Question

In Section 13.1 of your text, it is mentioned that equilibrium is reached in a "closed system." What is meant by the term "closed system," and why is it necessary to have a closed system in order for a system to reach equilibrium? Explain why equilibrium is not reached in an open system.

Short answer

A closed system is one where there is no exchange of matter with its surroundings but allows the exchange of energy. Equilibrium is reached in a closed system because the constant mass allows the system to reach a state of balance between processes occurring within it, maintaining constant properties like temperature, pressure, and concentration. On the other hand, an open system allows exchanges of both matter and energy with its surroundings. Equilibrium is not reached in an open system because the continuous exchange of matter causes ongoing changes in the system's properties, making it impossible to achieve a stable state of balance.

Step-by-step solution

Define Closed System

A closed system is a physical system that does not allow the exchange of matter with its surroundings but allows the exchange of energy. This means that the total mass in the system remains constant, but heat and work can be transferred between the system and its surroundings.

Define Open System

An open system, on the other hand, is one that can exchange both matter and energy with its surroundings. In an open system, there are no barriers to mass flow, so substances can freely enter or exit the system.

Explain the Necessity of a Closed System for Equilibrium

Equilibrium is reached when the rate of change in a system becomes zero, and the system is said to be at a state of balance. In a closed system, the total mass of the system remains constant, allowing the system to reach a state of balance between the processes occurring within it, and maintaining constant temperature, pressure, and concentration. If a system is not closed, the constant transfer of matter would cause a continuous change in the properties of the system, making it impossible to reach equilibrium.

Explain why Equilibrium is Not Reached in an Open System

In an open system, matter can continuously enter and leave the system, causing changes in the properties (such as concentration, pressure, and temperature) of the system. This continuous exchange of matter prevents the system from reaching a state of balance, as the processes within the system are constantly affected by the addition or removal of substances. Consequently, equilibrium cannot be achieved in an open system due to the ongoing changes and lack of stability.

Key concepts

Understanding a Closed System

A closed system is quite unique because it does not allow any exchange of matter with its surroundings. This might sound a bit restrictive, but it is a special condition that ensures the system remains isolated in terms of its mass. Despite this restriction on matter, energy exchanges such as heat or work can still occur with the surroundings. This means that while the overall mass inside remains the same, energy can enter or leave the system.
In chemistry, achieving equilibrium within such a system is crucial. Since no matter is being added or taken away, a closed system provides a stable environment. This stability is fundamental when studying chemical reactions that need to reach a state of equilibrium. The unchanging mass helps in maintaining consistent concentration, pressure, and temperature, facilitating equilibrium.

Exploring an Open System

An open system is characterized by its ability to freely exchange both matter and energy with its surroundings. Think of it as a system with no barriers for substances to pass in and out. This attribute makes open systems significantly more dynamic than closed ones.
Due to the continuous flow of matter, the conditions within an open system are subject to constant change. As substances enter or exit, properties such as pressure and concentration fluctuate. This makes achieving equilibrium challenging, as the conditions are always in flux. Thus, an open system does not offer the same stability needed to reach the delicate balance of equilibrium.

The Role of Exchange of Matter

Exchange of matter plays a pivotal role in distinguishing open systems from closed systems. In closed systems, the mass remains static because there is no exchange of matter.
In contrast, open systems allow substances to freely move in and out. This movement can cause substantial changes to the system's internal conditions. These changes make it harder for the system to settle into a stable state, preventing equilibrium from being attained.
The ongoing exchange in open systems leads to continual adjustments in properties like concentration and temperature. This aspect is a key reason why chemical equilibrium is not typically reached in open systems.

Understanding the Rate of Change

At equilibrium, the rate of change within a system falls to zero. In a closed system, this is possible as the conditions remain constant, without the interference of external matter. Such a rate of change is crucial as it allows reactions to occur at equal rates, leading to balance.
Without reaching a zero rate of change, substances would not achieve the balance necessary for equilibrium. It is this balance of reactions occurring at equal speeds that defines equilibrium. However, in an open system, constant addition and removal of matter prevent achieving a zero rate of change.

The State of Balance: Achieving Equilibrium

Equilibrium refers to a system being in a state of balance, where there are no net changes occurring. This balance is easier to achieve in a closed system where no new matter disrupts the existing conditions. In such systems, the conditions of pressure, concentration, and temperature are uninterrupted. This constancy supports the alignment of chemical processes within the system.
Conversely, an open system struggles to reach this desired state. With ongoing external influences, the internal balance is constantly disturbed. Such disturbances keep the system from reaching equilibrium, highlighting the critical role a closed system plays in attaining stable balance.