Question

Use any one of the phase changes to explain what is meant by dynamic equilibrium.

Short answer

Dynamic equilibrium is when the rates of evaporation and condensation are equal, keeping the amounts constant.

Step-by-step solution

Understanding Dynamic Equilibrium

Dynamic equilibrium occurs when a reversible reaction or phase change happens at the same rate in both directions. This means that the system is balanced, and there is no overall change in the concentrations of reactants and products, even though the molecules are constantly reacting.

Choosing a Phase Change Example

Let's consider the phase change of water between liquid and vapor, also known as evaporation and condensation. Imagine a closed container with water at a constant temperature.

Explaining Evaporation

In our container, some of the water molecules gain enough energy to transition into the vapor phase. This process is evaporation, where liquid water becomes water vapor.

Explaining Condensation

Simultaneously, water vapor molecules lose energy and transition back into the liquid phase through condensation. This is the reverse of evaporation.

Achieving Dynamic Equilibrium

In a closed system at equilibrium, the rate of evaporation of water molecules from the liquid to the gas phase equals the rate of condensation from the gas phase back to the liquid. At this point, the amount of water and vapor remains constant even though molecules continue to move between phases.

Key concepts

Phase Changes

Phase changes are transformations in the state of matter that involve energy exchanges. Common phase changes include melting, freezing, vaporization, condensation, sublimation, and deposition.
When a substance undergoes a phase change, its temperature remains constant as it absorbs or releases energy.

• Vaporization: Transition from liquid to gas. This includes boiling and evaporation.
• Condensation: Transition from gas to liquid.
• Melting: Transition from solid to liquid.
• Freezing: Transition from liquid to solid.

These changes are significant because they help establish dynamic equilibria under certain conditions, especially when reversible.
A great example is water in a closed container, where liquid water and vapor continuously exchange phases through evaporation and condensation.

Evaporation

Evaporation is a process where liquid molecules at the surface gain enough energy to enter the gaseous phase. This happens, for instance, when water is left in an open container and gradually turns into vapor. Even at temperatures below the boiling point, evaporation can occur at the surface.
Several factors affect the rate of evaporation:

• Temperature: Higher temperatures provide more energy for molecules to escape.
• Surface Area: Larger surface areas allow more molecules to evaporate.
• Humidity: Lower humidity levels accelerate evaporation as the surrounding air can take up more vapor.

In a closed system, evaporation contributes to dynamic equilibrium, balancing the rates of vaporization and condensation.

Condensation

Condensation is the process by which a gas turns into a liquid. It occurs when vapor molecules lose energy and change state, typically upon contact with a cooler surface. You might notice droplets forming on a cold glass, which is condensation in action.
Considerations that affect condensation include:

• Temperature: Lower temperatures enhance the likelihood of molecules losing energy and condensing.
• Pressure: Increased pressure can force gas molecules closer together, favoring condensation.

In nature, condensation is crucial for processes like cloud formation and precipitation. In closed systems, it balances evaporation, helping maintain a dynamic equilibrium.

Reversible Reactions

Reversible reactions are chemical processes in which the reactants form products that can revert back into reactants. They reach a state of dynamic equilibrium when the forward and reverse reactions occur at equal rates.
In the context of phase changes, reversible reactions are pivotal. They illustrate how no net change occurs in a closed system once equilibrium is reached. For example:

• Evaporation proceeds at the same rate as condensation in a closed container.
• Both changes continue, but the quantities of liquid and vapor remain consistent.

This dynamic behavior is a hallmark of systems in equilibrium, showcasing balance without any visible overall change.