Chapter 15, Problem 3A

Interpretation Introduction

Interpretation: The dissolution of ionic compounds in water and how positive and negative ions are prevented from getting recombined need to be explained

Concept introduction: Ionic substances are soluble in water and hence they are called electrolyte. Depending on nature of electrolyte (which element forms cation and anion) the extent of solubility varies.

Answer to Problem 3A

An ionic compound dissolves in water by the virtue of ionic interaction of ions and water. Water molecules wrap the ions and thus prevent the ions from getting recombined.

Explanation of Solution

When an ionic compound dissolves in water, the ions leave their ordered arrangement of crystal lattice. The energy needed to break the ions from 1 mol ionic compound from the crystal lattice is called lattice energy. After the ions get dispersed in water, it undergoes hydration with water molecules and the energy released when 1 mol of ionic compound undergoes hydration is called enthalpy of hydration.

When enthalpy of hydration is greater than lattice energy the ionic compound dissolves in water. Conversely, we can say when ion-water interaction is stronger than ion-ion and water-water interaction the solubility of ionic compound is favored.

Due to electronegativity difference of oxygen and hydrogen a dipole is generated in water molecule where oxygen acquires partial negative charge and hydrogen acquires partial positive charge. When ions dissolves in water, negative ion are attracted to positive end of hydrogen and positive ions are attracted to negative end of oxygen. Hence, the water molecule makes a shield around ions which prevents their recombination again into solid ionic compound.

Conclusion

An ionic compound dissolves in water by the virtue of ionic interaction of ions and water. Water molecules wrap the ions and thus prevent the ions from getting recombined.