The d- and f- Block Elements

Download CBSE Class 12th Standard CBSE Chemistry question papers, sample papers, important questions, and previous year solved papers in PDF format. Get free study materials, NCERT solutions, and exam preparation resources for Class 12th Standard CBSE Chemistry

Questions:

Generic 2

5 x 2 = 10

Why hydrated copper sulphate is blue while anhydrous copper sulphate is white ?

Describe the steps involved in the preparation of either potassium dichromate from sodium chromate or potassium permanganate from manganese dioxide.

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.

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

Predict which of the following will be coloured in aqueous solution. Ti³⁺, V³⁺, Cu⁺, Sc³⁺, Mn²⁺, Fe³⁺ and Co^2+. Give reason for each.

Generic 3

5 x 3 = 15

Compare the chemistry of the actinoids with that of lanthanoids with reference to the following :
(i) Electronic configuration
(ii) Oxidation states
(iii) Chemical reactivity

Complete the following reactions:
(a) Cr₂O₇^2-+14H⁺+6e^-\(\longrightarrow \).......... +7H₂O
(b) CrO₄^2-+..........\(\rightleftharpoons \)........\(\rightleftharpoons \).......+H₂O
(c) MnO₄^-+2H₂O+3e^-\(\overrightarrow { medium } \)........ +4OH^-

Identify the first row transition metal ions which have outer electronic configurations of 3d⁴ and 3d⁶ and describe their oxidation states.

(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?

10.

Explain the following:
(i) The transition elements have great tendency for complex formation.
(ii) There is a gradual decrease in the atomic sizes of transition elements in a series with increasing atomic numbers.
(iii) Lanthanum and Lutetium do not show colouration in solutions.
(At. No : La = 57, Lu = 71)

Generic 5

2 x 5 = 10

11.

Nuclear power station consists of nuclear reactor, in which a controlled chain reaction involving nuclear fission occurs using either U or Pu as fuel in reactor. The heat produced is extracted from the reactor and this heat is used to generate steam which drives a turbine and produces electricity. U or Pu can also be used in making atom bombs. The subcritical mass of U or Pu is carried out in aeroplanes and combine them to give a critical mass when dropped over the target. It leads to uncontrolled chain reaction which produces tremendous amount of energy and lot of destruction.Plutonium bomb was dropped over Hiroshima and Nagasaki, Japan in world War II which caused lot of destruction.
Answer the following questions.
(i) Should we use U and Pu for making atom bombs?
(ii) What is the long term disadvantages of nuclear radiation?
(iii) Why is U-235 used in nuclear reactor?
(iv) Why are breeder reactors more useful?

12.

Compare the general characteristics of the first series of the transition metals with those of the second and third columns. Give special emphasis on the following points:
(i) electronic configurations,
(ii) oxidation states,
(iii) ionisation enthalpies and
(iv) atomic sizes.

Multiple Choice Question

5 x 1 = 5

13.

Permanent magnets are generally made of alloys of

(a)

(b)

(c)

(d)

Any one of these

14.

Which metal has highest density ?

(a)

(b)

(c)

(d)

15.

Which of the following ion is colourless in aqueous solution ?

(a)

Fe²⁺

(b)

Mn²⁺

(c)

Ti³⁺

(d)

Sc³⁺

16.

The maximum oxidation state of osmium is

(a)

(b)

(c)

(d)

17.

Which of the following is not a transition element ?

(a)

(b)

(c)

(d)

Visit QB365-SLMS - https://www.qb365.in/slms/

Answers Key:

Generic 2

5 x 2 = 10

In hydrated copper sulphate, four water molecules are present as ligands. In the presence of these ligands d-orbitals are no longer degenerate in energy. Hence, d-d transition takes place absorbing red wavelength. The complementary colour, viz., blue is reflected. In anhydrous CuSO₄, d-orbitals remain degenerate.Hence, no d-d transition can occur. The white light is completely reflected back. Hence, it looks white.

\(2Na_2CrO_4 + H_2SO_4\longrightarrow Na_2SO_4 + Na_2Cr_2O_7 + H_20\)
\(Na_2Cr_2O_7 + 2KCI\longrightarrow K2Cr_2O_7 + 2NaCI\)
or
\(MnO_2+2KOH+{1\over 2}O_2\xrightarrow{heat}K_2MnO_4+H_2O\)
\(mNo_4^{2-}\xrightarrow{electrolysis}MnO_4^{-}+e^-\)

(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.

Copper exhibits +1 oxidation state frequently due to stable electronic configuration.

Ti³⁺, V³⁺, Mn²⁺, Fe³⁺ and Co²⁺ are coloured due to presence of unpaired electrons, they can undergo d-d transitions others Cu⁺, Sc³⁺ are colourless due to absence of unpaired electrons.

Generic 3

5 x 3 = 15

(i) Electronic configuration
Lanthanoids = [Xe] 4f^0-14 5d^0-1 6s²
Actinoids = [Rn] 5f^0-14 6d^0-1 7s²
(ii) Oxidation states In lanthanoids, +3 oxidation state is most common along with + 2 and + 4. While in actinoids, there is a greater range of oxidation states because 5f, 6d and 7s levels are of comparable energies.They show + 2, + 3, + 4, + 5, + 6 and + 7 oxidation states. Common oxidation state in actinoids is + 3.
(iii) Chemical reactivity Lanthanoids are less reactive than actinoids. Actually, earlier members of lanthanoids are quite reactive similar to calcium but with increasing atomic number, they behave more like aluminium. Lanthanoids react with dilute acids to liberate H₂ gas while actinoids react with boiling water and gives a mixture of oxide and hydride.

(a) Cr₂O₇^2-+14H⁺+6e^-\(\longrightarrow \)2Cr³⁺ +7H₂O
(b) CrO₄^2-+2H⁺\(\rightleftharpoons \)2HCrO₄^-\(\rightleftharpoons \)Cr₂O₇^2-+H₂O
(c) MnO₄^-+2H₂O+3e^-\(\overrightarrow { medium } \)MnO₂ +4OH^-

Cr²⁺ has electronic configuration 3d⁴ Chromium also shows +3 and +6 oxidation states. Fe²⁺ has electronic configuration 3d⁶. Iron has oxidation states +2 and +3.

(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.

10.

(i) It is due to the presence of vacant d-orbitals of suitable energy, the smaller size of cautions and higher charge.
(ii) It is due to increase in effective nuclear charge gradually because unpaired electrons increase in the beginning and then decreases. There is repulsion between paired electrons.
(iii) It is due to absence of unpaired electrons, they do not absorb light from visible region and do not radiate color.

Generic 5

2 x 5 = 10

11.

(i) We should avoid making atom bombs because they lead to the lot of destruction and lot of national money is spent on it.
(ii) Nuclear radiation may cause mutation in genes which leads to genetic disorders.
(iii) U-235 is fissionable material.
(iv) Breeder reactors are used to produce nuclear fuelic fissionable material needed for nuclear reactions.

12.

(i) Electronic configurations: In 1^st transition series, 3d orbitals are progressively filled, whereas, in 2nd transition series, 4d orbitals are progressively filled and in 3^rdtransition series, 5d-orbitals are progressively filled.
(ii) Oxidation states: Elements show variable oxidation states in both the. series. The highest oxidation state is equal to a total number of electrons in '5' as well as 'd' orbitals. The number of oxidation states shown is less in 5d transition series than 4d series. In 3d series +2, +3 oxidation states are common and they form stable complexes in these oxidation states. In other series, SO₄ and PtF₆ are formed which are quite stable in higher oxidation state.
(iii) ionization enthalpies: The ionization enthalpy of 5d series is higher than 3d and 4d series due to lanthanide contraction, the effective nuclear charge is more.
(iv) Atomic sizes: The atomic sizes of 4d and 5d series do not differ appreciably due to lanthanoid contraction. The atomic radii of second and third series are larger than 3d series.

Multiple Choice Question

5 x 1 = 5

13.