Question

Assertion (A): \[{\text{CsI}}\] is the least soluble of all the caesium halides in water.
Reason (R): It is due to low hydration energy.
A.Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B.Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C.Assertion is true but Reason is false
D.Both Assertion and Reason are false

Answer 1

Hint: We know that the hydration energy is dependent on the amount of attraction between ions and water molecules. As the size of caesium ions is very high, so the hydration enthalpy will be very less and the size of iodine ions is also very high, so its hydration enthalpy is also very low.

Complete step by step answer:
We know that from top to bottom the ionic size increases in the periodic table.
Now we know that Caesium is an alkali metal present in the sixth period of the periodic table. So its ionic size will be high.
We also know that the iodine is present in the fifth period and is the part of the halogen family. So it will also have a very high ionic size.
As we know that \[{\text{CsI}}\] is the ionic compound it will dissociate as follows:
\[CsI \to C{s^ + } + {I^ - }\]
Here the formation of caesium cation and iodide anion takes place.
As the dissociation takes place easily that means \[{\text{CsI}}\] has very low lattice energy.
We know that hydration energy is inversely proportional to the size of the ion and direct proportion to the charge on the ion.
Now as the size is high, the hydration energy will be very low.
And as the hydration energy is low so the \[{\text{CsI}}\] is least soluble in the water among all other caesium halides.

Therefore, we can conclude that the correct answer to the question is Option A.

Note: Here we must know that the hydration energy is inversely proportional to the size of the ions and among all halides the iodine has the largest size so it has the lowest hydration energy among all halides.