Question

The stability of the following alkali metal chlorides follows the order

• A. LiCl > KCl > NaCl > CsCl
• B. CsCl > KCl > NaCl > LiCl
• C. NaCl > KCl > LiCl > CsCl
• D. KC1 > CsCl > NaCl > LiCl

Answer 1

The Correct Option is D

Stability of compound depends upon its enthalpy of formation $\Delta H _{ f }$. The more negative value of $\Delta H _{ f }$ shows more stability of a compound. Thus, $KCl$ is more stable and $LiCl$ is least stable. $\Delta H _{ f }$ for $LiCl =-408.8\, kJ\, mol ^{-1}$ $\Delta H _{ f }$ for $NaCl =-412.5\, kJ\, mol ^{-1}$ $\Delta H _{ f }$ for $CsCl =-433\, kJ\, mol ^{-1}$ $\Delta H _{ f }$ for $KCl =-436\, kJ\,mol ^{-1}$

Answer 2

Ans: The enthalpy of formation refers to the change in the enthalpy, this is when one mole of a substance is formed from its element in their standard state and at a given temperature. It is a measure of the energy change during the formation of a compound.

The standard ethalpies of the formation of the alkali metal chlorides are: 

LiCl = -408.8kJmol⁻¹

NaCl= -411.15kJmol⁻¹

CsCl= -433kJmol⁻¹

KCl= -436kJmol⁻¹

The stable compounds have negative enthalpies of formation. The energy released during the formation of the compound from its constituent elements is much lower than the energy which is required to break the bonds in the elements.-ve enthalpy of formation means the compound is favourable to form.