1.Define Transition Elements.

Ans:
A transition element is defined as the one which has incompletely filled d orbitals in its ground state or in any one of its oxidation states.

2.Why is Zinc, Cadmium and Mercury of group 12 not considered as transition metals?

Ans:
Zinc, cadmium and mercury of group 12 have full d 10 configuration in their ground state as well as in their common oxidation states and hence, are not regarded as transition metals.

3.Transition metals form a large number of complexes.

Ans:
This is due to the comparatively smaller sizes of the metal ions, their high ionic charges and the availability of d orbitals for bond formation.

4.Transition metals and many of their compounds show paramagnetic behaviour.

Ans:
Transition metals and their compounds show paramagnetic behaviour is because of the unpaired electrons in atoms , molecules or ions. Paramagnetic behaviour is measured based on magnetic moment which is determined by the number of unpaired electrons and is calculated by using formula

where n is the number of unpaired electrons and µ is the magnetic moment in units of Bohr magneton (BM).

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5.The enthalpies of atomisation of the transition metals are high.

Ans:
Because of large number of unpaired electrons in their atoms they have stronger interatomic interaction and hence stronger bonding between atoms resulting in higher enthalpies of atomisation.

6.The transition metals generally form coloured compounds.

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Transition elements have partially filled d orbitals. When an electron from a lower energy d orbital is excited to a higher energy d orbital,the energy of excitation corresponds to the frequency of light absorbed.This is the reason why transition elements form colored compounds.

7.Transition metals and their many compounds act as good catalyst.

Ans:
Because they easily lend and take electrons from other molecules.Also for their ability to adopt multiple oxidation states and to form complexes.

8.Which metal in the first series of transition metals exhibits +1 oxidation state most frequently and why?

Ans:
Cu.This is because the electronic configuration of Cu is 3d ¹⁰4s ¹ and after losing one electron it acquires a stable 3d ¹⁰ configuration.

9.Give examples and suggest reasons for the following features of the transition metal chemistry:
(a) The lowest oxide of transition metal is basic, the highest is amphoteric/acidic.
(b) A transition metal exhibits highest oxidation state in oxides and fluorides.
(c) The highest oxidation state is exhibited in oxoanions of a metal.

Ans:
(a) Lowest oxide of transition metal is basic because of some of the valence electrons of the metal are not involved in bonding , due to that they can donate electrons and behave as a base.
The highest oxide of transition metal is amphoteric/acidic because the valence electrons are involved in bonding and are not available for donation of electrons.
(b) Both oxides and fluorides are highly electronegative and are very small in size, due to which they are able to exhibit highest oxidation state.
(c)Oxygen is a strong oxidising agent due to its high electronegativity and smaller size and because of that oxoanions of a metal have the highest oxidation state.

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10.What are alloys? Name an important alloy which contains some of the lanthanoid metals. Mention its uses.

Ans:
An alloy is a blend of metals prepared by mixing the components.An important alloy containing lanthanoid metals is misch metal which contains 95% lanthanoid metals (Ce, La and Nd) and 5% iron along with traces of S, C, Ca and Al. It is used in making parts of jet engines.

11.What is meant by "disproportionation" ? Give two examples of disproportionation reaction in aqueous solution.

Ans:
When a particular oxidation state becomes less stable relative to other oxidation states, one lower, one higher, it is said to undergo disproportionation. For example, manganese (VI) becomes unstable relative to manganese(VII) and manganese (IV) in acidic solution

12.Account for the following:
(i) Mn ²⁺ state is more stable than Fe2+ towards oxidation to +3 state.
(ii) In 3d series, the enthalpy of atomization is lowest for Zinc.
(iii) Actinoids show wide range of oxidation states as compare to Lanthanoids.

Ans:
(i) Because of stable half filled 3d ⁵ configuration Fe ²⁺ easily oxidizes to Fe ³⁺ to achieve stable 3d ⁵ configuration.
(ii) Because of no unpaired electrons in 3d orbital of Zn which causes weak metallic bonding.
(iii) Because of comparable energies of 5f , 6d and 7s orbitals.

13.Assign reasons for the following:
(i) Actinoids show irregularities in their electronic configuration.
(ii) In 3d series, E0 (Cu2+/Cu) has positive value.
(iii) Chromium metal is hard whereas Zinc metal is soft.

(i) Because of varying stability of 5f0, 5f7 and 5f14 configurations.
(ii) Because of its low enthalpy of hydration and high enthalpy of atomization.
(iii) Because of the presence of unpaired electrons in 3d orbitals of Cr, strong metallic bonding makes it hard whereas no unpaired electrons in 3d orbitals of Zn makes it soft.

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14.Name a transition element which does not exhibit variable oxidation states.

Ans:
Scandium (Z = 21) does not exhibit variable oxidation states.

15.Why is Cr ²⁺ reducing and Mn ³⁺ oxidising when both have d ⁴ configuration?

Ans:
Cr ²⁺ is reducing as its configuration changes from d ⁴ to d ³ , the latter having a half-filled t _2g level . On the other hand, the change from Mn ³⁺ to Mn ²⁺ results in the half-filled (d ⁵) configuration which has extra stability.

16.Why are Zn, Cd and Hg are not considered as transition elements?

Ans:
The orbitals in these elements are completely filled in the ground state as well as in their common oxidation states. Therefore, they are not regarded as transition elements.

17.On what ground can you say that scandium (Z = 21) is a transition element but zinc (Z = 30) is not?

Ans:
On the basis of incompletely filled 3d orbitals in case of scandium atom in its ground state (3d 1 ), it is regarded as a transition element. On the other hand, zinc atom has completely filled d orbitals (3d ¹⁰) in its ground state as well as in its oxidised state, hence it is not regarded as a transition element.

18.Describe the preparation of potassium dichromate from iron chromite ore.

Ans:
Potassium dichromate are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr2O4) with sodium or potassium carbonate in free access of air. The reaction with sodium carbonate occurs as follows:

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19.What is the effect of increasing pH on a solution of potassium dichromate?

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If pH of potassium dichromate is increased it is converted to yellow potassium chromate:

20.What are the use cases of potassium dichromate?

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Potassium dichromate is a very important chemical used in leather industry and as an oxidant for preparation of many azo compounds.
Sodium and potassium dichromates are strong oxidising agents; the sodium salt has a greater solubility in water and is extensively used as an oxidising agent in organic chemistry.Potassium dichromate is used as a primary standard in volumetric analysis.

21.What happens when potassium permanganate is heated to 513K?

Ans:
Potassium permanganate decomposes at 513K to potassium manganate, manganese dioxide and oxygen.

22.What is the effect of increasing pH on a solution of potassium dichromate?

Ans :
On increasing the pH of the solution Potassium dichromate (orange) becomes potassium chromate (yellow)

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23.Describe the preparation of potassium permanganate.

Ans:
Potassium permanganate is prepared by fusion of MnO2 with an alkali metal hydroxide and an oxidising agent like KNO3. This produces the dark green K2MnO4 which disproportionates in a neutral or acidic solution to give permanganate.

Commercially it is prepared by the alkaline oxidative fusion of MnO2 followed by the electrolytic oxidation of manganate (Vl).

24.Explain why Cu+ ion is not stable in aqueous solutions?

Ans:
Cu+ in aqueous solution undergoes disproportionation, i.e.,

The E ⁰ value for this is favourable.

25.Silver atom has completely filled d orbitals (4d ¹⁰) in its ground state. How can you say that it is a transition element?

Ans:
Silver (Z = 47) can exhibit +2 oxidation state wherein it will have incompletely filled d-orbitals (4d), hence a transition element.

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26.In the series Sc (Z = 21) to Zn (Z = 30), the enthalpy of atomisation of zinc is the lowest, i.e., 126 kJ mol ^–1. Why?

Ans:
In the formation of metallic bonds, no electrons from 3d-orbitals are involved in case of zinc, while in all other metals of the 3d series, electrons from the d-orbitals are always involved in the formation of metallic bonds.

27.How would you account for the irregular variation of ionisation enthalpies (first and second) in the first series of the transition elements?

Ans:
Irregular variation of ionisation enthalpies is mainly attributed to varying degree of stability of different 3d-configurations (e.g., d ⁰, d ⁵, d ¹⁰ are exceptionally stable).

28.Why is the highest oxidation state of a metal exhibited in its oxide or fluoride only?

Ans:
Because of small size and high electronegativity oxygen or fluorine can oxidise the metal to its highest oxidation state.

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29.Which is a stronger reducing agent Cr ²⁺ or Fe ²⁺ and why ?

Ans:
Cr ²⁺ is stronger reducing agent than Fe ²⁺. Reason: d ⁴ → d ³ occurs in case of Cr ²⁺ to Cr ³⁺ But d ⁶ → d ⁵ occurs in case of Fe ²⁺ to Fe ³⁺ In a medium (like water) d ³ is more stable as compared to d ⁵.

30.Zirconium and Hafnium are difficult to separate.Why?

They are difficult to separate because of their identical atomic and ionic radii which is a consequence of lanthanoid contraction.

31. Explain the following :
(a) A lanthonoid reacts with dilute acids ?
(b) A lanthonoid reacts with water?

Ans :
(a)When lanthonoid reacts with dilute acids , it liberates hydrogen gas.
(b)When lanthonoid reacts with water , it forms lanthanoid hydroxide and liberate hydrogen gas

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32.Account for the following:
(i) Mn shows the highest oxidation state of +7 with oxygen but with fluorine it shows the highest oxidation state of +4.
(ii) Cr2+ is a strong reducing agent.

(i) Because of the ability of oxygen to form multiple bond with Mn and single bond with fluorine.
(ii) Cr ²⁺ is a strong reducing agent as its configuration changes from d ⁴ to d ³ , the latter having a half-filled t ^2g level.

33.The element of 3d transition series are given as:
Sc Ti V Cr Mn Fe Co Ni Cu Zn
Answer the following:
(i) Write the element which shows maximum number of oxidation states.Give reason.
(ii) Which element has the highest m.p?
(iii) Which element shows only +3 oxidation state?
(iv) Which element is a strong oxidizing agent in +3 oxidation state and why?

Ans :
(i) Mn.It is because of maximum unpaired electrons.
(ii) Cr
(iii) Sc
(iv)Mn. Because Mn3+ and Mn2+ results in stable half filled d5 configuration.

34.How would you account for the following:
(a) Of the d ⁴ species, Cr ²⁺ is strongly reducing while manganese(III) is strongly oxidising.
(b) Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised
(c) The d ¹ configuration is very unstable in ions.

Ans:
(a) Cr ²⁺ is reducing as its configuration changes from d ⁴ to d ³, the latter having a half-filled t _2g level . On the other hand, the change from Mn ³⁺ to Mn ²⁺ results in the half-filled (d ⁵) configuration which has extra stability.
(b) Co ²⁺ is stable in aqueous solution because it get surrounded and weakly bonded to water molecules. In presence of strong ligands and air it gets oxidised to Co(III) as strong ligands get coordinated more strongly with Co(III). (c)Some species with d ¹ configuration are reducing and tends to loose one electron to acquire d ⁴ stable configuration .Others with d1 configuration like Cr(V) and Mn(VI) undergo disproportionation.

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35.Name the oxometal anions of the first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.

Ans:
(i) Vanadate , VO ^- ₃ Oxidation state of V is +5
(ii) Chromate , CrO ^2- ₄ Oxidation state of Cr is +6
(iii) Permanganate, MnO ^- ₄ Oxidation state of Mn is +7.

36.Name two characteristic properties exhibited by d – block elements due to their partly filled d orbitals.

Ans :
The characteristic properties exhibited by d – block elements due to their partly filled d orbitals are variable; (i) Oxidation states and (ii) Formation of coloured ions.

37.Give two characteristics of transition metal alloys.

Ans:
The alloys are hard and have high melting points.

38.What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

Ans:
Steady decrease in the size of lanthanides with increase in atomic number is known as lanthanoid contraction.
Due to lanthanoid contraction radii of members of 3rd transition series are very much similar to corresponding members of 2nd series.

39.Name the metal of the 1st row transition series that
i) has highest value for magnetic moment
ii) has zero spin only magnetic moment in its +2 oxidation state.
iii) exhibit maximum number of oxidation states.

Ans :
i) Chromium ii) Zinc iii) Manganese

40. What is Actinoid contraction?

Ans:
There is a gradual decrease in the size of atoms or M3+ ions across the series.This is known as the actinoid contraction.

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41. Actinoid contraction is more than lanthanoid contraction.Give reason.

Ans:
Because of poor shielding by 5f electrons from nuclear charge.