Sodium Chloride Energetics

Sodium chloride, like virtually all salts, is a more energetically favored configuration of sodium and chlorine than are these elements themselves; in other words, the reaction

is accompanied by a release of energy in the form of heat. How much heat, and why? To help us understand, we can imagine the formation of one mole of sodium chloride from its elements proceeding in these hypothetical steps in which we show the energies explicitly:

Step 1: Atomization of sodium (breaking one mole of metallic sodium into isolated sodium atoms)

Step 2: Same thing with chlorine. This requires more energy because it involves breaking a covalent bond.

Step 3: We strip an electron from one mole of sodium atoms (this costs a lot of energy!)

Step 4: Feeding these electrons to the chlorine atoms gives most of this energy back.

Step 5: Finally, we bring one mole of the ions together to make the crystal lattice — with a huge release of energy

If we add all of these equations together, we get

In other words, the formation of solid sodium chloride from its elements is highly exothermic. As this energy is released in the form of heat, it spreads out into the environment and will remain unavailable to push the reaction in reverse. We express this by saying that "sodium chloride is more stable than its elements".

Looking at the equations above, you can see that Equation 6 constitutes the big payoff in energy. The 787 kj/mol noted there is known as the NaCl lattice energy. Its large magnitude should be no surprise, given the strength of the coulombic force between ions of opposite charge.

It turns out that it is the lattice energy that renders the gift of stability to all ionic solids.Note that this lattice energy, while due principally to coulombic attraction between each ion and its eight nearest neighbors, is really the sum of all the interactions with the crystal. Lattice energies cannot be measured directly, but they can be estimated fairly well from the energies of the other processes described in the table immediately above.