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Question:

The plot of solubility of some salts against temperature is given on the right. A salt like $\ce{NaCl}$ can be recovered fully, when dissolved in a solvent and the solvent is removed by evaporation. Choose the correct statement(s):

  1. No heat is absorbed or released when $\ce{NaCl}$ is dissolved in water.
  2. More heat is released when the ions of $\ce{Yb2(SO4)3}$ are solvated than that absorbed when the ions are separated.
  3. The heat of solution is negative for $\ce{H3BO3}$.
  4. When $\ce{AlCl3}$ is dissolved in water and the water is evaporated, solid $\ce{AlCl3}$ is recovered.

My approach: (1) is clearly correct. (3) and (4) are clearly wrong. My confusion lies with (2).

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When the solubility of a salt decreases with temperature, this indicates that the enthalpy of dissolution is exothermic, i.e. heat is released during this process. This is the case when the lattice energy, which is at least required to separate the ions by breaking up the crystal lattice, is smaller than the energy which is released when the ions are solvated. The solubility curve of ytterbium(III) sulfate therefore proves the statement (B).

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