(a) Account for the following:

(i) Eu²⁺ is a strong reducing agent

(ii) Transition metals are coloured compounds

(iii) Zn has lowest enthalpy of atomization is the 3d series

(b) Complete the following equations:

(i) KMnO₄

(ii) Cr₂O₇^2- + 14H⁺ + 6Fe²⁺

OR

(a) Account for the following

(i) Transition elements form interstitial compounds.

(ii) Mn³⁺ (3d⁴) is strongly oxidising whereas Cr²⁺ (3d⁴) is strongly reducing.

(iii) Transition metals have high melting point.

(b) What is ‘Misch metal’? Write one use.

(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) Cr₂O₇^2- + 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.