PYQ + Trends

1. TITLE

NEET 2023 Q17: Fajans' Rules & Covalent Character — Full solution + Why they asked this

2. THE ACTUAL NEET QUESTION

NEET 2023 — Question
Given below are two statements:

Statement I: Hydrated chlorides and bromides of Ca, Sr and Ba on heating undergo hydrolysis.
Statement II: Hydrated chlorides and bromides of Be and Mg on heating undergo dehydration.

In the light of the above statements, choose the correct answer from the options given below:
A) Statement-I is correct but Statement-II is false.
B) Statement-I is incorrect but Statement-II is true.
C) Both Statement-I and Statement-II are true.
D) Both Statement-I and Statement-II are false.

3. OFFICIAL ANSWER

NEET Answer Key: (D)

4. COMPLETE SOLUTION

Concept: Fajans' Rules & Hydrolysis vs. Dehydration

This question tests the covalent vs. ionic nature of Group 2 halides, which is determined by Fajans' Rules.

  • Step 1: Apply Fajans' Rules (Polarization Power)

    • Rule: Smaller cations with high charge density cause high polarization of the anion, leading to Covalent Character.
    • Application: Beryllium ($Be^{2+}$) and Magnesium ($Mg^{2+}$) are very small cations. They polarize the chloride/bromide ions strongly, making $BeCl_2$ and $MgCl_2$ predominantly covalent.
    • Calcium ($Ca^{2+}$), Strontium ($Sr^{2+}$), and Barium ($Ba^{2+}$) are larger cations. They cause less polarization, making their halides predominantly ionic.

  • Step 2: Relate Bond Nature to Heating Effect

    • Covalent Halides ($BeCl_2 \cdot xH_2O$, $MgCl_2 \cdot xH_2O$): Because of the strong covalent interaction and high charge density, the water of crystallization is held tightly. On heating, they don't just lose water; they react with it (hydrolysis) to form oxides/hydroxides and release $HCl$.
      • Reaction: $MgCl_2 \cdot 6H_2O \xrightarrow{\Delta} MgO + 2HCl + 5H_2O$ (Hydrolysis occurs).
    • Ionic Halides ($CaCl_2 \cdot xH_2O$, etc.): These are stable ionic salts. The water molecules are just trapped in the crystal lattice. On heating, they simply lose the water molecules (Dehydration) to form anhydrous salts.
      • Reaction: $CaCl_2 \cdot 6H_2O \xrightarrow{\Delta} CaCl_2 + 6H_2O$ (Simple dehydration).

  • Step 3: Evaluate Statements

    • Statement I: Claims Ca, Sr, Ba undergo hydrolysis. FALSE (They undergo dehydration because they are ionic).
    • Statement II: Claims Be, Mg undergo dehydration. FALSE (They undergo hydrolysis because they are covalent).

Conclusion: Both statements are false. Option (D) is correct.

5. WHY NEET ASKED THIS

The examiner wanted to test if you can look beyond simple "trends" and apply Fajans' Rules to predict chemical properties.
* The Trap: Most students remember the trend of size ($Be < Mg < Ca...$) but forget how this size difference dictates the reaction with water (hydrolysis vs dehydration).
* The Concept: The shift from Covalent (Be, Mg) to Ionic (Ca, Sr, Ba) down the group is the key to solving this.

6. TREND ANALYSIS

PATTERN ALERT:
* 1990s-2010s: Questions were direct ordering. E.g., "Arrange in increasing covalent character: $LiCl < BeCl_2 < BCl_3 < CCl_4$".
* 2021: Questions focused on bond parameters like Bond Enthalpy (C-F vs C-I).
* 2023: The trend shifted to application-based questions. Instead of asking "Is $MgCl_2$ covalent?", they asked "Does it hydrolyse?", requiring you to know that Covalent = Hydrolysis prone.

Prediction for NEET 2025-26:
Expect a comparison question involving Melting Points or Solubility based on Fajans' Rules.
* Example: "Why is $AgI$ less soluble than $AgF$?" (Answer: $AgI$ has high covalent character due to large anion $I^-$, leading to lower water solubility compared to ionic $AgF$).

7. SIMILAR QUESTIONS TO PRACTICE

If you understood the logic above (Small Cation = Covalent), you can solve these variations:

  1. NEET 1990 (Modified): Which has the highest covalent character?

    • A) $LiCl$
    • B) $BeCl_2$
    • C) $BCl_3$
    • D) $CCl_4$
    • Hint: Smallest cation/Highest charge = Most Covalent ($CCl_4$).

  2. Practice Variation (LiCl vs NaCl):

    • "Which has a lower melting point: $LiCl$ or $NaCl$?"
    • Solution: $Li^+$ is smaller than $Na^+$. By Fajans' Rules, $LiCl$ is more covalent. Covalent compounds generally have lower melting points than ionic ones. Therefore, $LiCl$ melts at a lower temperature than $NaCl$.

8. QUICK REVISION FOR FAJANS' RULES

To predict Covalent Character in an ionic bond, check these Fajans' conditions:

  1. Small Cation: (e.g., $Li^+$ is more covalent than $Na^+$).
  2. Large Anion: (e.g., $LiI$ is more covalent than $LiF$).
  3. High Charge: (e.g., $Al^{3+}$ is more covalent than $Na^+$).
  4. Electronic Config: Cations with pseudo-noble gas config (like $Cu^+$, $Ag^+$) are more polarizing (covalent) than noble gas types ($Na^+$, $K^+$).

Mnemonic: Small Cation, Large Anion = Covalent (SCLA-C).

9. NCERT BASE

This question is directly based on the concepts of Ionic vs Covalent Bond character discussed in:
* NCERT Class 11 Chemistry, Part 1
* Chapter 4: Chemical Bonding & Molecular Structure
* Section 4.3.6: Polarity of Bonds & Fajans' Rules.

10. CTA

All PYQs from this chapter → Chemical Bonding Guide
Practice similar questions → Mock exams