Question

The lattice enthalpy and hydration enthalpy of four components are given below:

Compound	Lattice enthalpy (kJ/mol)	Hydration enthalpy (kJ/mol)
P	+780	-920
Q	+1012	-812
R	+828	-878
S	+632	-600

The pair of compounds which is soluble in water is:
a) P and Q
b) Q and R
c) R and S
d) Q and S
e) P and R

Answer 1

Hint: As we know that, the lattice enthalpy of a crystalline solid is a measure of the energy released when ions are combined to make a compound. Hydration enthalpy is the amount of energy released when one mole of ions undergo hydration.

Complete answer:
As we know that, the lattice enthalpy of a crystalline solid is a measure of the energy released when ions are combined to make a compound. It is a measure of the cohesive forces that bind ions. Lattice energy is relevant to many practical properties including solubility, hardness and volatility. The concept of lattice energy was originally developed for rock salt structured and sphalerite structured compounds like \[NaCl\] and \[ZnS\], where the crystal occupies high symmetry crystal lattice sites.
Hydration enthalpy is the amount of energy released when one mole of ions undergo hydration. Hydration energy is one component in the quantitative analysis of salvation. It is a particular special case of water. The value of hydration energies is one of the most challenging aspects of structural prediction.
If the hydration energy is greater than the lattice energy, then the enthalpy of solution is negative otherwise it is positive.
In the negative case, heat is released and in the positive case heat is absorbed.
In the above problem, the compound is only water soluble when the hydration enthalpy is greater than the lattice enthalpy. Hydration enthalpy is explained about the energy in which compounds get soluble and the lattice enthalpy is explained about the energy in which compound converts into its crystal form. If lattice enthalpy is less than it cannot form crystal. So, in compound P the hydration enthalpy is more than lattice enthalpy so it is soluble in water and same as in compound R also.

Hence, the correct option is (e).

Note: The compound only soluble when hydration enthalpy is greater than lattice enthalpy. Enthalpy is shown by negative and positive signs and this shows the rate of increments and decrements of the enthalpy or we say that about the energy released or absorbed.