Question

Define the crystallization enthalpy of a substance. How is it related to the substance's fusion enthalpy?

Short answer

Crystallization enthalpy is the negative of fusion enthalpy.

Step-by-step solution

Understand Crystallization Enthalpy

Crystallization enthalpy is the change in enthalpy when one mole of a substance transitions from its liquid state to its solid, crystalline state. It is an exothermic process since energy is released during the formation of the crystal lattice.

Understand Fusion Enthalpy

Fusion enthalpy refers to the enthalpy change when one mole of a substance transitions from its solid state to a liquid state. It is an endothermic process because energy is required to break the orderly structure of the solid so that it can become a liquid.

Relationship Between Crystallization and Fusion Enthalpy

The fusion enthalpy and crystallization enthalpy are equal in magnitude but opposite in sign. This means that the amount of energy absorbed during fusion is equal to the energy released during crystallization: \( \Delta H_{fusion} = -\Delta H_{crystallization} \).

Key concepts

Fusion Enthalpy

Fusion enthalpy, also known as the heat of fusion, is the amount of energy required to change one mole of a substance from the solid phase to the liquid phase at constant pressure and temperature. It symbolizes the energy needed to overcome the forces that hold the molecules together in a solid structure. This process is endothermic, meaning that it absorbs energy from the surroundings.To think of it in everyday terms, imagine melting an ice cube. The energy (often in the form of heat) needed to melt the ice doesn't just disappear. Instead, it is used to break the attractive forces between the water molecules, allowing them to move more freely in the liquid state.Some key aspects to remember are:

• The fusion enthalpy is specific to each substance, meaning different materials require different amounts of energy to change states.
• Units are generally expressed in kilojoules per mole (kJ/mol).
• Since it is an endothermic process, the enthalpy change (\( \Delta H \)) will have a positive value.

It's important to understand that the magnitude of the fusion enthalpy is vital for determining how a material behaves during phase changes.

Exothermic Process

An exothermic process is a chemical reaction or physical change that releases energy by light or heat into its surroundings. Exothermic reactions are observed in everyday phenomena like combustion, where energy is emitted as heat and light.Consider crystallization, for example. This process involves a liquid transforming into a solid and releasing heat during the formation. When water freezes into ice, the water molecules become orderly arranged in a crystalline structure, releasing energy in the form of heat to the surroundings.Key points to consider about exothermic processes include:

• They typically result in a temperature increase in the surrounding area.
• The enthalpy change for these processes (\( \Delta H \)) is negative because energy is released.

Exothermic processes are crucial in everyday applications such as heating systems and certain industrial reactions, making our lives warmer and more comfortable.

Endothermic Process

Endothermic processes are the opposite of exothermic ones. In an endothermic process, energy is absorbed from the surroundings, leading to a decrease in the external temperature. A common example is the process of fusion that we've discussed earlier.During fusion, heat is absorbed to provide enough energy for a substance to transition from a solid to a liquid state. This energy absorption helps to break down the structured lattice of the solid, allowing the molecules to move freely in a liquid.Notable considerations for endothermic processes:

• The surroundings usually cool down as the process absorbs heat.
• The enthalpy change (\( \Delta H \)) is positive, reflecting the energy intake.

Such processes are integral to many natural phenomena, driving actions like photosynthesis in plants and even some cooking practices where heat is absorbed to cook food. Understanding endothermic processes gives insights into how energy transformations occur in both natural and artificial environments.