Assign reasons for the following: (i) The enthalpies of atomisation of transition elements are high.

Question

Assign reasons for the following: (i) The enthalpies of atomisation of transition elements are high.

Assign reasons for the following:
(i) The enthalpies of atomisation of transition elements are high.
(ii) The transition metals and many of their compounds act as good catalysts.
(iii) From element to element the actinoid contraction is greater than the lanthanoid contraction.
(iv) The E° value for the Mn³⁺ / Mn²⁺ couple is much more positive than that of Cr³⁺ / Cr ^2+.
(v) Scandium (Z = 21) does not exhibit variable oxidation states and yet it is regarded as a transition element.

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Salim Ahmed Kawsar · Answer 1 · Feb 05, 2023 15:29:48

(i) This is because transition metals have strong metallic bonds as they have a large number of unpaired electrons.
(ii) The catalytic activity of transition metals is attributed to the following reasons:
(a) Because of their variable oxidation states transition metals form unstable intermediate compounds and provide a new path with lower activation energy for the reaction.
(b) In some cases, the transition metal provides a suitable large surface area with free valencies on which reactants are adsorbed.
(iii) This is due to poorer shielding by 5f electrons in actinoids than that by 4f electron in the lanthanoids.

(iv) This is because half filled d-subshell (3d⁵ ) in Mn ²⁺ is more stable.
(v) This is because scandium has partially filled d orbitals in the ground state (3d¹ 4s ² ).

Salim Ahmed Kawsar · Answer 2 · Feb 05, 2023 15:29:48

(i) The transition elements exhibit high enthalpies of atomisation because they have large number of unpaired electron in their atoms. Due to this they have stronger inter atomic interaction and hence stronger bonding between atoms.

(ii) Many transition metals and their compounds acts as catalyst the catalytic activity is due to their ability to exhibit multiple oxidation states, eg. V₂O₅ in contact process and finely divided iron in Haber process.

(iii) The actinoid contraction is similar to lanthanoid contraction. This contraction is due to poor shielding by 5f electrons in the actioids than that by 4f electrons in the lanthanoids.

(iv) Much larger third ionisation energy of Mn (where the required change is Cl₅ and d⁴) is mainly responsible for this. This also explains that +3 state of Mn is of little importance.

(v) Scandium (Z = 21) has incompletely filled 3d orbitals in the ground state. (3d) Hence, it is considered as a transition element.

Assign reasons for the following: (i) The enthalpies of atomisation of transition elements are high.

2 Answers