Question

When benzene is at its melting point, two processes occur simultaneously and balance each other. Describe these processes on the macroscopic and molecular levels.

Short answer

Melting and freezing processes occur and balance each other at both macroscopic and molecular levels, maintaining benzene at its melting point.

Step-by-step solution

- Identify the Melting Point

The melting point is the temperature at which a solid turns into a liquid. For benzene, this temperature is approximately 5.5 °C (41.9 °F).

- Understand the Macroscopic Processes

At the melting point, benzene exists in both solid and liquid forms. The macroscopic processes are melting (solid to liquid) and freezing (liquid to solid) occurring simultaneously. These two processes balance each other, maintaining a constant temperature.

- Analyze the Molecular Level

On the molecular level, during melting, benzene molecules absorb heat energy and overcome their fixed positions in the solid structure, transitioning into a more fluid, liquid state. Simultaneously, during freezing, some molecules in the liquid lose energy and re-form the structured, lattice arrangement of a solid.

- Balance of Energy Exchange

At the melting point, the energy absorbed by molecules to break free from the solid structure (melting) is equal to the energy released by molecules forming the solid structure from the liquid (freezing). This balance keeps the temperature constant.

Key concepts

Understanding Phase Change in Benzene

The concept of a phase change is crucial to understanding how substances transition between solid, liquid, and gas states. For benzene, the melting point is a key phase change where it converts from solid to liquid. When benzene reaches around 5.5 °C (41.9 °F), it undergoes this transition. During a phase change, the temperature remains constant as the system absorbs or releases energy to alter the state of the substance. In the case of melting, benzene molecules absorb energy to move from a fixed, orderly structure (solid) to a more fluid, disordered one (liquid). This input of energy disrupts the bonds holding the molecules in place. The phase change continues until all of the solid has melted into liquid.

Energy Exchange in Benzene's Melting

Energy exchange is a vital part of understanding the melting process. When benzene reaches its melting point, it requires a precise amount of heat energy known as the enthalpy of fusion. This energy is used to overcome the intermolecular forces that keep benzene molecules in a solid-state. During this process, no temperature change occurs, despite the continuous absorption of heat. Instead, all the absorbed energy goes into breaking the solid's structure. On the flip side, during freezing at the same temperature, benzene molecules release heat energy as they reassemble into a solid lattice. This release of energy is equal to the energy absorbed during melting, maintaining a balance and keeping the temperature steady.

Solid-Liquid Equilibrium at Benzene's Melting Point

At the melting point of benzene, the substance is at solid-liquid equilibrium. In this state, the rate of melting (solid to liquid) equals the rate of freezing (liquid to solid). When equilibrium is reached, the energy input (for melting) is perfectly balanced by the energy output (for freezing). This delicate balance ensures that the proportion of solid and liquid remains constant, keeping the system's temperature stable. As long as environmental conditions like pressure and temperature are maintained, benzene will remain in this equilibrium, where the processes of melting and freezing occur simultaneously and continuously.

Molecular Movement During Melting

On the molecular level, the movement and behavior of benzene molecules change significantly during melting. In the solid state, benzene molecules are packed tightly in a rigid, fixed structure due to strong intermolecular forces. As heat is applied, these molecules gain energy and start to vibrate more vigorously. When enough energy is absorbed at the melting point, the molecules break free from their fixed positions, transitioning into a liquid state where they move more freely and randomly. During the reverse process of freezing, some molecules in the liquid lose energy and slow down, allowing them to re-establish the tight, organized structure of the solid state. This interplay of molecular movement at the melting point is crucial for understanding the dynamic nature of phase changes in substances.