Question

Choose the correct statement from the following:

• A. Among the alkali metal halides, LiF is least soluble in water.
• B. LiF has least negative standard enthalpy of formation among alkali metal fluorides.
• C. The low solubility of CsI in water is due to its high lattice enthalpy.
• D. The standard enthalpy of formation for alkali metal bromides becomes less negative on descending the group.

Hint:

Solubility of ionic compounds is a tug-of-war between lattice enthalpy and hydration enthalpy. Remember the extreme cases: LiF has very high lattice enthalpy (low solubility), and CsI has very low hydration enthalpy (low solubility).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
We need to evaluate four statements regarding the properties of alkali metal halides and identify the correct one.
Step 2: Detailed Explanation:
- (A) Among the alkali metal halides, LiF is least soluble in water.
Solubility in water depends on the balance between lattice enthalpy (energy required to break the crystal lattice) and hydration enthalpy (energy released when ions are hydrated). For a substance to dissolve, hydration enthalpy should overcome lattice enthalpy. In LiF, both Li\(^+\) and F\(^-\) are very small ions, leading to an exceptionally high lattice enthalpy. Although its hydration enthalpy is also high, the lattice enthalpy is dominant, making LiF sparingly soluble in water. This statement is correct.
- (B) LiF has least negative standard enthalpy of formation among alkali metal fluorides.
The standard enthalpy of formation (\(\Delta_f H^\circ\)) of an ionic solid is largely determined by its lattice enthalpy. A more negative lattice enthalpy (stronger ionic bond) leads to a more negative \(\Delta_f H^\circ\). Since LiF has the highest lattice enthalpy among alkali fluorides due to the small size of its ions, it has the most negative standard enthalpy of formation, not the least. This statement is incorrect.
- (C) The low solubility of CsI in water is due to its high lattice enthalpy.
In CsI, both Cs\(^+\) and I\(^-\) are large ions. This results in a relatively low lattice enthalpy. The reason for its low solubility is that the hydration enthalpies of these large ions are also very low, and this small amount of released energy is insufficient to break the lattice, even though the lattice energy itself is not high. The statement claims a high lattice enthalpy, which is incorrect.
- (D) The standard enthalpy of formation for alkali metal bromides becomes less negative on descending the group.
On descending the group from Li to Cs, the lattice enthalpy of bromides decreases. This trend generally leads to the standard enthalpy of formation becoming more negative (i.e., the compounds become slightly more stable thermodynamically relative to their elements). For example, \(\Delta_f H^\circ(\text{LiBr}) = -351\) kJ/mol and \(\Delta_f H^\circ(\text{KBr}) = -394\) kJ/mol. The statement says it becomes less negative, which is incorrect.
Step 3: Final Answer:
The only correct statement is (A).