Question

Explain why the development of a vapor pressure above a liquid in a closed container represents an equilibrium. What are the opposing processes? How do we recognize when the system has reached a state of equilibrium?

Short answer

In a closed container with a liquid, the system reaches a state of equilibrium when the rates of evaporation and condensation become equal, leading to a constant vapor pressure above the surface. The opposing processes are evaporation, where liquid particles gain enough energy to change into a vapor, and condensation, where vapor particles lose energy and return to the liquid state. The state of equilibrium is recognized by the constant vapor pressure above the liquid and no net change in the amount of liquid and vapor within the container.

Step-by-step solution

Understanding vapor pressure and equilibrium

To understand why the development of vapor pressure above a liquid in a closed container represents an equilibrium, first, we need to understand the terms vapor pressure and equilibrium. Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. In simpler terms, it means the pressure exerted by the gas particles above a liquid surface. Equilibrium, in the context of chemical reactions, is a state in which the rate of the forward reaction is equal to the rate of the reverse reaction. In such a state, the reactants and products do not undergo any net change with time. Now that we understand the basic concepts, let's discuss the opposing processes and recognize the state of equilibrium.

Identifying the opposing processes

In a closed container with a liquid, there are two opposing processes that eventually determine the vapor pressure: 1. Evaporation (or Vaporization): It's the process through which a liquid is transformed into its vapor state. Evaporation increases with an increase in temperature as more energy is provided to the particles to escape the liquid surface. 2. Condensation: It's the process in which the vapor state of a substance is transformed back into its liquid state. In a closed container, some of the vapor particles above the liquid surface lose energy and return to the liquid state. Now let's discuss how these processes contribute to the establishment of equilibrium.

Recognizing equilibrium in a closed container

In a closed container containing a liquid, the system will eventually reach a state of equilibrium where the rates of evaporation and condensation become equal. The vapor pressure above the liquid, at this point, remains constant since there is no net change in the number of vapor particles. We can recognize when the system has reached a state of equilibrium by observing these two signs: 1. Constant vapor pressure: When the vapor pressure above the liquid remains constant over time, it indicates that the system has achieved a state of equilibrium. The constant pressure occurs because the rates of evaporation and condensation are balanced, and there is no net change in the number of vapor particles above the liquid surface. 2. No net change in the amount of liquid and vapor: At equilibrium, the amounts of liquid and vapor in the container will remain constant since the rate of vaporization is equal to the rate of condensation. Hence, there will be no net change in the phases of the substance within the closed system. In conclusion, the development of vapor pressure above a liquid in a closed container represents an equilibrium since the rates of evaporation and condensation become equal, and the system achieves a constant vapor pressure and no net change in the phases.