d-and f-Block Elements Previous Year Questions — JEE Main Chemistry
44 past-year d-and f-Block Elements questions from JEE Main (Chemistry) — page 4.
Q1311 (2025)
The correct option with order of melting points of the pairs (Mn, Fe), (Tc, Ru) and (Re, Os) is :
- $\mathrm{Fe} < \mathrm{Mn}$ , $\mathrm{Ru} < \mathrm{Tc}$ and $\mathrm{Re} < \mathrm{Os}$
- $\mathrm{Mn} < \mathrm{Fe}, \mathrm{Tc} < \mathrm{Ru}$ and $\mathrm{Re} < \mathrm{Os}$
- $\mathrm{Mn} < \mathrm{Fe}, \mathrm{Tc} < \mathrm{Ru}$ and $\mathrm{Os} < \mathrm{Re}$
- $\mathrm{Fe} < \mathrm{Mn}$ , $\mathrm{Ru} < \mathrm{Tc}$ and $\mathrm{Os} < \mathrm{Re}$
### Formulas Used
Melting point trends in $3d$, $4d$, and $5d$ series transition metals depend on the extent of metallic bonding and $d$-electron participation.
### Core Logic
According to NCERT transition element periodic trends:
* **$3d$ Series ($\mathrm{Mn}$ vs $\mathrm{Fe}$)**: Manganese ($\mathrm{Mn}$, $3d^5 4s^2$) has an abnormally low melting point compared to Iron ($\mathrm{Fe}$, $3d^6 4s^2$) because its stable, half-filled $d^5$ configuration holds $d$-electrons more tightly, reducing their participation in metallic bonding $\rightarrow \mathbf{\mathrm{Mn} < \mathrm{Fe}}$.
* **$4d$ Series ($\mathrm{Tc}$ vs $\mathrm{Ru}$)**: Technetium ($\mathrm{Tc}$, $4d^5 5s^2$) similarly shows a dip in melting point compared to Ruthenium ($\mathrm{Ru}$, $4d^7 5s^1$) due to the stable $4d^5$ configuration $\rightarrow \mathbf{\mathrm{Tc} < \mathrm{Ru}}$.
* **$5d$ Series ($\mathrm{Re}$ vs $\mathrm{Os}$)**: Rhenium ($\mathrm{Re}$, $5d^5 6s^2$) has optimal interatomic interaction and a higher melting point than Osmium ($\mathrm{Os}$, $5d^6 6s^2$) $\rightarrow \mathbf{\mathrm{Os} < \mathrm{Re}}$.
Combining these trends yields: **$\mathrm{Mn} < \mathrm{Fe}$, $\mathrm{Tc} < \mathrm{Ru}$, and $\mathrm{Os} < \mathrm{Re}$**
### Pattern Recognition
Stable half-filled $d^5$ configurations in $3d$ ($\mathrm{Mn}$) and $4d$ ($\mathrm{Tc}$) restrict $d$-electron delocalization, creating characteristic dips in melting point curves compared to adjacent metals.
**Correct Option:** **(C)**
Q1322 (2025)
The molar mass of the water insoluble product formed from the fusion of chromite ore $\mathrm{(FeCr_2O_4)}$ with $\mathrm{Na}_2\mathrm{CO}_3$ in presence of $\mathrm{O}_2$ is ________ $\mathrm{g \, mol^{-1}}$.
### Related Formula
$$\text{Balanced fusion reaction process description}$$
### Core Logic
Write the balanced chemical equation for the industrial preparation stage of chromate salts:
$$4\mathrm{FeCr_2O_4} + 8\mathrm{Na_2CO_3} + 7\mathrm{O_2} \rightarrow 8\mathrm{Na_2CrO_4} + 2\mathrm{Fe_2O_3} + 8\mathrm{CO_2}$$
Evaluating the solubilities of the products:
* $\mathrm{Na_2CrO_4}$ is highly soluble in water.
* $\mathrm{Fe_2O_3}$ (Iron(III) oxide) is water-insoluble.
Molar Mass of $\mathrm{Fe_2O_3}$:
$$M = (2 \cdot 55.85) + (3 \cdot 16.0) \simeq (2 \cdot 56) + (3 \cdot 16) = 112 + 48 = 160 \mathrm{~g/mol}$$
### Pattern Recognition
Transition metal oxides in high oxidation states with minimal ionic breakdown parameters reliably act as insoluble precipitates in water.
### Chapter Mix
Class 12 Chemistry: The d-and f-Block Elements
Q1383 (2024)
In acidic medium, $K_2Cr_2O_7$ shows oxidising action as represented in the half reaction
$$Cr_2O_7^{2-} + XH^+ + Ye^- \rightarrow 2A + ZH_2O$$
X, Y, Z and A are respectively are:
- $8, 6, 4 \text{ and } Cr_2O_3$
- $14, 7, 6 \text{ and } Cr^{3+}$
- $8, 4, 6 \text{ and } Cr_2O_3$
- $14, 6, 7 \text{ and } Cr^{3+}$
### Core Logic
The balanced half-reaction for the dichromate ion acting as an oxidising agent in an acidic medium is:
$$Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$$
### Step 1: Compare with Given Equation
Comparing this with the given equation $Cr_2O_7^{2-} + XH^+ + Ye^- \rightarrow 2A + ZH_2O$:
$X = 14$
$Y = 6$
$Z = 7$
$A = Cr^{3+}$
### Pattern Recognition
In acidic medium, dichromate ($Cr_2O_7^{2-}$) always requires $14H^+$ to balance $7O$ atoms, forming $7H_2O$. Chromium reduces from +6 to +3 state, taking $6e^-$ overall.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: The d-and f-Block Elements
Class 11 Chemistry: Redox Reactions
Q73 (2024)
Which of the following acts as a strong reducing agent? (Atomic number : Ce = 58, Eu = 63, Gd = 64, Lu = 71)
- $\mathrm{Lu}^{3+}$
- $\mathrm{Gd}^{3+}$
- $\mathrm{Eu}^{2+}$
- $\mathrm{Ce}^{4+}$
### Related Formula
$$\text{Electronic configuration of } \mathrm{Eu} = [\mathrm{Xe}] 4f^7 6s^2$$
### Core Logic
The most common and stable oxidation state for lanthanoids is $+3$. In the case of Europium:
$$\mathrm{Eu}^{2+} = [\mathrm{Xe}] 4f^7$$
This configuration possesses a highly stable half-filled $f$-subshell. However, because the $+3$ state is universally favored by thermodynamics in solution, $\text{Eu}^{2+}$ readily undergoes oxidation to lose one more electron:
$$\mathrm{Eu}^{2+} \rightarrow \mathrm{Eu}^{3+} + 1e^-$$
By releasing an electron to stabilize into the $+3$ state, it behaves as a potent reducing agent.
### Step 1: Evaluation
Conversely, $\text{Ce}^{4+}$ acts as a powerful oxidizing agent to return to $+3$, while $\text{Lu}^{3+}$ and $\text{Gd}^{3+}$ are already perfectly configured at their native stable limits.
### Pattern Recognition
Europium($II$) has a stable half-filled $f^7$ configuration, yet easily loses an electron to attain the highly stable $+3$ state typical of lanthanoids, making it a strong reducing agent.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: d and f Block Elements
Q75 (2024)
Which of the following statements are correct about Zn, Cd and $\mathrm{Hg}$ ?
A. They exhibit high enthalpy of atomization as the d-subshell is full.
B. Zn and Cd do not show variable oxidation state while Hg shows $+\mathrm{I}$ and $+\mathrm{II}$.
C. Compounds of Zn, Cd and Hg are paramagnetic in nature.
D. Zn, Cd and Hg are called soft metals.
Choose the most appropriate from the options given below:
- B, D only
- B, C only
- A, D only
- C, D only
### Related Formula
$$\text{General configuration of Group 12: } (n-1)d^{10} ns^2$$
### Core Logic
Analyzing each statement based on inorganic chemistry principles:
* **Statement A is false**: Because their $d$-subshell is completely full ($d^{10}$), these elements do not form strong metallic bonds. As a result, they exhibit the *lowest* enthalpy of atomization in their respective periods.
* **Statement B is true**: $\text{Zn}$ and $\text{Cd}$ show only a stable $+2$ oxidation state, whereas $\text{Hg}$ exhibits variable states forming both $+1$ (as $\text{Hg}_2^{2+}$) and $+2$.
* **Statement C is false**: With a fully paired $d^{10}$ subshell, their compounds lack unpaired electrons and are explicitly diamagnetic.
* **Statement D is true**: Due to weak metallic bonds, these elements have low melting points and are classified as soft metals.
### Step 1: Selection Verification
Statements B and D are true, matching choice (1).
### Pattern Recognition
Group 12 metals have a full $d^{10}$ subshell, leading to exceptionally weak metallic bonding, low enthalpies of atomization, and diamagnetic characteristics.
### Evaluation Rubric / Model Answer
null
### Chapter Mix
Class 12 Chemistry: d and f Block Elements
