Problem 21 - Chemistry The Central Science In Si Units

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 \(\mathrm{CaO}\) which are composed of doubly-charged ions?

Step-by-Step Solution

Answer

(a) Lattice energy is usually exothermic. (b) \( \text{Na}^+ (g) + \text{Cl}^- (g) \rightarrow \text{NaCl} (s) \). (c) \( \text{CaO} \) has a larger lattice energy than \( \text{NaCl} \).

Step 1: Understanding Lattice Energy

Lattice energy is the energy required to separate one mole of an ionic compound into its gaseous ions. It is usually exothermic, meaning it releases energy during the formation of the lattice. This is because energy is released when the oppositely charged ions attract each other and form a stable ionic structure.

Step 2: Writing the Chemical Equation for NaCl Lattice Energy

The lattice energy of NaCl can be represented by the formation of NaCl from its gaseous ions. The chemical equation for this process is: \[ \text{Na}^+ (g) + \text{Cl}^- (g) \rightarrow \text{NaCl} (s) \] This equation shows the combination of gaseous sodium cations and chloride anions to form solid sodium chloride, releasing energy in the process.

Step 3: Comparison of Lattice Energies in Ionic Compounds

Lattice energy is influenced by the charge and size of the ions. Doubly-charged ions, like in \( \text{CaO} \), usually have stronger ionic interactions compared to singly-charged ions like \( \text{NaCl} \). This leads to higher lattice energies for compounds like \( \text{CaO} \) because the electrostatic attractions between ions are stronger due to the higher charges.

Key Concepts

Ionic Compounds

Ionic compounds are formed from the attraction between positively and negatively charged ions. This bond is known for its strength and is the result of the transfer of electrons between atoms, leading to a stable electronic configuration. Typically, this happens between metals and non-metals. The metal loses electrons to become positively charged, while the non-metal gains those electrons to become negatively charged. Ionic compounds have several characteristics:

• They form crystalline solids at room temperature.
• They have high melting and boiling points due to the strong forces holding the ions together.
• They dissolve in water and conduct electricity when molten or in solution, as their ions are free to move.

Understanding these properties helps explain why these compounds are so stable. The orderly arrangement of ions is what contributes to this high stability and strength.

Chemical Equation

A chemical equation is a symbolic representation of a chemical reaction. It uses symbols and formulas to depict the reaction between substances, indicating the reactants and products involved in a chemical change. For lattice energy, the chemical equation demonstrates the conversion of gaseous ions into a solid ionic compound, showcasing energy release. In the case of sodium chloride (NaCl), the equation \[ \text{Na}^+ (g) + \text{Cl}^- (g) \rightarrow \text{NaCl} (s) \]represents the formation of solid NaCl from sodium and chloride ions in their gaseous states. This equation illustrates not just the formation of the compound but also the exothermic nature of lattice energy, where energy is released as the product forms.

Electrostatic Attractions

Electrostatic attractions are the forces that hold ions together in an ionic compound. These attractions arise because of the opposite charges on ions. The strength of this force is a primary factor in determining the properties of ionic compounds, including their lattice energy.Stronger electrostatic attractions occur in ionic compounds with:

• Doubly-charged ions, such as in \( \text{CaO} \), resulting in higher lattice energies as compared to compounds with singly-charged ions like \( \text{NaCl} \).
• Smaller ions, which allow closer proximity, leading to stronger interactions.

Such attractions are fundamental to the formation and stability of the compound's lattice, impacting their physical characteristics and the energy dynamics during formation.