Question

Why is the formation of solid sodium chloride from solid sodium and gaseous chlorine exothermic, even though it takes more energy to form the \(\mathrm{Na}^{+}\) ion than the amount of energy released upon formation of \(\mathrm{Cl}^{-} ?\)

Short answer

The formation of solid sodium chloride (NaCl) from sodium (Na) and chlorine (Cl_2) is exothermic because the large amount of energy released during the formation of the ionic lattice (lattice energy) exceeds the energy absorbed to form the Na+ and Cl- ions.

Step-by-step solution

Understand the Process of Ionic Bond Formation

The formation of sodium chloride (NaCl) from sodium (Na) and chlorine (Cl_2) involves several steps. First, sodium atoms must lose an electron to form Na+ ions, which is an endothermic process requiring energy. Second, the chlorine molecule must undergo homolytic cleavage to form two chlorine atoms, which is also an endothermic process. Lastly, when the Na+ and Cl- ions come together to form the ionic lattice of sodium chloride, a large amount of energy is released. This step is exothermic.

Compare Energy Changes

To determine the overall energy change, we compare the total energy taken in during the endothermic steps to the energy released in the exothermic step. The energy required to remove an electron from a sodium atom and the energy required to break the Cl_2 molecule into two Cl atoms are outweighed by the energy released when the positive Na+ and negative Cl- ions attract each other to form the solid ionic lattice.

Examine the Lattice Energy

The key factor in the exothermic nature of the reaction is the lattice energy, which is the energy released when the ions form a solid lattice. The lattice energy is typically very large due to the strong electrostatic attraction between the oppositely charged ions in the lattice, and it more than compensates for the energy required to form the individual ions.

Conclude the Exothermic Nature

Overall, even though energy is absorbed to create the Na+ and Cl- ions individually, the process of sodium chloride (NaCl) formation is exothermic because the energy released during the formation of the ionic lattice (lattice energy) is far greater than the energy absorbed during the formation of Na+ and Cl- ions.

Key concepts

Ionic Bond Formation

Understanding the formation of an ionic bond is essential to grasp the chemical processes that lead to the creation of common compounds like sodium chloride (NaCl). Ionic bonds are formed when electrons are transferred from one atom to another, which typically occurs between metals and non-metals. In this case, sodium (Na), a metal, loses an electron to become a positively charged ion, Na+. Chlorine (Cl₂), a non-metal, undergoes a reaction whereby a molecule of Cl₂ splits into two Cl atoms, and each atom gains an electron to become negatively charged Cl- ions.

This electron transfer results in the formation of ions with opposite charges. They naturally attract each other due to electrostatic forces. Once formed, these oppositely charged ions adhere to a precise, repeating pattern known as a crystal lattice. The completion of this structure marks the formation of an ionic compound, showcasing a pivotal exchange of electrons leading to a stable ionic bond.

Lattice Energy

Lattice energy is a critical concept when discussing ionic compounds like sodium chloride. It represents the energy released when ions bond together to form a solid lattice structure. The solid lattice is highly organized and maximizes the attraction between oppositely charged ions.

Think of lattice energy like the strong glue holding the ions together. It is a measure of the strength of the bonds that form the crystal lattice of an ionic solid. In the sodium chloride example, after Na loses an electron and Cl gains an electron, the resulting Na+ and Cl- ions are brought together. The force of attraction between these opposite charges is so strong that, when they form the ionic solid, a significant amount of energy is released.

This release of energy is substantial because it is proportional to the product of the charges of the ions and inversely proportional to the distance between the ions. Therefore, smaller ions or ions with higher charges will usually have higher lattice energies.

Exothermic Reaction

The formation of sodium chloride is a classic example of an exothermic reaction, where the system releases energy to its surroundings. This can often be felt as heat. An exothermic process contrasts with an endothermic process, which absorbs energy and typically feels cold to the touch.

During the creation of sodium chloride, the total energy needed to ionize sodium and to break apart the chlorine molecules into atoms (both endothermic processes) is less than the energy released when these ions come together to form the solid lattice (an exothermic process). The overall reaction results in a net release of energy, which is why the process can be described as exothermic, becoming energetically more favorable as the ionic solid forms.

This energy release is primarily due to the high lattice energy that was described earlier. As Na+ and Cl- form a stable ionic lattice, the lattice energy overcomes the energy required to ionize sodium and cleave chlorine molecules, thereby demonstrating an exothermic reaction in this chemical process.