soln H for solid lithium perchlorate (LiClO 4 ) is -26.55 kJmol -1 . When LiClO

Question

solnH for solid lithium perchlorate (LiClO4) is -26.55 kJmol-1. When LiClO4 is dissolved in water:

a. The solution gets HOT, because interactions between H2O and LiClO4 are weaker than those inH2O and LiClO4 alone.

b. The solution gets COLD, because interactions between H2O and LiClO4 are weaker than those inH2O and LiClO4 alone.

c. The solution gets HOT, because interactions between H2O and LiClO4 are stronger than those inH2O and LiClO4 alone.

d. The solution gets COLD, because interactions between H2O and LiClO4 are stronger than those inH2O and LiClO4 alone.

Please give explain your answer

Explanation / Answer

Ans. Correct option- C. The solution gets hot, because interactions between H2O and LiClO4 are stronger than those in H2O and LiClO4 alone.

LiClO4 solvates in water forming hydration shell around the ions Li+ and ClO4-. The hydration shell involves dipole-dipole, ion-dipole, ion-induced dipole, hydrogen bonding, etc. interaction. The greater extent of intermolecular interaction and subsequent stabilization of hydration shell is represented by release of large amount heat (large, negative enthalpy of solvation). Note that greater is the extent and stabilization of intermolecular interactions, greater is the amount of energy released during the interaction.

# Option B, D- Incorrect. The enthalpy of solvation is –ve, it means that heat is released into solution during solvation. Therefore, the resultant solution get hot by absorbing the heat released during salvation.   

# Option A- Incorrect. If homo-molecular interaction between H2O- H2O , and between LiClO4 – LiClO4 alone are greater than hetero-molecular interactions (H2O-LiClO4) in the solution phase, then the solubility of solute is drastically low, i.e. very low or little LiClO4 would be dissolved. Due to poor solubility, and very low intermolecular interactions, very little amount of heat would be released into the solution. In summary, in case of weaker heterotrophic intermolecular interactions, the enthalpy of solvation is usually positive, but NOT large, negative.

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