Dissolution of NACl
Explanation:
Water is a highly ordered material. You made models of parts of the ice/water lattice in class showing that each oxygen atom is connected to others around it through bridging hydrogen atoms (an extreme case of hydrogen bonding). When something dissolves in water, some of these O-H bonds are broken. This requires heat energy. The water molecules can then form attractive interactions to solute ions or molecules, releasing heat energy.
- A crystal of NaCl is also highly ordered. The chloride anions form a cubic close packed lattice and the sodium cations fit into the octahedral holes in the lattice. Strong ionic bonding holds the anions and cations together in the crystal. When NaCl dissolves in water the strong ionic bonds are broken (requiring heat energy) and the ions interact with water molecules (releasing heat energy).
- The solution of NaCl in water has much less order than the pure water and the crystalline salt. Entropy increases every time a solute dissolves in a solvent.
- When NaCl dissolves in water the heat required just about balances the heat released so the temperature of the solution changes very little.
