CBSE Class 12 Chemistry D And F Block Elements VBQs

Question. Which one of the following is diamagnetic ion?
(a) Co2+
(b) Ni2+
(c) Cu2+
(d) Zn2+
Answer. D

Question. Zr and Hf have almost equal atomic and ionic radii because of
(a) diagonal relationship
(b) lanthanoid contraction
(c) actinoid contraction
(d) belonging to the same group
Answer. B

Question. Which of the following is likely to form white salts?
(a) Cu2+
(b) Sc3+
(c) Ti3+
(d) Fe3+
Answer. B

Question. Which one of the following characteristics of the transition metals is associated with higher catalytic activity?
(a) High enthalpy of atomisation
(b) Paramagnetic behaviour
(c) Colour of hydrate ions
(d) Variable oxidation states
Answer. D

Question. Colour of transition metal ions are due to absorption of some wavelength. This results in
(a) d-s transition
(b) s-s transition
(c) s-t/transition
(d) d-d transition
Answer. D

Question. Which of the following pairs of ions have the same electronic configuration?
(a) Cu²⁺, Cr²⁺
(b) Fe³⁺, Mn²⁺
(c) Co³⁺, Ni³⁺
(d) Sc³⁺, Cr³⁺
Answer. B

Question. Anomalous electronic configuration in the 3d series are of
(a) Cr and Fe
(b) Cu and Zn
(c) Fe and Cu
(d) Cr and Cu
Answer. D

Question. Which metal has lowest melting point?
(a) Cs
(b) Na
(c) Hg
(d) Sn
Answer. C

ASSERTION REASON TYPE QUESTIONS

Read the passage given below and answer the following questions:

The transition elements have incompletely filled d-subshells in their ground state or in any of their oxidation states. The transition elements occupy position in between s- and p-blocks in groups 3-12 of the Periodic table. Starting from fourth period, transition elements consists 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 outer orbitals of these elements is (n - 1) d^1-10 ns^1-2. The electronic configurations of outer orbitals of Zn, Cd, Hg and Cn are represented by the general formula (n - 1) d¹⁰ n². All the transition elements have typical metallic properties such as high tensile strength, ductility, malleability. Except mercury, which is liquid at room temperature, other transition elements have typical metallic structures. The transition metals and their compounds also exhibit catalytic property and paramagnetic behaviour. Transition metal also forms alloys. In these questions (i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.

Question. Assertion: Cu²⁺ iodide is not known.
Reason : Cu²⁺ oxidises I^– to iodine.
Answer. A

Question. Assertion: Actinoids form relatively less stable complexes as compared to lanthanoids.
Reason: Actinoids can utilize their 5f orbitals along with 6d orbitals in bonding but lanthanoids do not use their 4f orbital for bonding
Answer. D

Question. Assertion: Cu cannot liberate hydrogen from acids.
Reason: Because it has positive electrode potential.
Answer. A

Question. Assertion: The highest oxidation state of osmium is +8.
Reason: Osmium is a 5d-block element
Answer. B

SHORT ANSWERS TYPE QUESTIONS

Question. How would you account for the increasing oxidising power in the series: V0₂⁺ < Cr₂0₇ ^2-< Mn0₄^– ?
Answer. This is due to the increasing stability of the lower species to which they are reduced.

Question. Which metal in the first transition series exhibits a +1 oxidation state most frequently and why?
Answer. Copper has electronic configuration 3d¹⁰4s¹. It can easily lose one (4s¹) electron to give stable 3d10 configuration.
Metal ion  Magnetic moment (BM)
Sc3+           0.00
Cr2+           4.90
Ni2+            2.84
Ti3+            1.73

Question. The magnetic moments of a few transition metal ions are given below: (atomic no. Sc = 21, Ti = 22, Cr = 24, Ni = 28) Which of the given metal ions: (i) has the maximum number of unpaired electrons?
Answer. Cr²⁺

(ii) forms colourless aqueous solution?
Answer. Sc³⁺

(iii) exhibits the most stable +3 oxidation state?
Answer. Sc³⁺

Question. Out of Fe and Cu, which has a higher melting point and why?
Answer. Fe has a higher melting point. The metallic bond is formed due to the interaction of electrons in the outermost orbitals. The strength of bonding is related to the number of unpaired electrons. Fe has more unpaired electrons leading to stronger metallic bonding. So, it has a higher melting point.

Question. How would you account for the following:
(i) Cr²⁺ is reducing in nature while with the same d-orbital configuration (d⁴), Mn³⁺ is an oxidizing agent.
Answer. Cr²⁺ is reducing as its configuration changes from d⁴ to d³, the latter having a half-filled t2g level. On the other hand, the change from Mn³⁺ to Mn²⁺ results in the half-filled (d⁵) configuration which has extra stability.

(ii) In a transition series of metals, the metal which exhibits the greatest number of oxidation states occurs in the middle of the series.
Answer. This is because, in the middle of the transition series, the maximum number of electrons are available for sharing with others. The small number of oxidation states at the extreme left side is due to the lesser number of electrons to lose or share. On the other hand, at the extreme right-hand side, due to a large number of electrons, only a few orbitals are available in which the electrons can share with others for higher valence.

Question. Manganese exhibits the highest oxidation state of +7 among the 3d-series of transition elements.
Answer. Manganese has the electronic configuration [Ar] 3d⁵ 4s². It can lose seven electrons due to the participation of 3d and 4s electrons and therefore, exhibits the highest oxidation state of +7 in its compounds.

LONG ANSWER TYPE QUESTIONS

Question. [i]What happens, when [a] manganate ion reacts with thiosulphate? [b] dichromate ion reacts with iron in presence of acid?
[ii] Explain the following trends in the properties of the members of the transition elements. [a] Mn²⁺ is more stable than Fe²⁺ towards oxidation to +3 state.
[b] The enthalpy of atomization is lowest for Zn in 3d-seriesof the transition elements. [c] The E⁰ value for the Mn³⁺/Mn²⁺ couples much more positive than that forCr³⁺/Cr²⁺ couple.
Answer. [i][a] Manganate ion reacts with thiosulphate
8MnO₄^-[aq]+3S₂O₃^2- [aq]+ H₂O 8MnO₂⁺ +6SO₄^2- +2OH^-
[b] Dichromate ion reacts with iron in the presents of acid
Cr₂O₇^2- [aq]+6Fe²⁺[aq]+14 H⁺[aq] 2Cr³⁺ +6Fe³⁺ +7H₂O [ii] [a] Electronic configuration of Mn²⁺ =[Ar] 3d⁵ Electronic configuration of Fe²⁺ =[Ar]3d⁶ Mn²⁺ having half filled d orbitals will be more stable than Fe²⁺ as it has partially filled d orbitals
[b]Zinc has completely filled d orbitals, which limits its tendency to form metallic bonds. Thus it requires least enthalpy to get atomized. [c] Mn³⁺[3d⁴] is less stable than Mn²⁺[3d⁵] because Mn²⁺ has stable half filled configuration, Cr³⁺ has stable 3d³[t2g³] configuration, therefore Cr³⁺ cannot be reduced toCr²⁺.

Question. Assign reasons for the following:
a) The enthalpies of atomization of transition elements are high.
b) The transition metals and many of their compounds act as good catalysts.
c) There is a gradual decrease in the atomic sizes of transition elements in a series with increasing atomic numbers.
d) The transition elements have a great tendency for complex formation.
e) Transition metals generally form coloured compounds.
Answer. 
a) This is because transition elements have strong metallic bonds as they have a large number of unpaired electrons, therefore they have greater interatomic overlap.
b) The catalytic activity of transition metals is attributed to the following reasons‐
i) Because of their variable oxidation state, transition metals form unstable intermediate compounds and provide a new path with the lower activation energy for the reaction.
ii) In some cases, the transition metal provides a suitable large surface area with free valencies on which reactants are adsorbed.
c) There is a gradual decrease in the atomic sizes of transition elements in a series with increasing atomic numbers due to poor shielding effects of the d‐electrons, the net electrostatic attraction between the nucleus and the outermost electrons increases
d) orbitals of suitable energy, the small size of cations, and higher nuclear charge.
e) Due to the presence of unpaired electrons in d‐orbitals which undergoes a d‐d transition.

CASE BASED QUESTIONS

Read the passage given below and answer the following questions:

The transition elements have incompletely filled d-subshells in their ground state or in any of their oxidation states. The transition elements occupy position in between s- and p-blocks in groups 3-12 of the Periodic table. Starting from fourth period, transition elements consists 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 outer orbitals of these elements is (n - 1) d1-10 ns^1-2. The electronic configurations of outer orbitals of Zn, Cd, Hg and Cn are represented by the general formula (n - 1) d¹⁰ n². All the transition elements have typical metallic properties such as high tensile strength, ductility, malleability. Except mercury, which is liquid at room temperature, other transition elements have typical metallic structures. The transition metals and their compounds also exhibit catalytic property and paramagnetic behaviour. 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.

The following questions are multiple choice questions. Choose the most appropriate Answer.

Question. Which of the following characteristics of transition metals is associated with higher catalytic activity?
(a) High enthalpy of atomisation
(b) Variable oxidation states
(c) Paramagnetic behaviour
(d) Colour of hydrated ions
Answer. B

Question. 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.
Answer. C

Question. Which one of the following outer orbital configurations may exhibit the largest number of oxidation states?
(a) 3d⁵4s¹
(b) 3d⁵4s²
(c) 3d²4s²
(d) 3d³4s²
Answer. B

Question. The oxidation state of vanadium in V₂O₅ is
(a) +5/2
(b) +7
(c) +5
(d) +6
Answer. C

Question. Predict magnetic behaviour of manganate ion?
(a) ferromagnetic
(b) diamagnetic
(c) paramagnetic
Answer. C

d and f-block elements

Question. People residing near villages have a tendency to dispose waste in water A person was disposing mercury cells in water. A student Raju, asked the person not to do so.(a)What are the harmful effects of mercury?
(b)What values are associated with the above discussion?
Answer:
(a) impaired neurological development, carcinogenic
(b) Care for environment and mankind 

Question. KMnO₄ is commercially obtained from ore Pyrolusite. It exists in the form of dark purple crystals. It is moderately soluble in water at room temperature and its solubility increases with the rise in temperature. KMnO₄ is useful oxidising agent and oxidises under neutral, acidic and basic conditions.
i) What happens when KMO₄ is heated?
ii) What is Baeyer’s reagent?
iii) How does it act as a test for unsaturation?
iv) What is value associated with the use of KMnO₄ in our daily life?
Answer:
i) 2KMnO₄ ∆ K₂ MnO4 + MnO₂ + O₂
ii) Baeyer’s reagent is dil cold alkaline KMnO4(aq) solution.
iii) It is decoloured (pink to colourless) by the reaction of any unsatured organic compound (alkenes or alkynes or unsaturated fat etc)
iv) It useful to purify water .It destroys undesirable bacteria by oxidation. It is an antiseptic for washing wounds. It has bleaching action too for textile industries.

Question. Nickel is the second most abundant element by weight in earth crust. The mond’s process provides us high purity of nickel. Most of the nickel produced is used to make ferrous and non ferrous alloys. Nickel provides both the strength of steel and its resistance to chemical attack. Nickel is used in making alloys for coins in USA. It is widely used as catalyst especially in hydrogenation of vegetable oils to get vegetable ghee. Nickel is also used in Ni-Cd cell?
i) What is the use of Nickel steel and why?
ii) How is Nickel of high purity, obtained by Mond’s process?
iii) Why is Ni-Cd cell preferred over lead storage cell?
iv) Why is vegetable oil better than vegetable ghee? What values are possessed by people, taking vegetable oils?
v) Why is nickel used for alloys used in making coins?
Answer:
i) It is used for making clock pendulums because it has coefficient of thermal expansion.
ii) Nickel when heated in steam of carbon monoxide forms Ni(CO)₄ which decomposes to from pure Ni.
iii) Lead creates pollution therefore; Ni-Cd cell is preferred.
iv) Vegetable oils are unsaturated and they do not lead to the formation of cholesterol where as vegetable ghee is saturated and leads to the formation of high cholesterol. Nickel gets mixed up with vegetable oil which is carcinogenic. People taking vegetable oils are more conscious about their health.
 v) It is because it does not get corroded and has lustre like silver.