Question

The lattice energy of sodium chloride, \(\mathrm{NaCl}\), is \(-787.5 \mathrm{kJ} / \mathrm{mol}\) . The lattice energy of potassium chloride, \(\mathrm{KCl}\), is \(-715 \mathrm{k} / / \mathrm{mol}\) . In which compound is the bonding between ions stronger? Why?

Short answer

\text{NaCl} has stronger bonding because it has a more negative lattice energy.

Step-by-step solution

Understand Lattice Energy

Lattice energy is the amount of energy released when ions are combined to form a crystalline lattice. It is an indicator of the strength of the bonding between ions in an ionic compound.

Compare Lattice Energies

Compare the lattice energies of \(\text{NaCl}\) and \(\text{KCl}\). \(\text{NaCl}\) has a lattice energy of \(-787.5 \text{ kJ/mol}\) while \(\text{KCl}\) has a lattice energy of \(-715 \text{ kJ/mol}\).

Determine the Stronger Bonding

The more negative the lattice energy, the stronger the bonding between the ions. Since \(\text{NaCl}\) has a more negative lattice energy \(-787.5 \text{ kJ/mol}\) compared to \(\text{KCl}\) \(-715 \text{ kJ/mol}\), the bonding between ions in \(\text{NaCl}\) is stronger.

Key concepts

ionic bonding

Ionic bonding is the force of attraction between oppositely charged ions. In ionic compounds such as \(\text{NaCl}\) and \(\text{KCl}\), atoms transfer electrons to achieve a full outer shell of electrons. Sodium (Na) loses an electron to become \(\text{Na}^+\) while Chlorine (Cl) gains an electron to become \(\text{Cl}^-\). This transfer of electrons creates ions which are then attracted to each other due to their opposite charges. This strong electrostatic attraction forms what we call an ionic bond.
Ionic bonds are generally formed between metals and non-metals. Metals tend to lose electrons and become positively charged cations, while non-metals gain these electrons to become negatively charged anions. The strength of these bonds can be evaluated by looking at the lattice energy of the compounds, which indicates how strong the bonds between the ions are in the crystalline structure.

crystalline lattice

A crystalline lattice is the ordered arrangement of ions in an ionic compound. This structure results due to the repetitive pattern in which ions arrange to balance the opposite charges effectively. Each positive ion is surrounded by negative ions and vice versa, which stabilizes the structure. For example, in the crystalline lattice of \(\text{NaCl}\), each sodium ion (\(\text{Na}^+\)) is surrounded by six chloride ions (\(\text{Cl}^-\)) and each chloride ion is surrounded by six sodium ions.
This neat, orderly structure is essential for the stability and strength of an ionic compound. The lattice energy is a measure of this stability; it represents the amount of energy released when the ions are brought together from an infinite distance to form the compound. A higher (or more negative) lattice energy typically means the crystalline lattice is more stable and the ionic bonds are stronger.

bond strength

Bond strength in ionic compounds is primarily determined by the lattice energy. The more negative the lattice energy, the stronger the bond between the ions. This is because more energy is released when the ions come together, indicating a stronger attraction. For instance, the lattice energy of \(\text{NaCl}\) is \(-787.5 \text{kJ/mol}\), which is more negative compared to the lattice energy of \(\text{KCl}\), which is \(-715 \text{kJ/mol}\). This tells us that the ionic bonds in \(\text{NaCl}\) are stronger than those in \(\text{KCl}\).
Several factors affect lattice energy including:

• Ion Size: Smaller ions can get closer together, which increases the electrostatic attraction and thus the lattice energy.
• Ion Charge: Higher charged ions have stronger attractions, increasing the lattice energy.
  In summary, \(\text{NaCl}\) has stronger bonds due to its more negative lattice energy, indicating a more stable crystalline structure and stronger ionic bonding between the ions.