Problem 19 - Chemistry: The Central Science in SI Units, Global Edition

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?

Step-by-Step Solution

Answer

(a) In Equation 13.1, the lattice energy corresponds to the term \(ΔH_{lattice}\). (b) In this equation, the energy term that is always exothermic is \(ΔH_{hydration}\).

Step 1: Recognizing the Lattice Energy Term

In the equation, we are given two enthalpy terms. The lattice energy is the energy required to separate the ions in an ionic solid, which occurs when the ionic solid dissolves. This corresponds to \(ΔH_{lattice}\), as it represents the energy needed to break the ionic bonds in the solid lattice.

Step 2: Identifying the Exothermic Energy Term

In the given equation, there are two energy terms: 1. \(ΔH_{lattice}\): the energy required to break the ionic bonds in the solid lattice (endothermic). 2. \(ΔH_{hydration}\): the energy released when the ions are hydrated (surrounded by water molecules), which is always exothermic. Therefore, \(ΔH_{hydration}\) is the energy term that is always exothermic. To sum up the answers to the exercise: (a) The lattice energy corresponds to the term \(ΔH_{lattice}\). (b) The energy term that is always exothermic is \(ΔH_{hydration}\).

Key Concepts

Lattice Energy

Lattice energy is a key concept in chemistry, especially when studying ionic compounds. It represents the energy required to separate an ionic solid into its individual ions. Imagine an ionic solid as a tightly packed three-dimensional arrangement of positive and negative ions. To break apart this structure, significant energy is needed, often measured in kilojoules per mole
. The stronger the attraction between the ions, the greater the lattice energy, making the compound more stable.

• The strength of the lattice energy affects the compound's solubility in water. A high lattice energy means the compound is less likely to dissolve.
• Lattice energy is often endothermic, requiring energy input, because it involves breaking attractive forces between ions in the lattice.

Understanding lattice energy helps in determining properties like melting and boiling points and predicting how a compound will behave in solution.

Exothermic Process

An exothermic process is a chemical reaction or physical change that releases energy, typically in the form of heat. These processes occur when the energy needed to break bonds in the reactants is less than the energy released when new bonds form in the products.

• Common examples include combustion reactions, like the burning of gasoline or wood.
• In the context of dissolving ionic solids, exothermic processes involve the release of energy when new interactions are formed.
  

Exothermic reactions often make the surroundings warmer as energy is released into the environment. In dissolution, exothermic processes improve solubility by compensating for the endothermic lattice energy required to break apart the solid.

Hydration Energy

Hydration energy, also known as the heat of hydration, is the energy released when ions become surrounded by water molecules. If you imagine an ion entering water, water molecules arrange themselves around the ion to stabilize it. This process releases energy and is always exothermic.

• Hydration energy is a crucial factor in determining the solubility of ionic compounds in water. Greater hydration energy generally increases solubility.
• It competes with lattice energy; if the hydration energy is sufficient to overcome the lattice energy, the ionic compound will dissolve.

In equation form, hydration energy can be represented as
\[ \Delta H_{hydration} = \sum \Delta H_i \] where \( \Delta H_i \) can include various hydration energies for individual ions. Balancing hydration and lattice energies dictates how easily a compound dissolves.