Question

Draw a picture of solid NaCl, and describe what kinds of interaction are present in NaCl.

Draw a picture of NaCl after it dissolved in water, and describe what kinds of interaction are present in NaCl(aq).

What interactions had to be overcome to form this solution? (Nacl)Was this process exothermic or endothermic?

What interactions had to be formed to form this solution? (Nacl)Was this process exothermic or endothermic?

Answer 1

Sodium chloride is formed when sodium atoms interact with chlorine atoms. When this occurs, sodium will donate an electron (which is a negatively-charged particle) to chlorine. This makes sodium slightly positive and chlorine slightly negative. In solid chloride the bond between sodium and chlorine is ionic bond.

If we dissolve in water, the solution contains sodium ions (Na⁺) and chloride ions (Cl^-), rather than neutral units of NaCl. In water oxygen will have partially negative charge and hydrogens will have partially positive charge. So oxygens will attract Na⁺ ions and hydrogens will attract Cl^- ions.

If we dissolve NaCl in water and if it fully dissolves (that is, the ions all become evenly dispersed within the water), two exchanges of energy need to happen:

1. Lattice Energy: Energy is required to the solution to pull the ions away from each other: in order to pull the positively and negatively charged ions apart, energy must be added. This energy needed to pull the ions apart is called the Lattice Energy.
2. Hydration Energy:  Energy is required to solution when the water molecules surround the ions: as the water molecules are attracted to and surround the ions, energy is released into the solution. This energy released as water molecules surround the ions is called the Hydration Energy.
   
   exothermic or endothermic depends on lattice energy and hydration energy, which is greater, the Lattice Energy, or the Hydration Energy.

For example, if we dissolve NaCl in water the Lattice Energy is 779 kJ/mol, and the Hydration Energy is 774 kJ/mol (1). If we subtract the Hydration Energy from the Lattice Energy, we get a change of +5 kJ/mol:

779 kJ/mol – 774 kJ/mol = +5 kJ/mol

means it requires just slightly more energy to separate the ions from one another than is released from the water molecules surrounding the ions. This means just slightly more energy must be put into the solution than is released back into the solution; therefore dissolving table salt in water is endothermic.