Question

When an ionic solid dissolves in water, which of the processes involved in the solution process require energy? Which release energy?

Short answer

The breaking of ionic bonds in the solid and the breaking of hydrogen bonds in water require energy, while the formation of ion-dipole attractions releases energy.

Step-by-step solution

Identify the processes in dissolving an ionic solid

When an ionic solid dissolves in water, there are three main processes involved: \n 1. Breaking the ionic bonds in the lattice structure of the solid. This process requires energy to overcome the bonds holding the solid together. It's also known as lattice dissociation energy. \n 2. Breaking of hydrogen bonds in water to make room for ions. This process also requires energy, referred to as hydration energy. \n 3. The formation of ion-dipole attraction between the ions and water molecules. This process releases energy, known as the heat of hydration.

Differentiate the energy-requiring processes from the energy-releasing process

The processes that require energy include the breaking of ionic bonds in the solid and the breaking of hydrogen bonds in the water. The process that releases energy is the formation of ion-dipole attractions.

Key concepts

Lattice Dissociation Energy

When we talk about the lattice dissociation energy in an ionic dissolution process, we're discussing the energy needed to break apart the organized structure of the solid ionic lattice. The lattice is a highly ordered and stable arrangement of ions held together by strong electrostatic forces known as ionic bonds. These bonds are the "glue" keeping the ions in their solid formation.
To convert the lattice into individual ions, significant energy must be supplied to overcome these bonds.
This energy demand is what's known as lattice dissociation energy. Let's delve deeper:

• The stronger the ionic bonds, the higher the lattice dissociation energy. This means more energy will be required to break them.
• The size and charge of the ions play a crucial role. Smaller ions and ions with higher charges typically result in stronger ionic bonds.

Understanding lattice dissociation energy helps explain why some salts dissolve readily in water, while others do not.

Hydration Energy

Hydration energy refers to the energy involved in the interaction between ions and water molecules during dissolution. In simpler terms, it's the sum of energies required for the ions to interact with and be surrounded by water molecules.
This process is crucial for the dissociation of ions, and is necessary when the ionic compound dissolves.
Here's how it works:

• To dissolve, ions first need space in the water. This involves breaking some hydrogen bonds between water molecules, requiring energy supply.
• Next, the water molecules orient themselves to surround the ions; this positioning is energetically favorable.

Thus, hydration energy encompasses both energy-consuming and energy-releasing steps, making it a complex but essential part of the dissolving process.

Heat of Hydration

The heat of hydration is released energy when ions form new bonds with water molecules during dissolution. Once ionic bonds in the solid are broken and ions are free in the solution, water molecules quickly surround them, creating what we call ion-dipole interactions.
This bonding results in energy being released, known as the heat of hydration.
Here's why it matters:

• The released energy from ion-dipole formation often compensates for the energy used in breaking the ionic lattice and the hydrogen bonds in water.
• This energy release typically makes the dissolution process feel warm, or, if the lattice and hydration energy perfectly balance each other, the temperature remains relatively constant.

The heat of hydration is crucial because it helps determine whether a solution will form spontaneously and provides insight into the energetics of the dissolving process.