definition of lattice energy
Answers
Lattice Energy is a type of potential energy that may be defined in two ways.
In one definition, the lattice energy is the energy required to break apart an ionic solid and convert its component atoms into gaseous ions. This definition causes the value for the lattice energy to always be positive, since this will always be an endothermic reaction.
The other definition says that lattice energy is the reverse process, meaning it is the energy released when gaseous ions bind to form an ionic solid. As implied in the definition, this process will always be exothermic, and thus the value for lattice energy will be negative. Its values are usually expressed with the units kJ/mol.
Lattice energy is a type of potential energy that relates to the stability of ionic solids. Ionic solids are very stable, which means that it takes a lot of energy to break their bonds. One place we can see these effects is in their melting point. You may have once used a recipe that calls for melting sugar, which is not an ionic solid. However, table salt, which looks very similar, would never melt at a temperature suitable for cooking. This is because table salt, or sodium chloride, is an ionic solid with incredibly stable bonds. You need an enormous amount of energy to break those bonds so salt is difficult to melt. When melting sugar, however, you only have break weak interactions between the sugar molecules.
The type of energy we're discussing is called lattice energy because it refers to ionic solids where the ions are arranged in what is known as a crystal lattice.
Arrangement of ions in the crystal lattice of sodium chloride (NaCl)
image by Eyal Bairey, public domain, wikimedia commons
Lattice energy is also known as lattice enthalpy and can be stated in two ways. One way is the energy released when gaseous ions combine to form an ionic solid. The other is the energy required to transform an ionic solid into its component gaseous ions. Either way, we can see that lattice energy is a measure of the forces between the ions in an ionic solid.
To avoid confusion, we can say that the lattice energy required to break the bonds in an ionic solid and form gaseous ions is the lattice dissociation enthalpy, and the energy released when gaseous ions bind to make an ionic solid is the lattice formation enthalpy.
Ionic solids are always more stable than gaseous ions, meaning that ionic solids are in a lower energy state than gaseous ions. This means that the lattice dissociation enthalpy will always be a positive number, and the lattice formation enthalpy will always be a negative number.
Let's consider the lattice energy of sodium chloride. Sodium chloride is very stable and requires 788 kJ to entirely convert one mole of it into gaseous ions. Thus, we can say that the lattice dissociation enthalpy is +788 kJ/mol and the lattice formation enthalpy is -788 kJ/mol. Overall, sodium chloride's lattice energy is 788 kJ/mol.
The more stable the ionic solid, the larger the lattice energy will be. This is because the more stable bonds are, the harder they are to break. This is like how you'd need more energy to break apart a sturdy chair than a flimsy one.