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cbse 12th Standard Chemistry d- and f- Block Elements Important Questions And Answers Study Material - QB365

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Questions:

Generic 2

16 x 2 = 32

What is misch metal? Mention its two important uses.

Compare the relative stability of +2 oxidation states in aqueous solutions for the metals having in their atoms the outer electron configurations, 3d³, 4s², 3d⁶ 4s² and 3d⁵ 4s².

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.

Explain the following observations:
(i) Generally there is an increase in density of elements from titanium (Z = 22) to copper (Z = 29) in the first series of transition elements.
(ii) Transition elements and their compounds are generally found to be good catalysts in chemical reactions.

Assign reasons for each of the following :
(i) Transition metals generally form coloured compounds.
(ii) Manganese exhibits the highest oxidation state of +7 among the 3d series of transition elements.

Explain giving reasons:
(i) transition metals and their compounds generally exhibit a paramagnetic behaviour.
(ii) The chemistry of actinoidsis not so smooth as that of lanthanoids.

Explain giving a suitable reason for each of the following:
(i) Transition metals and their compounds are generally found to be good catalysts.
(ii) Metal-metal bonding is more frequent for the 4d and the 5d series transition metals than that for the 3d series.

The \(E^{\ominus}\left(\mathrm{M}^{2+} / \mathrm{M}\right)\) value for copper is positive (+0.34 V). What is possibly the reason for this? (Hint: consider it's high \({ \Delta }_{ a }{ H }^{ o }\) and low \({ \Delta }_{ hyd }{ H }^{ o } \))

Which of the 3d series of the transition metals exhibits the largest number of oxidation states and why?

10.

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

11.

Write the highest oxidation state shown by element with atomic number 23.

12.

Out of Cr²⁺ and Cr^3+, which one is stable in aqueous solution?

13.

Write the outer electronic configuration of Cr atom (Z=24).

14.

There is hardly any increase in atomic size with increasing atomic numbers in a series of transition metals. Give reason.

15.

Among landthanoids, Ln(III) compounds are predominant. however, occasionally in solutions or in solid compounds, +2 and +4 ions are also obtained. Give reason.

16.

Lanthanoids form primarily +3 ions, while the actinoids usually have higher oxidation states in their compounds, +4 or even +6 being typical. Give reason.

Generic 3

7 x 3 = 21

17.

When an oxide of manganese (A) is fused with KOH in the presence of an oxidising agent and dissolved in water, it gives a Compound (B) disproportionates in neutral or acidic solution to give purple compound (C). An alkaline solution of compound (C) oxidises potassium iodide solution to a compound (D) and compound (A) is also formed. Identify compound A to D and also explain the reactions involved.

18.

Indicate the steps in the preparation of :
(a) K₂Cr₂O₇ from chromite ore.
(b) KMnO₄ from pyrolusite ore.

19.

(a) Comlete the following chemical equations for reactions in aqueous media:
(i) Cr₂O₇^2-+H⁺+Fe2+ \(\longrightarrow \)
(ii) MnO₄^-+I^-+H⁺\(\longrightarrow \)
(b) How many unpaired electrons are present in Mn²⁺ ion (At. no. of Mn =25)? How does it influence magnetic behaviour of Mn²⁺ ions?

20.

How would you account for the following?
(a) Of the d⁴ species, Cr²⁺ 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 d¹ configuration is very unstable in ions.

21.

What are the characteristics of the transition elements and why are they called transition elements ? Which of the d-block elements may not be regarded as the transition elements?

22.

Which is the last element in the series of the actinoids? Write the electronic configuration of this element. Comment on the possible oxidation state of this element.

23.

Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Generic 5

4 x 5 = 20

24.

(a) (i) Account for the following:
(1) Zn²⁺ salts are colourless while Ni²⁺ salts are coloured.
(2) Cr²⁺is a strong reducing agent.
(3) Transition metals and their compounds show catalytic activities.
(ii) Write the ionic equations for the oxidizing action of MnO, in acidic medium with
(1) I^-ion and
(2) Fe²⁺ion

25.

What is the cause of actinoid contraction? Describe some similarities and differences between lanthanoids and actinoids.

26.

(i) Distinguish the structure of chromate ion from that of dichromate ion.
(ii) Between the dichromates of sodium and potassium, which one is preferred for oxidising organic compound and why?
(iii) Give two equations that explains the nature of K₂Cr₂O₇ as reducing agent.

27.

(i) Name a compound of
(a) transition metal which is used in the manufacture of sulphuric acid.
(b) transition metal that is used in Haber's process for the manufacture of ammonia.
(c) transition metal that have light sensitive properties and act as valuable source in photographic industry.
(ii) Write the equations which are involved in the oxidation of hydrogen sulphide to sulphur by potassium permanganate solution.

Case Study Questions

3 x 4 = 12

28.

The unique behavior of Cu, having a positive Eo accounts for its inability to liberate H2 from acids. Only oxidizing acids (nitric and hot concentrated sulphuric acid) react with Cu, the acids being reduced. The stability of the half-filled (d⁵) subshell in Mn²⁺ and the completely filled (d¹⁰ ) configuration in Zn²⁺ are related to their E^o (M³⁺ /M²⁺) values. The low value for Sc reflects the stability of Sc³⁺ which has a noble gas configuration. The comparatively high value for Mn shows that Mn²⁺(d⁵) is particularly stable, whereas a comparatively low value for Fe shows the extra stability of Fe³⁺ (d⁵). The comparatively low value for V is related to the stability of V²⁺ (half-filled t_2g level).
1. Standard reduction electrode potential of Zn²⁺ /Zn is – 0.76 V. This means
(a) ZnO cannot be reduced to Zn by H₂ under standard conditions
(b) Zn cannot liberate H₂ with concentrated acids
(c) Zn is generally the anode in an electrochemical cell
(d) Zn is generally the cathode in an electrochemical cell.
2. E^ovalues for the couples Cr³⁺/Cr²⁺ and Mn³⁺ /Mn²⁺ are -0.41 and +1.51 volts respectively. These values suggest that
(a) ZnO cannot be reduced to Zn by H2 under standard conditions
(b) Cr²⁺ is more stable than Cr³⁺ state
(c) Mn³⁺ is more stable than Mn²⁺
(d) Cr²⁺ acts as an oxidizing agent whereas Mn³⁺ acts as a reducing agent
3.The reduction potential values of M, Nand O are +2.46, -1.13 and -3.13 Y respectively. Which of the following order is correct regarding their reducing property?
(a) O>N>M
(b) O>M>N
(c) M>N>O
(d) M>O>N
4. Which of the following statements are true?
(a) Mn²⁺ compounds are more stable than Fe²⁺ towards oxidation to +3 state.
(b) Titanium and copper both in the first series of transition metals exhibits +1 oxidation state most frequently.
(c) Cu⁺ ion is stable in aqueous solutions.
(d) The E⁰ value for the Mn³⁺ /Mn²⁺ couple is much more positive than that for Cr³⁺/Cr²⁺ or Fe³⁺/Fe²⁺.
(a) (b) and (c)
(b) (a) and (d)
(c) (a) and (c)
(d) (b) and (d)

29.

Transition metal oxides are compounds formed by the reaction of metals with oxygen at high temperatures. The highest oxidation number in the oxides coincides with the group number. In vanadium, there is a gradual change from the basic V2O3 to less basic V2O4, and to amphoteric V2O5⋅ V2O4 dissolves in acids to give VO2+ salts. Transition metal oxides are commonly utilized for their catalytic activity and semiconductive properties. Transition metal oxides are also frequently used as pigments in paints and plastic. Most notably titanium dioxide. One of the earliest applications of transition metal oxides to the chemical industry involved the use of vanadium oxide for the catalytic oxidation of sulfur dioxide to sulphuric acid. Since then, many other applications have emerged, which include benzene oxidation to maleic anhydride on vanadium oxides; cyclohexane oxidation to adipic acid on cobalt oxides. An important property of the catalyst material used in these processes is the ability of transition metals to change their oxidation state under a given chemical potential of reductants and oxidants.
1.Which oxide of vanadium is most likely to be basic and ionic?
(a) VO
(b) V₂O₃
(c) VO₂
(d) V₂O₅
2.Vanadyl ion is
(a) VO₂⁺
(b) VO₂⁺
(c) V₂O⁺
(d) VO₄^3–
3.Which of the following statements is false?
(a) With fluorine vanadium can form VF₅.
(b) With chlorine vanadium can form VCl₅.
(c) Vanadium exhibits the highest oxidation state in oxohalides VOCl₃, VOBr₃ and fluoride VF₅.
(d) With iodine vanadium cannot form VI⁵ due to oxidising power of V⁵+ and reducing nature of I–
4.The oxidation state of vanadium in V₂O₅ is
(a) +5/2
(b) +7
(c) +5
(d) +6
5.Identify the oxidizing agent in the following reaction.

(a) V₂O₅
(b) Ca
(c) V
(d) None of these

30.

The transition elements have incompletely filled d-subshells in their ground state or in any of their oxidation states. The transition elements occupy positions in-betweens- and p-blocks in groups 3-12 of the Periodic table. Starting from the fourth period, transition elements consist of four complete series: Sc to Zn, Y to Cd and La, Hf to Hg and Ac, Rf to Cn. In general, the electronic configuration of the outer orbitals of these elements is (n – 1)d1–10 ns0–2. The electronic configurations of outer orbitals of Zn, Cd, Hg, and Cn are represented by the general formula(n – 1)d10 ns2. All the transition elements have typical metallic properties such as high tensile strength, ductility, and malleability. Except for mercury, which is liquid at room temperature, other transition elements have typical metallic structures. The transition metals and their compounds also exhibit catalytic properties and paramagnetic behavior. Transition metal also forms alloys. An alloy is a blend of metals prepared by mixing the components. Alloys may be homogeneous solid solutions in which the atoms of one metal are distributed randomly among the atoms of the other.
1.Which of the following characteristics of transition metals is associated with higher catalytic activity?
(a) High enthalpy of atomization
(b) Variable oxidation states
(c) Paramagnetic behavior
(d) Colour of hydrated ions
2.Transition elements form alloys easily because they have
(a) same atomic number
(b) same electronic configuration
(c) nearly same atomic size
(d) same oxidation states.
3.The electronic configuration of tantalum (Ta) is
(a) [Xe]4f⁰5d¹6s²
(b) [Xe]4f ¹4 5d² 6s²
(c) [Xe]4f¹4 5d³6s²
(d) [Xe]4f¹4 5d⁴ 6s²
4.Which one of the following outer orbital configurations may exhibit the largest number of oxidation states?
(a) 3d⁵4s¹
(b) 3d⁵4s²
(c) 3d²4s²
(d) 3d³4s²

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Answers Key:

Generic 2

16 x 2 = 32

Misch metal consists of lanthanoids (95%), iron, traces of S, C, Ca and Al.
Uses:
(i) It is used to produce bullets.
(ii) It is used in shells and lighter flints.

Mn²⁺ (3d⁵) > V²⁺ (3d³) > Fe²⁺ (3d⁶)

3d⁵ 4s² > 3d³ 4s² > 3d⁶ 4s²

It is because reduction potential of Mn²⁺ is lowest and it has half filled d-orbitals, therefore it is more stable and reduction potential of Fe²⁺ is highest because it has t_2g orbitals which are half filled and more stable.

In MnO₄^-, oxidation state of Mn is +7 which is equal to its group number.
In CrO₄^2-, oxidation state of Cr is +6 which is equal to its group number.

(i) It is because atomic mass increases more than atomic volume, therefore density increases from titanium (Z = 22) to copper (Z = 29).
(ii) It is because they show variable oxidation states and have vacant d-orbitals forming unstable intermediates which readily change into products.

(i) It is because transition metals have unpaired electron in d-orbitals and undergo d-d transitions by absorbing light from visible region and rediate complementary colour.
(ii) Mn has electronic configuration (Ar)4s²3d⁵ and all the electrons in 's' as well as 'd' can take part in bond formation, therefore, it shows +7 highest oxidation state.

(i) It is because of presence of unpaired electrons in d-orbitals of transition metal.
(ii) It is because all of them radioactive and some of them have short half life, therefore, the chemistry of actinodes is not smooth.

(i) It is because these metals show variable oxidation states and form unstable intermediate which readily change into product.
(ii) It is due to less energy required for more number of electrons to take part in metal-metal bonding in 4d and 5d series.

It is because hydration energy and lattice energy of Cu²⁺ is more than that of Cu⁺.

Mn exhibits largest number of oxidation states because it has 7 electrons in 's' as well as 'd' orbitals which can take part in bond formation.

10.

It is because F₂ and O₂ are strong oxidising agents due to high electronegativity and lattice energy of fluoridesand oxide is high.

11.

Highest oxidation state shown by an element with atomic no.23 is +5.

12.

Highest oxidation state shown by an element with atomic no.23 is +5.

13.

Cr(24): [Ar] 3d⁵ 4s¹

14.

It is because effective nuclear charge does not increase appreciably as pairing of electrons in d-orbitals take place which cause repulsion after Mn(25).

15.

It is because effective nuclear charge does not increase appreciably as pairing of electrons in d-orbitals take place which cause repulsion after Mn(25).

16.

In Actinoids 5f, 6d and 7s orbitals have comparable energies and electrons from these orbitals can take part to show higher oxidation states.

Generic 3

7 x 3 = 21

17.

\({ MnO }_{ 2 }+4KOH+{ O }_{ 2 }\rightarrow 2{ K }_{ 2 }{ MnO }_{ 4 }+2{ H }_{ 2 }O\)
\(\\ { 3MnO }_{ 4 }^{ 2- }+4{ H }^{ + }\rightarrow { 3MnO }_{ 4 }^{ - }+{ MnO }_{ 2 }+2{ H }_{ 2 }O\)
\(\\ { 2MnO }_{ 4 }^{ - }+{ H }_{ 2 }O+KI\rightarrow 2{ MnO }_{ 2 }+2{ OH }^{ - }+{ KIO }_{ 3 }\)

18.

(a)\(4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \longrightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2\)
\(2Na_2CrO_4 + 2H+ \longrightarrow Na_2Cr_2O_7 + 2Na^+ + H_2O\)
\(Na_2Cr_2O_7 + 2KCI \longrightarrow K_2Cr_2O_7 + 2NaCI\)
(b)\(2MnO_2 + 4KOH + O_2 \xrightarrow{\Delta} 2K_2MnO_4 + 2H_2\)
\(\underset{Magnate\ ion}{MnO_4^{2-}}\xrightarrow{electrolysis}\underset{Permagnate\ ion}{MnO_4^-}+e^-\)

19.

(a)\((i) Cr_2O_7^{2-} + 14 H^+ + 6Fe^{2+}\longrightarrow\ 2Cr^{3+} + 6Fe^{3+} + 7H_2O\)
\((ii) 2 MnO_4^- + 10 I^- + 16 H^+\longrightarrow 2Mn^{2+} + 8H_2O + 5I_2\)
(b) Mn²⁺: 3d⁵4s⁰ has S unpaired electrons.It is highly paramagnetic and it is attracted by magnet.

20.

(a) E values for Cr³⁺/Cr²⁺is negative (- 0.41 V) and for Mn³⁺/Mn²⁺is positive (+1.57V).Thus, Cr²⁺ can undergo oxidation and, therefore, is reducing agent. On the other hand, Mn(III) can undergo reduction, and therefore, acts as an oxidizing agent.
(b) In the presence of complexing agents, cobalt gets oxidized from,+2 to +3 state because Co (III) is more stable than Co (II).
(c) After the loss of ns electrons, a d1 electron can easily be lost to give a stable configuration. Therefore, the elements having d¹ configuration are either reducing or undergo disproportionation.

21.

Characteristics:
(i) They show variable oxidation states.
(ii) They form coloured ions
They are called transition elements because they are less electropositive elements because they are less electropositive than p-block elements. Zn, Cd, Hg are not regarded as transition elements.

22.

Lawrencium (103) is the last elementin series of actinoids.
Its electronic configuration is [Rn] 5f¹⁴6d¹7s².
The possible oxidation state of Lr is +3.

23.

\(4FeCr_2O_4 + 8Na_2CO_3 + 7O_2\longrightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2\)
\(2Na_2CrO_4 + 2H^+\longrightarrow Na_2Cr_2O_7 + 2Na^+ + H_2O\)
\(Na_2Cr_2O_7 + 2KCI \longrightarrow K_2Cr_2O_7 + 2NaCI\)
\(\underset{dihromate\ ion\ medium}{\underset{(orange)}{Cr_2O_7^{2-} }+\underset{Basic} {2OH^-} }\longrightarrow \underset{chromate\ ion\\ yellow}{2CrO_4^{2-}} + H_2O\)

Generic 5

4 x 5 = 20

24.

(a) (i)
(1) Zn²⁺ with configuration [As] \({ }^{18} 3 d^{10}\) has all filled orbitals. Therefore its salts are colourless. Ni²⁺ with configuration \([A r]^{18} 3 d^8\) has some half filled orbitals. So form coloured salts.
(2) As after the loss of one electron Cr²⁺ becomes \(\mathrm{Cr}^3\) which has more stable half filled ( \(\mathrm{t}_{2 \mathrm{~g}}^3\) ) configuration.
(3) due to variable oxidation state.
(ii) (1) \(\mathrm{MnO}^{4-}+10 \mathrm{I}^{-}+16 \mathrm{H}^{+} \longrightarrow 2 \mathrm{Mn}^{2+}+5 \mathrm{I}_2+8 \mathrm{H}_2 \mathrm{O}\)
(2) \(\mathrm{MnO}_4^{-}+5 \mathrm{Fe}^{2+}+8 \mathrm{H}^{+} \longrightarrow \mathrm{Mn}^{2+}+5 \mathrm{Fe}^{3+}+4 \mathrm{H}_2 \mathrm{O}\)

25.

Actinoid Contraction. The decrease in atomic and ionic size along actinoid series, is called actinoid contraction.
Cause: It is due to poor shielding effect of 5f-electrons due to which effective nuclear charge increases.
Comparison Between Lanthanoids and Actinoids: Similarities:
(i) In bothf-orbitals are progressively filled.
(ii) Both form co loured ions.
(iii) Both are paramagnetic in nature.
(iv) Both show contraction, i.e. lanthanoid contraction and actinoid contraction.
Differences between Lanthanoids and Actinoids:

Lanthanoids	Actinoids
(i) 4f-orbital is progressively filled.	(i) 5f-orbital is progressively filled
(ii) +3 oxidation state is most common along with +2 and +4.	(ii) They show +2, +3,+4,+5,+6,+7 o xidation states
(iii) Only promethium (Pm) is radioactive	(iii) All are radioactive
(iv) They are less reactive than actinoids.	(iv) They are more reactive.
(v) The magnetic properties are less complex	(v) Magnetic properties are more complex.

26.

(ii) Potassium

27.

(i) (a) Vanadium
(b) Iron
(c) Silver
(ii) \(\begin{equation} 2 \mathrm{MnO}_{4}^{-}+5 \mathrm{H}_{2} \mathrm{~S}+16 \mathrm{H}^{+} \longrightarrow 2 \mathrm{Mn}^{2+}+5 \mathrm{~S}+8 \mathrm{H}_{2} \mathrm{O} \end{equation}\)

Case Study Questions

3 x 4 = 12

28.

1.(a) ZnO cannot be reduced to Zn by H2 under standard conditions
2.(a) ZnO cannot be reduced to Zn by H2 under standard conditions
3.(a) O>N>M
4.(b) (a) and (d)

29.

1. (a) VO
2. (a) VO2+
3. (b) With chlorine vanadium can form VCl5.
4. (c) +5
5. (a) V2O5

30.

1.(b) Variable oxidation states
2.(c) nearly same atomic size
3.(c) [Xe]4f14 5d36s2
4.(b) 3d54s2