Question

(a) In Equation 13.1 which of the enthalpy terms for dissolving an ionic solid would correspond to the lattice energy? (b) Which energy term in this equation is always exothermic?

Short answer

(a) In Equation 13.1, the lattice energy term corresponds to ΔH_1. (b) The energy term in Equation 13.1 that is always exothermic is ΔH_3 (solvation energy).

Step-by-step solution

Recall Equation 13.1

Equation 13.1 represents the dissolving of an ionic solid, and it is given as follows: ΔH = ΔH_1 + ΔH_2 + ΔH_3 ΔH_1: Represents the energy required to break the ionic solid into its constituent ions (Lattice energy) ΔH_2: Represents the energy required to overcome the forces holding the solvent molecules together (Solvent separation energy) ΔH_3: Represents the energy released when the ions are solvated, i.e., the energy change upon the interaction between solute ions and solvent molecules (Solvation energy)

Identify the lattice energy term

Given the components of Equation 13.1, we can now identify the lattice energy term. The lattice energy corresponds to the energy required to break the ionic solid into its constituent ions. In Equation 13.1, this is represented by ΔH_1. (a) The lattice energy term in Equation 13.1 is ΔH_1.

Determine the exothermic energy term

Next, we need to identify which energy term in Equation 13.1 is always exothermic. An exothermic process is one in which energy is released. Among the terms in Equation 13.1: - ΔH_1 (lattice energy) is endothermic since energy is required to break the ionic solid. - ΔH_2 (solvent separation energy) is also endothermic since energy is needed to overcome the forces holding the solvent molecules together. Since energy is released when the ions are solvated (when solute ions interact with the solvent molecules), we can conclude that: (b) The energy term in Equation 13.1 that is always exothermic is ΔH_3 (solvation energy).

Key concepts

Lattice Energy

Lattice energy is a key concept in the study of ionic compounds. It refers to the amount of energy required to separate one mole of an ionic solid into its constituent ions in the gas phase. This process is endothermic, meaning it requires energy input. In other words, energy must be provided to break apart the ionic lattice structure due to the strong electrostatic forces binding the ions together. Understanding lattice energy helps in predicting the stability of ionic compounds and their behaviors in various processes. For example, compounds with high lattice energy tend to have higher melting and boiling points because more energy is needed to overcome the ionic bonds. Lattice energy is influenced by:

• The charges of the ions: Higher charges lead to a greater lattice energy due to stronger attractions.
• The size of the ions: Smaller ions can pack more closely together, resulting in a higher lattice energy.

Equation 13.1 from the original exercise lists lattice energy as ΔH_1, the energy term necessary to break an ionic compound into ions.

Solution Process

The solution process involves the mixing of solute and solvent to form a homogeneous mixture, known as a solution. When an ionic solid dissolves in a solvent, several energy changes take place, represented in Equation 13.1 by the terms ΔH_1, ΔH_2, and ΔH_3. Understanding each energy term helps to grasp how dissolving occurs:

• ΔH_1: Refers to the lattice energy, necessitating energy to dissociate the ionic compound's lattice.
• ΔH_2: The energy used to separate solvent molecules, also an endothermic process since energy is required to break the interactions between solvent molecules, such as hydrogen bonds or van der Waals forces.
• ΔH_3: Solvation energy, which describes the exothermic process where energy is released as solute ions interact with solvent molecules, stabilizing the ions in the solution.

The overall energy change in the solution process, ΔH, is the sum: ΔH = ΔH_1 + ΔH_2 + ΔH_3. Whether the solution process is endothermic or exothermic overall depends on these energy balance interactions.

Enthalpy Changes

Enthalpy changes refer to the heat absorbed or released during a chemical process at constant pressure. In thermochemistry, it serves as an important tool to measure the energetic aspects of chemical reactions and physical changes. During the dissolution of an ionic solid, the enthalpy changes are captured by ΔH_1, ΔH_2, and ΔH_3 in Equation 13.1:

• ΔH_1 (Lattice Energy): Represents the endothermic input required to break the ionic bonds within a solid.
• ΔH_2 (Solvent Separation Energy): Another endothermic change, taking energy to overcome solvent interactions.
• ΔH_3 (Solvation Energy): Represents an exothermic change; energy is released when ions are solvated by solvent molecules.

The net enthalpy change, ΔH, determines whether the solution process overall absorbs heat (endothermic) or releases heat (exothermic). In the original exercise, we identify ΔH_3 as always exothermic, as it consistently releases energy when ions and solvent molecules interact, hinting at the natural tendency of systems to seek lower-energy, more stable states.