Question

What do we call the energies required, respectively, to melt and to vaporize 1 mole of a substance? Which of these energies is always larger for a given substance? Why?

Short answer

The energies required to melt and vaporize 1 mole of a substance are called Molar Enthalpy of Fusion (ΔHfus) and Molar Enthalpy of Vaporization (ΔHvap) respectively. For a given substance, ΔHvap is always larger than ΔHfus because vaporization requires overcoming the intermolecular forces completely, while fusion only involves weakening these forces.

Step-by-step solution

Define the energies required to melt or vaporize 1 mole of a substance

The energy required to melt one mole of a substance is called the Molar Enthalpy of Fusion, denoted as ΔHfus. The energy required to vaporize one mole of a substance is called the Molar Enthalpy of Vaporization, denoted as ΔHvap.

Compare the energies and identify which is larger

For a given substance, the Molar Enthalpy of Vaporization (ΔHvap) is always larger than the Molar Enthalpy of Fusion (ΔHfus).

Explain why the Molar Enthalpy of Vaporization is larger than the Molar Enthalpy of Fusion

The reason ΔHvap is larger than ΔHfus is due to the differences in the phase changes. When a substance changes from a solid to a liquid during fusion, the intermolecular forces holding the particles together are weakened, but they still remain to a certain extent. However, when a substance changes from liquid to vapor during vaporization, the intermolecular forces are completely overcome, and the particles are not held together anymore. As a result, separating particles in a liquid to form a vapor requires more energy than breaking the force connections in a solid to form a liquid, which is why ΔHvap will always be larger than ΔHfus for any given substance.

Key concepts

Molar Enthalpy of Fusion

Molar Enthalpy of Fusion refers to the amount of energy required to convert one mole of a solid substance into a liquid at constant temperature and pressure. This is a critical value because it signifies the energy needed to overcome the intermolecular forces that hold the molecules together in the solid state.

During the melting process, the molecules within the solid gain energy, which increases their movement. This increased movement weakens the intermolecular forces, allowing the solid to change into a liquid. However, these forces are not entirely disrupted; they are merely less rigid than in the solid form.

Key points to remember about Molar Enthalpy of Fusion include:

• It is denoted by $abla{H}_{extfus}$.
• It's typically measured in kilojoules per mole (kJ/mol).
• The $abla{H}_{extfus}$ for a substance can vary significantly with the strength of the intermolecular forces present within that substance.

Understanding the Molar Enthalpy of Fusion is essential as it provides insights into the stability and properties of solids, and their transformations into liquids.

Molar Enthalpy of Vaporization

The Molar Enthalpy of Vaporization is the energy necessary to change one mole of a liquid into its vapor phase at constant temperature and pressure. This concept is key to understanding the amount of energy needed to overcome the forces holding molecules in the liquid.

When a substance undergoes vaporization, the energy supplied is used to completely overcome these intermolecular forces. As a result, molecules are no longer held together and can move freely as a vapor. This energy requirement explains why vaporization generally involves more energy than melting.

Essential facts about Molar Enthalpy of Vaporization:

• It is symbolized as $abla{H}_{extvap}$.
• This quantity is usually higher than Molar Enthalpy of Fusion due to the greater energy needed to break intermolecular attractions completely.
• Like fusion, vaporization values are expressed in kJ/mol.

By analyzing the Molar Enthalpy of Vaporization, one can infer the strength of the interactions within the liquid and the energy contributions required for the substance to enter the vapor state.

Phase Changes

Phase changes refer to the transformations between different states of matter: solid, liquid, and gas. These transformations involve energy changes as substances absorb or release heat to rearrange their molecular structures.

Two important phase changes include:

• **Fusion:** The process of changing from a solid to a liquid. Here, the substance reaches its melting point, requiring energy to weaken but not completely break intermolecular forces.
• **Vaporization:** The transition from liquid to gas, which requires additional energy to completely overcome molecular attractions, resulting in free-moving particles.

During these changes, the temperature of the substance remains constant even though energy is being absorbed or released. The energy changes are instead used to alter the molecular structure, explaining why certain substances have higher enthalpy requirements based on their phase change.

Understanding phase changes is crucial for grasping how different states of matter interact and the thermodynamic principles that govern these transformations. Overall, these concepts not only define the nature of substances and their interactions but also provide insights into energy transfer within chemical processes.