Question

When equilibrium is established in the evaporation of a liquid into a sealed container, we refer to it as a dynamic equilibrium. Why?

Short answer

Dynamic equilibrium in the context of evaporation in a sealed container is referred to as such because, while the rates of evaporation and condensation are equal, resulting in no net change, the molecules continue to move between phases dynamically.

Step-by-step solution

Understanding Dynamic Equilibrium

In a dynamic equilibrium, the process of evaporation and the reverse process of condensation occur at the same rate. Although individual molecules are in motion, changing from the liquid to the gas phase and vice versa, the overall number of liquid and gas molecules remains constant over time.

Continuous Process

The term 'dynamic' refers to the continuous movement and exchange of molecules between the liquid and vapor phases. Unlike static equilibrium, where no movement occurs, the dynamic nature signifies ongoing activity at the molecular level.

Macroscopic Stability

From a macroscopic perspective, the system appears stable because the amount of liquid and vapor does not change. This apparent stability is why we say the equilibrium is established, though on a microscopic level, individual molecules are in constant motion.

Key concepts

Understanding Evaporation and Condensation

Evaporation and condensation are two processes that go hand in hand, especially when talking about dynamic equilibrium. Simply put, evaporation is the process where liquid particles gain enough energy to escape into the gaseous state. On the other hand, condensation is the process by which gas particles lose energy and convert back into a liquid. Imagine a sealed juice bottle left in the sun; some of the liquid inside will vaporize and occupy the space above the liquid. Later, when the temperature drops, the vapor will condense back into liquid.

These processes are driven by molecular motion; the higher the temperature, the more energetic the molecules are, hence more evaporation. When energy decreases, so does molecular speed, leading to condensation. In a closed system, such as a sealed container, an equilibrium is established when the rate of evaporation equals the rate of condensation. This is the dynamic balance between two opposing processes that ensures the number of molecules in the liquid phase stays relatively constant over time.

The Nature of Molecular Motion

Molecular motion is a concept central to understanding dynamic equilibrium. At the microscopic level, molecules are always in motion, and the speed and direction of this motion determine the state of the matter. In solids, molecules vibrate in place, while in liquids, they move more freely but remain close together. In gases, molecules travel in straight lines until they collide with either another molecule or the walls of their container.

Kinetic energy is pivotal here; it's the energy of motion that molecules possess. When the kinetic energy is high due to increased temperature or decreased intermolecular forces, molecules can break free from their liquid state and become a gas, resulting in evaporation. Conversely, when kinetic energy is low, gas molecules slow down, allowing attractions to pull them closer together and condense into a liquid. Thus, the ceaseless dance of molecules contributes to the dynamic nature of the equilibrium.

Macroscopic Stability in Dynamic Equilibrium

When discussing dynamic equilibrium from a macroscopic perspective—that is, what we see without a microscope—stability is a key factor. It might seem like nothing is changing when we look at a glass of water sitting in a closed container, but there's a lot of activity at the molecular level. This is because the macroscopic quantity of liquid and gas does not reveal the individual changes of phase that molecules are constantly undergoing.

So the term ‘macroscopic stability’ reflects a state where the large-scale properties, such as pressure, color, and the amount of liquid, do not change over time. Nonetheless, this doesn't mean that the system is static—a dynamic equilibrium is always swaying back and forth on the microscopic scale. Even when we can't see it, molecules are ceaselessly evaporating and condensing in a delicate balancing act that makes it look as though nothing changes to the naked eye.