E, 3. Solubilities of alkaline-earth cations. a. Solubility of the hydroxides. P
ID: 1043104 • Letter: E
Question
E, 3. Solubilities of alkaline-earth cations. a. Solubility of the hydroxides. Place 10 drops of 0.1 M MgCl2, 0.1 M CaCl2, and netal ed in um foil 0.1 M Sr(NO)2 in three separate, clean test tubes. Count and add drops of 0.050 M NaOH until a cloudiness appears in each test tube. Predict the trend in the solubility of the hydroxides of the Group 2A cations. b. Solubility of the suljates. Place 10 drops of 0.1 M MgCl2, 0.1 M CaCl, and 0.1 M Sr(NOs)2 in three separate, clean test tubes. Count and add drops of 0.10 M Na,SO, until a cloudiness appears in each test tube. Predict the trend in the solubility of the sulfates of the Group 2A cations.Explanation / Answer
3. Solubilities of hydroxide and sulfates of group 2A cations depends on the two key concept: Lattice Enthalpy & Hydration Enthalpy.
Lattice Enthalpy: Energy released when one mole of ionic compound formed from its gaseous constituents under normal condition.
Hydration enthalpy: Energy released when one mole of gaseous ions is completly surrounded by water molecule under normal condition. Smaller the size of ion,more will be hydration and more will be solubility.
a) Solubilities of hydroxides of Group 2A cations (Mg2+, Ca2+& Sr2+) increases as we move down the group .(means from magnesium to strontium)because as we move down the group 2, cation size increases . Due to this, distance between the cation and -OH ion also increases and by defination lattice enthalpy decreases. Hydration enthalpy also decreases since its inversly proportional to the size of cation. However, decrease in lattice enthalpy is far greater than the decrease in hydration enthalpy.thats why the solubility of cation 2A hydroxide increases down the group.
b) Similarly in this case also,Lattice enthalpy and Hydration enthalpy both are decreasing as we move down the group 2 but due to shape and ionic charge of sulfate ion SO42- ,lattice decrease is much slower than group 2 Hydroxides.while hydration enthalpy remains the same as in previous trend .therefore ,decrease in hydration enthalpy is much greater in this case than the lattice enthalpy decrease.That's why solubilities of sulfate of Group 2A decreases down the group.
c) In Experimental procedure discussed above,tests used to define the periodic trend in the chemical properties of alkaline earth metal ions are solubility tests in water of hydroxides and sulfates of alkaline earth metals.
Test Observations that leads to the conclusion of trend :
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