Explaining Solubility of Group 2 Sulfates & Hydroxides : Part 1

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https://megalecture.com/courses/ In the following lecture, we will discuss the solubility of Group 2 Hydroxides and Sulfates and the factors that will affecg them.

Dissolving a substance is a two step process.

Step 1 : Giant Ionic Lattice of an zionic Compound neess to be broke. Ions have strong electrostatic force that needs to be overcome to free or dissociate the ions in the lattice. Vigorous stirring helps to break this ionic lattice.
NaCl(s) ----- Na+ (g) + Cl (g)
It is an endothermic process and definition wise is the oppositte of Lattice Energy. Smaller ions have stronger lattice hence lattice energy is higher. Down group 2 as cationic size increases, lattice energy decreases.

Step 2 : One ions dissociate, the now bond with water molecules. This is enthalpy of hydration where gaseous ions turn into aqueous ions.
Na+ (g) + (aq) ----- Na+ (aq)
Cl- (g) + (aq) ---- Cl (aq)
Bond formation occurs, hence Enthalpy of Hydration is exothermic and down the group enthalpy of hydration becomes less exothermic as cationic radius increases.

For Hydroxides the solubility increases down the group, so enthalpy of solution becomes more exothermic. Since lattice enthapy and enthalpy of hydration both decrease due to larger cationic size down group 2, this indicates that Lattice Energy decreases more so that the overall Enthalpy of solution becomes exothermic which makes hydroxides more soluble.

For Sulfates the solubility decreases down the group, so enthalpy of solution becomes more endothermic. Since lattice enthapy and enthalpy of hydration both decrease due to larger cationic size down group 2, this indicates that Lattice Energy decreases less so that the overall Enthalpy of solution becomes exothermic which makes hydroxides more soluble down group 2.


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