Class 12 Chemistry Chapter 8 Ncert Solutions | Download Free PDF

Class 12 Chemistry Chapter 8 NCERT Solutions in English 

Adda247 provides NCERT Solutions for class 12 Chemistry Chapter 8. These Class 12 Chemistry Chapter 8 Ncert Solutions will not only help students to boost their board exams and score brilliant marks but for competitive exams. The Solutions are according to the NCERT guidelines.

Complete 16 chapters wise solutions were provided for the benefit of students.12^th class sets the base for higher education for every student. This makes it the most crucial class for any student who is aiming for his/her dream of quality education. Scoring good marks in the 12^th class is equivalent to a quality high education. So, it becomes extremely important for students to give a boost to their class 12 Chemistry preparation with Adda247 NCERT solutions.

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Advantages of NCERT Solutions of Class 12 Chemistry Chapter 8

• In – depth explanation of all the questions with logical reason.
• Numericals are solved with step by step process.
• Free PDF download option.
• All the answers are given to the point of theoretical questions.

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NCERT Solution for class 12 Chemistry Chapter 8: The d and F Block Elements

This chapter deals with the elements of D and F block in table. In this chapter students shall first deal with the electronic configuration, occurrence and general characteristics of the transition elements with special emphasis on the trends in the properties of the first row transition metals along with the preparation and properties of some important compounds. This will be followed by the consideration of certain general aspects such as electronic configurations, oxidation states and chemical reactivity of the inner transition metals.

The d-block of the periodic table contains elements of the groups 3-12 in which the d orbitals are progressively filled in each of the four long periods. The f – block consists of elements in which 4 f and 5 f orbitals are progressively filled. They are placed in a separate panel at the bottom of the periodic table.

Characteristics of d and f block Elements.

• These are metallic in nature.
• They are hard and have high densities.
• They have high melting and boiling point.
• They shows variable oxidation states.
• They form coloured ions and compounds.
• The atomic radii decreases with increase in atomic number.

The main difference between d block elements and f block elements is that d block elements are chemical elements having electrons filled to their d orbitals whereas f block elements are chemical elements having electrons filled to their f orbitals.

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Key features of NCERT Solution of Chemistry Class 12 Chapter 8

• The NCERT Solution provide clear and precise answer.
• The columns are used wherever necessary.
• Covers the basic of a topic.
• Adheres to the pattern of CBSE.

Important Questions of Class 12 Chemistry Chapter 8 Ncert Solutions

Question: 1 To what extent do the electronic configuration decide the stability of oxidation states in the first series of the transition elements? Illustrate your answer with examples.

Answer:

The elements in the first half of the transition series exhibit many oxidation states with Mn exhibiting maximum no. of oxidation state (+2 to +7). The stability of +2 oxidation state increases with the increase in atomic no. this happens as more electrons are getting filled in the d orbital. However, Sc does not show +2 oxidation state. Its electronic configuration is 4s²3d¹. It loses all the 3 electrons to form Sc³⁺. +3 oxidation state of Sc is very stable as by losing all three electrons, it attains stable noble gas configuration, [Ar]. Ti(+4) and V(5) are very stable for the same reason. For Mn, +2 oxidation state is very stable as after losing two electrons, its d orbital is exactly half filled, [Ar]3d⁵.

Question: 2 What may be the stable oxidation state of the transition element with the following d elements d electron configuration in the ground state of their atoms.

Answer:

Question: 3 Name the oxometal anions of the first series of the transition element with the following d electron configurations in the ground state of their atoms: 3d³, 3d⁵.3d⁸ and 3d⁴?

Answer:

Question: 4 What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

Answer:

The steady decrease in the size of lanthanoid ions (M3+) with the increase in atomic number is called lanthanoid contraction.

Consequences of lanthanoid contraction :

Due to lanthanoid contraction, the size of M³⁺ ions decreases and there is increase in the covalent character in M – OH bond.

Question: 5 In what way is the electronic configuration of the transition elements different from that of the non-transition elements?

Answer:

Transition elements have partially filled d orbital but the non-transition elements have no d orbital or have completely filled d orbitals.

Question: 6 What are the characteristics of the transition elements and why are they called transition elements? Which of the d block element may not be regarded as the transition element?

Answer:

Transition elements are those elements in which the atoms or ions (in stable oxidation state) contain partially filled d-orbital. These elements lie in the d-block and show a transition of properties between s-block and p-block. Therefore, these are called transition elements.

Elements such as Zn, Cd, and Hg cannot be classified as transition elements because these have completely filled d-subshell.

Question:7 Explain giving reasons:

• Transition metals and many of their compounds show paramagnetic behaviour.
• The enthalpies of atomisation of the transition metals are high.
• The transition metals generally form coloured compounds.
• Transition metals and their many compounds act as good catalyst.

Answer:

i.)Transition metals show paramagnetic behaviour. Paramagnetism arises due to the presence of unpaired electrons with each electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. However, in the first transition series, the orbital angular momentum is quenched. Therefore, the resulting paramagnetism is only because of the unpaired electron.

(ii) Transition elements have high effective nuclear charge and a large number of valence electrons. Therefore, they form very strong metallic bonds. As a result, the enthalpy of atomization of transition metals is high.

(iii) Most of the complexes of transition metals are coloured. This is because of the absorption of radiation from visible light region to promote an electron from one of the d-orbitals to another. In the presence of ligands, the d-orbitals split up into two sets of orbitals having different energies. Therefore, the transition of electrons can take place from one set to another. The energy required for these transitions is quite small and falls in the visible region of radiation. The ions of transition metals absorb the radiation of a particular wavelength and the rest is reflected, imparting colour to the solution.

(iv) The catalytic activity of the transition elements can be explained by two basic facts.

(a) Owing to their ability to show variable oxidation states and form complexes, transition metals form unstable intermediate compounds. Thus, they provide a new path with lower activation energy, Ea, for the reaction.

(b) Transition metals also provide a suitable surface for the reactions to occur.

Question:8 What are the interstitial compounds? Why are such compounds well known for transition metals?

Answer:

The compounds formed when small atoms of H, C or N get trapped inside the crystal lattice of metals is known as interstitial compounds. A number of interstitial compounds are formed by the transition metals. As vacant spaces of the transition metals are filled up by small atoms, these compounds are hard and rigid.

Interstitial compounds are well known for transition compounds due to its closed crystalline structure with voids in them. The atomic size of transition metals are very large hence have large voids to occupy these small atoms.

Question:9 How is the variability in oxidation states of transition metals different from that of the non-transition metals? Illustrate with example.

Answer:

The non-transition elements also show variable oxidation states. However these differ from the variable oxidation states shown by transition elements. Variable oxidation states shown by transition elements can differ by one unit while the oxidation states shown by non-transition elements differ by 2 unit due to inert pair effect. For example, thallium exhibits oxidation states +1, +3 and lead exhibits +2, +4 oxidation states. Moreover, in case of transition elements of the same group, higher oxidation state is more stable for heavier elements. e.g., Mo (VI) and W (VI) are more stable than Cr (VI). In non-transition elements of p-block, lower oxidation state is more stable due to inert pair effect e.g., Pb2+ is more stable than Pb4+.

Question:10 Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Answer:

Potassium dichromate is prepared from chromate in the following steps:

Step 1:

Preparation of sodium chromate

4FeCr2O7 + 16NaOH + 7O2 -à 8NaCrO4 +2Fe2O3 +8H2O

Step 2:

Conversion of sodium chromate into sodium dichromate

2Na2Cro4 + Conc.H2SO4 -à  Na2Cr2O7 + Na2SO4 + H2O

Step 3:

Conversion of sodium dichromate to potassium dichromate

Na2Cr2O7 + 2KCl à K2Cr2O7 + 2NaCl

Potassium dichromate being less soluble than the sodium chloride is obtained in the form of crystals and can be removed.

The dichromate ion exist in equilibrium with chromate ion at pH and however, by changing the pH, they can be interconverted.

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