NCERT Exemplar Class 12 Chemistry Chapter 8: Higher Order Thinking Skills (HOTS) Questions
Some tricky Class 12 chemistry chapter 8 questions and answers are given below that will help you tackle complex problems. Students can follow class 12 d and f-block notes to learn the concepts in detail.
Question 1: The correct decreasing order of spin only magnetic moment values $(\mathrm{BM})$ of $\mathrm{Cu}^{+}, \mathrm{Cu}^{2+}, \mathrm{Cr}^{2+}$ and $\mathrm{Cr}^{3+}$ ions is:
1) $\mathrm{Cu}^{+}>\mathrm{Cu}^{2+}>\mathrm{Cr}^{3+}>\mathrm{Cr}^{2+}$
2) $\mathrm{Cu}^{2+}>\mathrm{Cu}^{+}>\mathrm{Cr}^{2+}>\mathrm{Cr}^{3+}$
3) $\mathrm{Cr}^{2+}>\mathrm{Cr}^{3+}>\mathrm{Cu}^{2+}>\mathrm{Cu}^{+}$
4) $\mathrm{Cr}^{3+}>\mathrm{Cr}^{2+}>\mathrm{Cu}^{+}>\mathrm{Cu}^{2+}$
Answer:
$\mathrm{Cu}^{+} \Rightarrow 3 \mathrm{~d}^{10} \Rightarrow$ 
$\Rightarrow 0 \mathrm{BM}$
$\mathrm{Cu}^{2+} \Rightarrow 3 \mathrm{~d}^9 \Rightarrow$ 
$\Rightarrow 1.73 \mathrm{BM}$
$\mathrm{Cr}^{2+} \Rightarrow 3 \mathrm{~d}^4 \Rightarrow$ 
$\Rightarrow 4.89 \mathrm{BM}$
$\mathrm{Cr}^{3+} \Rightarrow 3 \mathrm{~d}^3 \Rightarrow$ 
$\Rightarrow 3.87 \mathrm{BM}$
So order :
$\mathrm{Cr}^{2+}>\mathrm{Cr}^{3+}>\mathrm{Cu}^{2+}>\mathrm{Cu}^{+}$
Hence, the correct answer is option (3).
Question 2:The number of unpaired electrons responsible for the paramagnetic nature of the following complex species are respectively :
$\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-},\left[\mathrm{FeF}_6\right]^{3-},\left[\mathrm{CoF}_6\right]^{3-},\left[\mathrm{Mn}(\mathrm{CN})_6\right]^{3-}$
1) $1,5,4,2$
2) $1,5,5,2$
3) $1,1,4,2$
4) $1,4,4,2$
Answer:
$\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-} \Rightarrow \mathrm{Fe}^{3+}, \mathrm{d}^5, \mathrm{t}_{2 \mathrm{~g}}^5 \mathrm{e}_{\mathrm{g}}^0$
$\quad$ $\Rightarrow 1$ unpaired electron
$\left[\mathrm{FeF}_6\right]^{3-} \Rightarrow \mathrm{Fe}^{3+}, \mathrm{d}^5, \mathrm{t}_{2 \mathrm{~g}}^3 \mathrm{e}_{\mathrm{g}}^2 \Rightarrow 5$ unpaired electrons $\left[\mathrm{CoF}_6\right]^{3-} \Rightarrow \mathrm{Co}^{3+}, \mathrm{d}^6, \mathrm{t}_{2 \mathrm{~g}}^4 \mathrm{e}_{\mathrm{g}}^2 \Rightarrow 4$ unpaired electrons
$\begin{aligned} {\left[\mathrm{Mn}(\mathrm{CN})_6\right]^{3-} \Rightarrow } & \mathrm{Mn}^{3+}, \mathrm{d}^4,\end{aligned} \mathrm{t}_{2 \mathrm{~g}}^4 \mathrm{e}_{\mathrm{g}}^0 \mathrm{C}$.
$\quad$ $\Rightarrow 2$ unpaired electron
Hence, the correct answer is option (1).
Question 3. The first transition series metal ' M ' has the highest enthalpy of atomisation in its series. One of its aquated ion $\left(\mathrm{M}^{\mathrm{n}}\right)$ exists in green colour. The nature of the oxide formed by the above $\mathrm{M}^{\mathrm{n}-}$ ion is :
(1) neutral
(2) acidic
(3) basic
(4) amphoteric
Answer:
* In 3d series Vanadium has highest enthalpy of atomization and colour of $\mathrm{V}^{+3}$ is green.
* Oxide form by $\mathrm{V}^{+3}$ is $\mathrm{V}_2 \mathrm{O}_3$ (Basic oxide)
Hence, the correct answer is option (3).
Approach to Solve Questions of Class 12 Chemistry Chapter 8
The approach should be simple and time-saving. The following are the points that can help you build a good question-solving strategy
1. Understand the basics
Start by clearly understanding the electronic configuration, oxidation states and general properties of d- and f-block elements. These concepts form the foundation for most questions. The NCERT Exemplar Class 12 Chemistry Solutions Chapter 8 will help you understand these concepts better.
2. Memorize important trends
Make your focus on periodic trends such as atomic size, ionization enthalpy, melting/boiling points and magnetic properties within the transition and inner transition elements.
3. Learn colour and complex formation
Try to remember common colored compounds and their oxidation states, especially for transition metals. You can make flowcharts or flashcards to revise it. You can learn these concepts in class 12 chemistry chapter 8 notes to learn more.
4. Practice reactions and uses
Give proper attention to the important reactions like the preparation and properties of potassium dichromate (K₂Cr₂O₇) and potassium permanganate (KMnO₄), and their oxidizing behaviour in acidic, basic, and neutral media.
5. Use NCERT examples and exercises
You can refer to the NCERT solved examples and try solving in-text questions for a better understanding of the concept. Also, solve the textbook exercise questions as they are often directly asked in board exams. You can also refer to the NCERT exemplar for better learning. Practice previous year questions and solve mock tests.
NCERT Exemplar Class 12 Chemistry Chapter 8: Important Formulas and Key Points
Some Important formulas and key points of the d-block and f-block are given below. Practice more at NCERT Exemplar Class 12 Chemistry Solutions Chapter 8 d and f-block elements.
1. General electronic configuration of the d-block:
=(n−1)d1−10ns0−2
2. General electronic configuration of the f-block:
=(n−2)f1−14(n−1)d0−1ns2
3. Magnetic moment μ
=μ=n(n+2)B.M.
Where n = number of unpaired electrons.
4. Most f-block elements are paramagnetic due to unpaired f-electrons.
5. Lanthanides are known for the +3 oxidation state.
6. Actinides show a wider range of oxidation states (+3 to +6 or more).
7. Mostly d-block elements are paramagnetic due to unpaired d-electrons.
8. D-block elements act as good catalysts.
