Question

(a) Is lattice energy usually endothermic or exothermic? (b) Write the chemical equation that represents the process of lattice energy for the case of NaCl. (c) Would you expect salts like NaCl, which have singly charged ions, to have larger or smaller lattice energies compared to salts like CaO which are composed of doubly-charged ions?

Short answer

Lattice energy is usually exothermic, as energy is released upon ion combination. The chemical equation for the lattice energy of NaCl is: \(Na^+(g) + Cl^-(g) \rightarrow NaCl(s) + Lattice \, Energy\). Salts with singly-charged ions, like NaCl, typically have smaller lattice energies compared to salts with doubly-charged ions, like CaO, due to the stronger electrostatic attraction between ions with higher charges.

Step-by-step solution

(a) Lattice energy: endothermic or exothermic?

Lattice energy is the energy required to separate one mole of an ionic compound into its gaseous ions, which is an endothermic process, or the energy released when one mole of an ionic compound is formed from its gaseous ions, which is an exothermic process. Typically, when we refer to lattice energy, we're considering the energy released when the ions come together to form the lattice structure. Therefore, lattice energy is usually exothermic since energy is released upon ion combination.

(b) Chemical equation for lattice energy of NaCl

For the case of NaCl, the chemical equation representing the process of lattice energy can be written as follows: \[Na^+(g) + Cl^-(g) \rightarrow NaCl(s) + Lattice \, Energy\] This equation shows that the gaseous ions, Na⁺ and Cl⁻, come together to form a solid NaCl lattice structure, with the release of lattice energy.

(c) Comparing lattice energies of singly-charged and doubly-charged ions

Lattice energy largely depends on the charges of the ions involved in forming the compound and their sizes. The greater the charges of the ions and the smaller their sizes, the larger the lattice energy. Salts like NaCl, which have singly charged ions (Na⁺ and Cl⁻), typically have smaller lattice energies compared to salts like CaO, which have doubly charged ions (Ca²⁺ and O²⁻). This is because salts with doubly charged ions, such as CaO, have stronger electrostatic attraction between their ions due to their higher charges, leading to a higher lattice energy.

Key concepts

Endothermic Process

An endothermic process is a chemical reaction or physical change that absorbs heat energy from its surroundings. This means that the system takes in energy, and the surroundings cool down. In the context of lattice energy, the process of separating an ionic solid into its gaseous ions requires an input of energy, hence it is endothermic. For instance, when an ionic compound like NaCl is separated into Na⁺ and Cl⁻ ions in the gas phase, energy must be supplied to overcome the strong electrostatic forces holding the ions together. This increase in potential energy corresponds to the lattice energy which must be added for the separation to occur.

It is crucial to understand that an endothermic process does not necessarily mean that the entire system is non-spontaneous or unfavorable. Endothermic reactions can occur spontaneously if the resultant increase in entropy (disorder) of the system and its surroundings compensates for the energy absorbed.

Exothermic Process

Contrary to endothermic processes, an exothermic process releases energy to the surroundings, usually in the form of heat. It results in the warming of the immediate environment. The formation of an ionic compound, such as NaCl from its individual gaseous ions, is an exothermic process because energy is given off when the ions come together to form the solid lattice. This release of energy is directly related to the electrostatic attraction between oppositely charged ions.

During the formation of NaCl, the exothermic reaction can be represented by the chemical equation involving a release of lattice energy. The notion that these reactions are often spontaneous—due in part to the fact that they release energy—can help to develop a deeper understanding of chemical thermodynamics and the factors that contribute to the stability of ionic compounds.

Ionic Compound Formation

Ionic compound formation involves the transfer of electrons from one atom to another, resulting in the creation of positively and negatively charged ions. These ions then attract each other through strong electrostatic forces to form a crystalline lattice. The formation process is typically exothermic due to the release of energy when the ions come together.

For example, in the case of NaCl, a sodium atom (Na) donates an electron to a chlorine atom (Cl), forming Na⁺ and Cl⁻ ions. These ions then attract and arrange themselves into a repeating three-dimensional pattern known as a crystal lattice. In this lattice, each positively charged sodium ion is surrounded by six negatively charged chloride ions, and vice versa, leading to an energetically favorable and stable arrangement.

Chemical Equation Representation

A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants on the left side, the products on the right side, and the direction of the reaction with an arrow. For lattice energy, the chemical equation illustrates the transformation from gaseous ions to the solid lattice, which can include the energy change as part of the equation.

In the case of the lattice energy for NaCl, the chemical equation is as follows: \[Na^+(g) + Cl^-(g) \rightarrow NaCl(s) + Lattice \, Energy\]This equation indicates that gaseous sodium ions (Na⁺) and chloride ions (Cl⁻) react to form solid sodium chloride (NaCl), with the release of energy (lattice energy). The '(g)' denotes that the species are in the gaseous state, while '(s)' signifies a solid. The inclusion of 'Lattice Energy' in the equation highlights that this is an exothermic process, with the energy term conventionally placed on the product side for such reactions.