Discuss the interpretation of the following: (a) fHo (298 K) becomes less negative along the series LiF, NaF, KF, RbF, CsF, but more negative along the series LiI, NaI, KI, RbI, CsI. (b) The thermal stability of the isomorphous sulfates of Ca, Sr and Ba with respect to decomposition into the metal oxide (MO) and SO3 increases in the sequence CaSO4 < srso4=””><>
The Correct Answer and Explanation is :
(a) The trend in the standard enthalpy of formation (( \Delta H_f^\circ (298 K) )) for ionic compounds along the series LiF, NaF, KF, RbF, CsF and LiI, NaI, KI, RbI, CsI reflects the influence of lattice energies and ionization energies. In the case of the alkali metal fluorides (LiF to CsF), the ( \Delta H_f^\circ ) becomes less negative as we move down the group. This is because the ionic radii of the alkali metal ions increase as we go from Li+ to Cs+, and the lattice energy (which is proportional to the inverse of the sum of the ionic radii) decreases. This weaker lattice energy means the ionic bond is less strong, and the formation of the crystal lattice is less exothermic, making ( \Delta H_f^\circ ) less negative.
For the alkali metal iodides (LiI to CsI), the trend reverses, and ( \Delta H_f^\circ ) becomes more negative as we move down the group. This occurs because the iodide ion (I-) is much larger than the fluoride ion (F-), leading to a weaker lattice energy in the case of fluorides. However, in iodides, the ion-dipole interaction and polarizability of the larger iodide ion play a role in stabilizing the lattice as the ionic radii increase. The larger ions allow for better packing, and the stabilization increases with the increasing size of the alkali metal ions, making the enthalpy of formation more negative.
(b) The thermal stability of the sulfates of calcium (CaSO4), strontium (SrSO4), and barium (BaSO4) with respect to their decomposition into the metal oxide (MO) and SO3 follows the trend CaSO4 < SrSO4 < BaSO4. This trend is attributed to the ionic sizes of the metal ions involved. As the ionic radius of the metal ion increases, the lattice energy decreases, making the bond between the metal and the sulfate anion weaker and easier to break. Therefore, BaSO4, with the largest ion (Ba2+), has the highest thermal stability, while CaSO4, with the smallest ion (Ca2+), is the least stable.
The lattice energy is weaker for larger ions, and thus, the decomposition temperature for sulfates decreases as the ionic radius of the metal increases. This trend aligns with the principles of thermodynamics where the stability of a compound is related to the strength of its ionic bonds and lattice energy.
Let me generate the image for this concept.
Here is the visual representation of the trends in the thermal stability and enthalpy of formation for the ionic compounds you mentioned. It illustrates the changes in the enthalpy of formation along the series LiF to CsF and LiI to CsI, as well as the trend in thermal stability for the sulfates of Ca, Sr, and Ba. Let me know if you’d like further details or clarification!
