Q. 24

(a) Account for <

Answer :

(a)


(i)


• A reducing agent is defined as a substance that reduces other compounds but itself gets oxidised.


• Upon oxidation Eu can easily lose 2 electrons to attain the stable half-filled 4f7 configuration. Thus, Eu2+ is a strong reducing agent.


(ii)


• As we know, transition metals have either partially filled or half-filled d orbitals. According to crystal field theory, upon excitation the d orbitals get split into two energy levels; one higher orbital and another lower orbital.


• Thus, the electrons can make a ‘jump’ from one orbital to another making the transition elements coloured.


(iii)


• Zinc possesses the stable fully filled configuration of 3d104s2 which makes it highly stable and relatively unreactive.


• Because of the absence of unpaired electrons, the intermolecular interactions are weak and thus the enthalpy of atomisation is less.


(b)


(i) KMnO4 K2MnO4 + MnO2 + O


• Potassium permanganate upon heating decomposes to give potassium manganate, manganese dioxide and oxygen gas.


ii) Cr2O72- + 14H+ + 6Fe2+ 2Cr3+ + 6Fe3+ + 7H2O


OR


(a)


(i)


• Many interstitial compounds are formed by transition metals. Transition metals react with elements such as hydrogen, carbon, nitrogen and boron where small atoms of H, C or N get trapped inside the crystal lattice of the metals.


• As these vacant spaces are filled up these atoms, this makes the compounds hard and rigid.


(ii)


• Cr2+ is a strong reducing agent as it gets oxidised to Cr3+ having a d3 configuration.


• This configuration can also be written as t23g configuration which is highly stable.


• Similarly, Mn3+ acts as a strong oxidising agent as it gets reduced to Mn2+ having a d5 configuration which being half-filled is highly stable.


• Thus, Mn3+ is oxidising whereas Cr2+ is reducing though both have d4 configuration.


(iii)


• The transition metals have 3d electrons available for metallic bonding and the density of these elements are also relatively high.


• Thus, the melting and boiling points of transition elements are high since 3d electrons are available for metallic bonding.


(b)


• Misch metal is an alloy of rare earth metals such as cerium, lanthanum etc. Its typical composition is 55% cerium, 25% lanthanum and the rest are made up of other earth metals.


• An alloy of misch metal with iron is used a spark producing flint in cigarette lighters and similar devices.


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