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

Let's analyze each statement to determine the correct statement regarding alkali metal halides:

1. The statement "Among the alkali metal halides, LiF is least soluble in water." is correct. Lithium fluoride (LiF) is known to have very low solubility in water compared to other alkali metal fluorides. This is attributed to its high lattice energy due to the small size of the lithium ion.
2. Consider the statement "LiF has least negative standard enthalpy of formation among alkali metal fluorides." This statement is incorrect. LiF has a more negative enthalpy of formation due to its strong ionic bonds, which is typical when smaller cations and anions are involved.
3. The statement "The low solubility of CsI in water is due to its high lattice enthalpy." is also incorrect. The solubility of a salt in water is influenced by its lattice enthalpy and hydration enthalpy. In the case of CsI, the lattice enthalpy is relatively low because of the large size of the ions involved, making this statement incorrect. The low solubility is more a factor of low hydration energy than high lattice energy.
4. Lastly, the statement "The standard enthalpy of formation for alkali metal bromides becomes less negative on descending the group." This statement is not true. As one descends the group, the size of the cations increases, generally resulting in less exothermic standard enthalpies of formation due to the lower lattice energies of the resulting bromides.

Based on the analysis, the correct statement is: "Among the alkali metal halides, LiF is least soluble in water." because of its high lattice energy resulting from the small size of the lithium ion.