Question

Observe the following statements
Statement - I: Both LiF and CsI have low solubility in water
Statement - II: Low solubility of LiF in water is due to smaller hydration enthalpy of ions and that of CsI is due to its high lattice enthalpy
The correct answer is

• A. Both statements I and II are correct
• B. Statement I is correct, but statement II is not correct
• C. Statement I is not correct, but statement II is correct
• D. Both statements I and II are not correct

Hint:

For solubility of ionic compounds: 1. Small cation + Small anion \(\rightarrow\) High Lattice Energy \(\rightarrow\) Low Solubility (e.g., LiF). 2. Large cation + Large anion \(\rightarrow\) Low Hydration Energy \(\rightarrow\) Low Solubility (e.g., CsI). Mismatch in sizes (Small+Large or Large+Small) usually leads to high solubility.

Answer 1

The Correct Option is B

Step 1: Analyzing Statement I:
Lithium Fluoride (LiF) has very low solubility in water compared to other alkali metal fluorides. Cesium Iodide (CsI) also has lower solubility compared to other alkali metal halides (like LiI, NaI, KI). In the context of alkali halides, LiF and CsI are often cited as the least soluble examples at the extremes. Statement I is considered correct in the context of standard textbook descriptions (e.g., NCERT).
Step 2: Analyzing Statement II:
The solubility depends on the balance between Lattice Enthalpy (\(\Delta H_{lattice}\)) and Hydration Enthalpy (\(\Delta H_{hyd}\)). For a salt to dissolve, \(\Delta H_{hyd}\) should generally overcome \(\Delta H_{lattice}\). * For LiF: Both \(\text{Li}^+\) and \(\text{F}^-\) are small ions. This leads to a very high Lattice Enthalpy which the Hydration Enthalpy (though high for \(\text{Li}^+\)) cannot overcome significantly. Thus, low solubility of LiF is due to high lattice enthalpy. * For CsI: Both \(\text{Cs}^+\) and \(\text{I}^-\) are large ions. Large ions have low hydration enthalpy. While the lattice enthalpy is also low, the hydration enthalpy is smaller to an extent that it doesn't support dissolution well. Thus, low solubility of CsI is due to smaller hydration enthalpy. Comparison with Statement II: Statement II claims LiF has low solubility due to *smaller hydration enthalpy* (Incorrect, it's due to high lattice enthalpy) and CsI due to *high lattice enthalpy* (Incorrect, it's due to small hydration enthalpy). The reasons are swapped/incorrect. Final Answer:
Statement I is correct, but Statement II is not correct.