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Question #44181

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The Question

Consider the following thermodynamic and structural observations for Group 1 and Group 11 compounds: \textbf{I.} The magnitude of the experimental lattice energy ( $|U_{exp}|$ ) of \ce{AgI} is significantly greater than its theoretical Born-Landé lattice energy ( $|U_{th}|$ ), whereas for \ce{AgF}, $|U_{exp}| \approx |U_{th}|$ . \textbf{II.} The thermal decomposition of \ce{LiBF4(s)} to \ce{LiF(s)} and \ce{BF3(g)} occurs at a much lower temperature than that of \ce{CsBF4(s)}. \textbf{III.} The \ce{B-F} bond length in the \ce{BF4^-} ion is significantly longer than in the \ce{BF3} molecule.

Which of the following options correctly identifies the primary chemical principles responsible for these observations?

Options

A.
I: Covalent character via polarization increases $|U_{exp}|$ ; II: Large $U(\ce{LiF}) - U(\ce{LiBF4})$ drives decomposition; III: Absence of $p\pi-p\pi$ back-bonding in \ce{BF4^-}.
B.
I: $d\pi-p\pi$ back-bonding strengthens the lattice; II: Lower polarizing power of \ce{Cs^+} stabilizes the lattice kinetically; III: Steric repulsion between fluorines lengthens the bond.
C.
I: Born-Landé overestimates electrostatic repulsion; II: High sublimation energy of Li metal drives decomposition; III: \ce{BF3} possesses $d\pi-p\pi$ back-bonding.
D.
I: \ce{AgI} has lower absolute lattice energy due to size; II: \ce{LiBF4} is kinetically labile due to small \ce{Li^+}; III: Hybridization change to $sp^3$ inherently shortens the bond.

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Lattice Energy Factors|Back Bonding Strength

Why This Question Is Tricky

Common Mistakes

The question targets the 'Lattice Energy Trap' where students erroneously assume the Born-Landé equation perfectly describes all ionic solids.

It forces the recognition that Fajans' Rules (covalent character via polarization) create a measurable thermodynamic deviation ( $|U_{exp}| > |U_{th}|$ ).

Furthermore, it tests the 'Hidden Variable' of lattice energy differences driving the thermal decomposition of polyatomic salts, rather than simple cation polarizing power.

Finally, it uses $p\pi-p\pi$ back-bonding as a structural determinant, trapping students who misattribute bond length changes to steric hindrance or incorrect orbital overlaps (like $d\pi-p\pi$ for Boron).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Lattice Energy Factors
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free