies (IE 1 + IE 2 ) is less f Ni. This means that Ni (II) compounds are easily fmed as compared to Pt (II) compounds. Thus, Ni (II) compounds are me stable than Pt (II) compounds. Now, the sum of the third and fourth ionization energies (IE 3 + IE 4 ) is less f Pt. Thus, Pt (IV) is me easily attained while me energy would be required f obtaining Ni (IV) ion. Hence, Pt (IV) compounds are me stable than Ni (IV) compounds. F example, K 2 PtCl 6 (Pt in IV oxidation state) is a well-known compound while cresponding nickel salt is not known. Electrode potential The stability of compounds in solution does not depend upon ionization energy solely. It also depends upon facts such as energy of sublimation of the metal and the hydration energy. All these facts can be chemically depicted as follows: Where, subh is the enthalpy of sublimation. Where, IE is the ionization energy Where, hydh is the enthalpy of hydration Thus, the total enthalpy change ( H T ) when solid metal, M, is brought in the aqueous medium in the fm of monovalent ion, M + (aq),

4 will be the sum of the enthalpies of the three reactions involved, i.e. H T = subh + IE + hydh The electrode potentials f elements are a measure of H T. Thus, the stability of the compounds in solution depends upon the value of electrode potential. Since transition elements can occur in me than one oxidation states, the oxidation state f which the value of H T is the lowest will be the most stable oxidation state f that metal in aqueous solution. In other wds, the lower the electrode potential (i.e. me negative the standard reduction potential) of the electrode, me 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 f first row transition metals Element V Cr Mn Fe Co Ni Cu As seen in the table above, there is no regular trend in the values of the reduction potentials as the ionization energies (IE 1 + IE 2 ) and sublimation energies of these metals do not show any regular trend. Oxidation states The valence electrons in transition elements are placed in two sets of bitals, viz. (n 1) d-bital and ns-bital. The difference between the energies of these two bitals is very less and hence both energy levels are used f bond fmation. In ground state, ns electrons are used to give an oxidation state of +2 while f 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 elements Table 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 , covalent character increases f higher oxidation states since bonds are fmed by sharing of electrons in higher oxidation states. F 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. F example, the common oxidation state f iron (Fe) are + 2 an + 3 whereas f 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 fmed 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 bitals to accommodate the metal donated d-electrons. It is imptant 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 reduce in solution under appropriate conditions. F example, Cu + is unstable in water and gets oxidized to Cu 2+, while Cr 3+ is stable in water. Similarly, Fe 2+ 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 fluine and oxygen exhibit the highest oxidation state as fluine and oxygen have high electronegativities. Catalytic properties Many transition metals and their compounds can be used as effective catalysts in various processes. These metals their compounds provide catalytic effect simply because they can attain variable oxidation states. This fact is made me clear in the following points.

5 i. Transition elements have unpaired d-electrons and can have variable oxidation states. This property of showing multiple oxidation state helps them to absb an electron lose an electron to attain low and high oxidation states, respectively. Thus, transition metals are capable of absbing and re-emitting wide range of energies ( electrons) and providing suitable activation energy f a particular reaction. ii. In some cases, transition elements act as catalysts by fming 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. Accding to adsption they of catalysis, the transition metal provides a large surface area with free valencies where the reactants are adsbed 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 ions Most 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 absption of light of a particular wavelength in visible region. Electrons absb energy and get promoted to higher energy level. This transition of electrons is responsible f coloured ions. The transition elements have incomplete d-bitals. Though these d-bitals are degenerate, i.e. have the same energy, f an isolated ion, but when in compounds these bitals are affected by the combining atoms molecules and lose their degeneracy. This loss of degeneracy makes transition of electron from one d-bital to another d-bital which is otherwise fbidden. Such an effect on the central metal by incoming ligands is called crystal field effect. The difference in energy f these d-bitals is very less and the radiation of light cresponding 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-bitals and hence has no electron f promotion to higher level. Cu (I) and Zn (II) are also colourless since they have completely filled d-bitals leaving no empty bital f promotion of electrons. Magnetic properties Transition metal ions and their compounds show magnetic behaviour due to the presence of unpaired electrons in (n 1) d-bitals. A spinning electron has spin as well as bital motion. Thus, a spinning electron revolving around the nucleus creates a magnetic field amount itself. In other wds, 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 me 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

6 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 bital 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 fmation Transition metal ions fm a variety of complexes unlike, s and p-block elements. Complexes are the binary compounds fmed by the donation of lone pairs by a negative ion neutral molecule (called bonds) to the central metal ion. Transition elements fm complexes easily because: They have small size and thus can accommodate a number of ligands around them. They have large effective nuclear charge due to po shielding effect of d-bitals. They have vacant d-bitals of suitable energy so as to accept lone pairs of electrons from ligands. Interstitial compounds Interstitial compounds are those compounds in which small atoms (like hydrogen, nitrogen, carbon, etc.) occupy empty spaces ( interstitial sites) within the lattice framewk of the compound. Transition metals fm a large number of interstitial compounds. These compounds are hard and rigid. F 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 fmation Transition 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 me hard than the constituent metals and have high melting points. Alloys are me resistant to crosion. The metals chromium, vanadium, molybdenum, tungsten and manganese are used in the fmation of alloy steel and stainless steel.

7 Topic Name : Potassium Dichromate Preparation of potassium dichromate Potassium dichromate is prepared from chromates which in turn are prepared from chrome iron, FeCr 2 O 4. The various steps involved are as follows: i. Preparation of sodium chromate: The e, FeCr 2 O 4 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: 4FeO.Cr 2 O 3 + O 2 2Fe 2 O 3 + 4Cr 2 O 3 4Na 2 CO 3 + 2Cr 2 O 3 + 3O 2 4Na 2 CrO 4 + 4CO 2 ] 2 4FeO.CrO 3 + 8Na 2 CO 3 + 7O 2 8Na 2 CrO 4 + 2Fe 2 O 3 + 8CO 2 The roasted mass is then extracted with water when sodium chromate is completely dissolved while Fe 2 O 3 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: 2Na 2 CrO 4 + H 2 SO 4 Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O Sodium sulphate being less soluble crystallizes and is then filtered off. iii. Conversion of sodium dichromate into potassium dichromate: Sodium dichromate is me soluble and less stable than potassium dichromate and it is converted to potassium dichromate easily upon treatment with potassium chlide. Na 2 Cr 2 O 7 + 2KCl K 2 Cr 2 O 7 + 2NaCl Potassium dichromate being much less soluble than sodium salt, crystallizes out on cooling. Properties of potassium dichromate Potassium dichromate is an ange red crystalline solid which melts at 669 K. It is moderately soluble in cold water but freely soluble in hot water. Some imptant properties are as follows: i. Action of heat: When heated potassium dichromate decomposes with evolution of oxygen. ii. Action of alkalis: An ange red solution of potassium dichromate turns yellow upon treatment with alkali due to the fmation of chromate. K 2 Cr 2 O KOH 2K 2 CrO 4 + H 2 O The reaction is reversed on acidification, i.e. K 2 CrO 4 + H 2 SO 4 K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O Thus, the inter conversion of chromate and dichromate is an equilibrium process which is ph dependent. 2 2CrO 4 + 2H + 2HCrO 4 2 Cr 2 O 7 + H 2 O Chromate (Yellow) Hydrogen Chromate Dichromate (Orange) In acidic conditions (low ph), dichromate is me stable while under alkaline conditions (high ph) chromate is me stable. Structure of chromate and dichromate ions are shown below:

8 Chromate ion Dichromate ion iii. Action of concentrated sulphuric acid: Red crystals of chromic anhydride (chromium trioxide) are fmed when potassium dichromate is treated with cold concentrated sulphuric acid. K 2 Cr 2 O 7 + 2H 2 SO 4 2CrO 3 + 2KHSO 4 + H 2 O Red On the other hand, heating of potassium dichromate with concentrated sulphuric acid results in evolution of oxygen. 2K 2 Cr 2 O 7 + 8H 2 SO 4 2K 2 SO 4 + 2Cr 2 (SO 4 ) 3 + 8H 2 O + 3O 2 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. K 2 Cr 2 O 7 + 4H 2 SO 4 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3O Cr 2 O H + + 6e 2Cr H 2 O Where chromium in + 6 oxidation state in Cr 2 O 2 7 ion is being reduced to Cr 3+ (+ 3 oxidation state.) Let us examine the action of acidified potassium dichromate solution as an oxidizing agent by taking few examples: 1. It liberates I 2 from KI. K 2 Cr 2 O 7 + 4H 2 SO 4 6KI + 3H 2 SO 4 + 3O K 2 Cr 2 O 7 + 7H 2 SO 4 + 6KI Cr 2 O H + + 6I K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3O 3K 2 SO 4 + 3I 2 + 3H 2 O 4K 2 SO 4 + Cr 2 (SO 4 ) 3 + 3I 2 + 7H 2 O 2Cr I 2 + 7H 2 O 2. It oxidizes ferrous salts to ferric salts. K 2 Cr 2 O 7 + 4H 2 SO 4 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3O 2FeSO 4 + H 2 SO 4 + O Fe 2 (SO 4 ) 3 + H 2 O] 3 K 2 Cr 2 O 7 + 7H 2 SO 4 + 6FeSO 4 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 3Fe 2 (SO 4 ) 3 + 7H 2 O Cr 2 O H + + 6Fe 2+ 2Cr Fe H 2 O The above two reactions are used in the volumetric estimation of iodine and iron (II). 3. It oxidizes H 2 S to sulphur. K 2 Cr 2 O 7 + 4H 2 SO 4 H 2 S + O H 2 O + S] 3 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3O K 2 Cr 2 O 7 + 4H 2 SO 4 + 3H 2 S K 2 SO 4 + Cr 2 (SO 4 ) 3 + 3S + 7H 2 O

9 Cr 2 O H + + 3H 2 S 2Cr S + 7H 2 O 4. It oxidizes ethyl alcohol to acetaldehyde and acetic acid. K 2 Cr 2 O 7 + 4H 2 SO 4 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3O CH 2 CH 2 OH + O CH 3 CHO + H 2 O Ethyl alcohol Acetaldehyde CH 3 CHO + O CH 3 COOH Acetaldehyde Acetic acid 5. It oxidizes sulphites to sulphates. K 2 Cr 2 O 7 + 4H 2 SO 4 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3O Na 2 SO 3 + O Na 2 SO 4 ] 3 K 2 Cr 2 O 7 + 4H 2 SO 4 + 3Na 2 SO 3 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3Na 2 SO 4 Cr 2 O H + + 2SO 3 2 2Cr SO H 2 O 6. It oxidizes SO 2 to sulphuric acid. K 2 Cr 2 O 7 + 4H 2 SO 4 K 2 SO 4 + Cr 2 (SO 4 ) 3 + 4H 2 O + 3O SO 2 + O + H 2 O H 2 SO 4 ] 3 K 2 Cr 2 O 7 + H 2 SO 4 + 3SO 2 K 2 SO 4 + Cr 2 (SO 4 ) 3 + H 2 O Cr 2 O H + + 3SO 2 Cr 2 (SO 4 ) 3 + H 2 O 7. It oxidizes halogen acids to halogen. K 2 Cr 2 O HCl 2KCl + 2CrCl 3 + 7H 2 O + 3Cl 2 8. Chromyl chlide test: When potassium dichromate is treated with a strong (concentrated) sulphuric acid and a chlide, reddish brown vapours of chromyl chlide are fmed. K 2 Cr 2 O 7 + 2H 2 SO 4 KCl + 4HCl KHSO 4 + HCl] 4 2CrO 3 + 4HCl 2KHSO 4 + 2CrO 3 + H 2 O 2CrO 2 Cl 2 + 2H 2 O K 2 Cr 2 O 7 +4KCl +6H 2 SO 4 2CrO 2 Cl 2 + 6KHSO 4 + 3H 2 O Chromyl chlide (Red) This test is used in detection of chlide ions in qualitative analysis.

10 Topic Name : Potassium Permanganate Preparation of potassium permanganate Potassium permanganate is prepared on a large scale from mineral pyrolusite (MnO 2 ). The preparation involves following steps. i. Conversion of MnO 2 to potassium magnate: Pyrolusite is fused with potassium hydroxide and the molten liquid is stirred well in the presence of air. 2MnO 2 + 4KOH + O 2 2K 2 MnO 4 + 2H 2 O Oxidizing agents like potassium nitrate can also be used instead of air: MnO 2 + 2KOH + KNO 3 K 2 MnO 4 + KNO 2 + H 2 O ii. Oxidation of potassium manganate to potassium permanganate: Potassium manganate can be chemically oxidized to permanganate by bubbling CO 2, Cl 2 etc., through the solution of potassium manganate. 3K 2 MnO 4 + 2CO 2 2KMnO 4 + MnO 2 + 2K 2 CO 3 2K 2 MnO 4 + Cl 2 2KMnO 4 + 2KCl These 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: 2K 2 MnO 4 + H 2 O + O 2KMnO 4 + 2KOH MnO 4 2 Green At cathode: 2H + + 2e H 2 MnO 4 Purple + e Examples of oxidation by potassium permanganate in neutral solution are discussed below: i. It oxidizes hot manganese sulphate to manganese dioxide. 2KMnO 4 + H 2 O 2KOH + 2MnO 2 + 3O 3MnSO 4 + 3H 2 O + 3O 3MnO 2 + 3H 2 SO 4 2KOH + H 2 SO 4 K 2 SO 4 + 2H 2 O ii. iii. 3MnSO 4 + 2KMnO 4 + 2H 2 O 5MnO 2 + K 2 SO 4 + 2H 2 SO 4 It oxidizes sodium thiosulphate to sodium sulphate. 3Na 2 S 2 O 3 + 8KMnO 4 + H 2 O 3Na 2 SO 4 + 8MnO 2 + 3K 2 SO KOH It oxidizes hydrogen sulphide to sulphur. 2 KMnO 4 + 4H 2 S 2 MnS + S + K 2 SO 4 + 4H 2 O Properties of potassium permanganate Potassium 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.

11 2KMnO 4 K 2 MnO 4 + MnO 2 + O 2 ii. Action of concentrated sulphuric acid: When treated with cold concentrated sulphuric acid potassium permanganate is converted to Mn 2 O 7 (green oil) which decomposes on warming to MnO 2 (it is highly explosive). 2KMnO 4 + 2H 2 SO 4 Mn 2 O 7 + 2KHSO 4 + H 2 O 2Mn 2 O 7 4MnO 2 + 3O 2 iii. Oxidizing properties: Potassium permanganate is a strong oxidizing agent and the reaction is ph dependent. In alkaline solution: In strongly alkaline solution, MnO 4 2 ion is produced as shown in the reaction. 2KMnO 4 + 2KOH 2K 2 MnO 4 + H 2 O + O MnO 4 + e MnO 4 2 The MnO 4 2 ion gets further reduced to MnO 2, K 2 MnO 4 + H 2 O MnO 2 + 2KOH + O MnO H 2 O + 2e MnO 2 + 4OH Thus, the complete reaction is: 2KMnO 4 + H 2 O 2MnO 2 + KOH + 3O MnO 4 + 2H 2 O + 3e MnO 2 + 4OH A few examples of oxidation by KMnO 4 in alkaline medium are: a. potassium iodide is oxidized to potassium iodate. 2KMnO 4 + H 2 O + KI 2MnO 2 + 2KOH + KIO 3 I + 6OH IO 3 + 3H 2 O + 6e In acidic medium: In the presence of dilute sulphuric acid, the following reaction takes place, 2KMnO 4 + 3H 2 SO 4 K 2 SO 4 + 2MnSO 4 + 3H 2 O + 5O MnO 4 + 8H + + 5e Mn H 2 O Potassium permanganate acts as a very strong oxidizing agent in acidic media. Few examples of oxidation by acidic potassium permanganate solution are: oxidation of H 2 S to S.

12 2KMnO 4 + 3H 2 SO 4 H 2 S + O H 2 O + S] 5 K 2 SO 4 + 2MnSO 4 + 3H 2 O + 5O 2KMnO 4 + 3H 2 SO 4 + 5H 2 S K 2 SO 4 + 2MnSO 4 + 8H 2 O + 5S 2MnO H + + 5S 2 2Mn H 2 O + 5S oxidation of ferrous sulphate to ferric sulphate. 2KMnO 4 + 3H 2 SO 4 + O K 2 SO 4 + 2MnSO 4 + 3H 2 O + 5O 2FeSO 4 + H 2 SO 4 + O Fe 2 (SO 4 ) 3 + H 2 O] 5 2KMnO 4 + 8H 2 SO FeSO 4 K 2 SO 4 + 2MnSO 4 + 5Fe 2 (SO 4 ) H 2 O 2MnO H Fe 2+ 2Mn H 2 O + 10Fe 3+ Oxidation of potassium iodide to iodine: 2KMnO 4 + 3H 2 SO 4 2KI + H 2 SO 4 K 2 SO 4 + 2HI ] 5 2HI + O H 2 O + I 2 ] 5 K 2 SO 4 + 2MnSO 4 + 3H 2 O + 5O 2KMnO 4 + 3H 2 SO KI K 2 SO 4 + 2MnSO 4 + 8H 2 O + 5I 2 2MnO H I 2Mn H 2 O + 5I 2 In neutral medium: In neutral medium, potassium permanganate is weakly oxidizing and the reaction involved is: 2KMnO 4 + H 2 O 2KOH + 2MnO 2 + 3O MnO 4 + 2H 2 O + 3e MnO 2 + 4OH The alkali (KOH) produced renders the solution basic as the reaction proceeds and the reaction given above is then essentially same as that f alkaline medium. Structure of permanganate ion The four oxygen atoms are arranged tetrahedrally around manganese in MnO 4 as manganese is sp 3 hybridized. Structure of permanganate ion

13 Topic Name : Inner Transition elements (Lanthanides and Actinides) The elements in which the last electron enters (n 2) f-bitals are called f-block elements. These elements are also known as inner transition elements. This is because they fm a series between the transition metal series. The filling of 4f and 5f-bitals takes place in f-block elements. This is the basis of classification of f-block elements as lanthanides actinides. Lanthanides are the elements in which the last electron enters the 4f-bital. Lanthanides constitutes the first inner transition series. These elements are called lanthanides lanthanons since the series starts from the element lanthanum. The elements in which the last electron enters the 5f-bital constitute the second inner transition series. They are also called actinides 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 fm alloys with other metals especially with iron. These alloys are used extensively since the presence of rare earth elements is found to improve the wkability of steel at high temperatures. Two imptant 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. Pyrophic alloys: Pyrophic 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, Sm 2+, Eu 2+ and Yb 2+ ions in solution act as good reducing agents since they tend to oxidize to me stable + 3 oxidation state. Similarly, Ce 4+ acts as a good oxidizing agent in solution due to its ability to get reduced to Ce 3+ 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-bital. Now as the atomic number increases along the lanthanide series, the ionic radii decreases as is nmally 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-bitals 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.

14 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 po 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 labaty. Their chemistry is studied using tracer techniques. Comparison of lanthanides and actinides Similarities: Lanthanides and actinides involve filling of f-bitals and thus are similar in many respects. i. The most common oxidation state is +3 f 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 fmation as compared to lanthanides. iv. Lanthanide compounds are less basic while actinide compounds have appreciable basicity. v. Actinides fm few imptant oxocations such as UO 2+ 2, PuO 2+ 2, etc, while such oxocations are not known f 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 imptant 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. Pyrophic alloys find their uses in the preparation of ignition devices such as tracer bullet shells and flints f lighters. iii. Lanthanide oxides can absb ultraviolet rays. Some such oxides are used as additives in glasses f special purposes like: o Sunglasses (by adding Nd 2 O 3 ) o Goggles f glass blowing and welding wk (by adding Nd 2 O 3 and Pr 2 O 3 ) o Glasses protecting eyes from neutron radiation (by adding Ce 2 O 3 + Sm 2 O 3 ) iv. Lanthanide oxides are also used as abrasives f polishing glasses. v. Lanthanide compounds are used in manufacture of dyes and paints. F example, cerium molybdate is used in yellow dyes and salts of Nd are used f red colour. vi. Certain compounds of lanthanides are used as catalyst f reactions like hydrogenation, dehydrogenation and f reactions like hydrogenation, dehydrogenation and oxidation of ganic 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 reacts. ix. Aqueous solution of Ce +4 is used as an oxidizing agent while those of Sm 2+, Eu 2+ and Yb 2+ are used as reducing agents. x. Many radioactive actinides are used in radiotracer techniques f detection of cancer and many metabolic pathways. Thium salts are also used in treatment of cancer.