NCERT Class XII Chemistry
Chapter - The d- and f-Block Elements
Questions with Solutions
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Question : 11
Total: 49
11.Explain giving reasons :
(i) Transition metals and many of their compounds show paramagnetic behaviour.
(ii) The enthalpies of atomisation of the transition metals are high.
(iii) The transition metals generally form coloured compounds.
(iv) Transition metals and their many compounds act as good catalysts.
(i) Transition metals and many of their compounds show paramagnetic behaviour.
(ii) The enthalpies of atomisation of the transition metals are high.
(iii) The transition metals generally form coloured compounds.
(iv) Transition metals and their many compounds act as good catalysts.
Solution:
(i) Paramagnetism arises from the presence of unpaired electrons,each such electron has magnetic moment associated with its spin angularmomentum and orbital angular momentum. For the compounds of thefirst series of transition metals, the contribution of the orbital angularmomentum is effectively quenched and hence is of no significance. Forthese, the magnetic moment is determined by the number of unpairedelectrons and is calculated by using the ‘spin-only’ formula,i . e . , µ = √ n ( n + 2 ) B . M
Where n is the number of unpaired electrons and μ is the magneticmoment in units of Bohr magneton (BM). A single unpaired electron hasa magnetic moment of 1.73 Bohr magneton (BM).
(ii) Because of large number of unpaired electrons in their atoms theyhave stronger interatomic interactions and hence stronger metallicbonding between atoms resulting in higher enthalpies of atomisation.
(iii) Due to presence of unpaired electrons and d-d transitions, thetransition metals are generally coloured. When an electron from a lowerenergy d orbital is excited to a higher energy d-orbital, the energy ofexcitation corresponds to the frequency of light absorbed. This frequencygenerally lies in the visible region. The colour observed corresponds tothe complementary colour of the light absorbed. The frequency of thelight absorbed is determined by the nature of the ligand.
(iv) The transition metals and their compounds are known for theircatalytic activity. This activity is ascribed to their ability to adopt multipleoxidation states and to form complexes. Vanadium(V) oxide (in ContactProcess), finely divided iron (in Haber’s Process), and nickel (in CatalyticHydrogenation) are some of the examples. Catalysts at a solid surfaceinvolve the formation of bonds between reactant molecules and atoms ofthe surface of the catalyst.
Where n is the number of unpaired electrons and μ is the magneticmoment in units of Bohr magneton (BM). A single unpaired electron hasa magnetic moment of 1.73 Bohr magneton (BM).
(ii) Because of large number of unpaired electrons in their atoms theyhave stronger interatomic interactions and hence stronger metallicbonding between atoms resulting in higher enthalpies of atomisation.
(iii) Due to presence of unpaired electrons and d-d transitions, thetransition metals are generally coloured. When an electron from a lowerenergy d orbital is excited to a higher energy d-orbital, the energy ofexcitation corresponds to the frequency of light absorbed. This frequencygenerally lies in the visible region. The colour observed corresponds tothe complementary colour of the light absorbed. The frequency of thelight absorbed is determined by the nature of the ligand.
(iv) The transition metals and their compounds are known for theircatalytic activity. This activity is ascribed to their ability to adopt multipleoxidation states and to form complexes. Vanadium(V) oxide (in ContactProcess), finely divided iron (in Haber’s Process), and nickel (in CatalyticHydrogenation) are some of the examples. Catalysts at a solid surfaceinvolve the formation of bonds between reactant molecules and atoms ofthe surface of the catalyst.
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