Question

In Section 17.3 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 for a system to reach equilibrium? Explain why equilibrium is not reached in an open system.

Short answer

A closed system is a physical system that does not exchange matter with its surroundings, while energy can still be exchanged. Equilibrium is reached in a closed system because reactants and products are confined within the system, allowing the rates of forward and reverse reactions to become equal, resulting in constant concentrations of reactants and products. In contrast, an open system exchanges both matter and energy with its surroundings. Equilibrium is not reached in an open system because the constant exchange of particles prevents the establishment of a stable state with equal reaction rates and constant concentrations.

Step-by-step solution

Definition of a Closed System

In the context of physics and chemistry, a closed system refers to a physical system that does not exchange any matter with its surroundings. It means that particles within the system cannot enter or leave the system. However, energy can still be exchanged between the closed system and its surroundings in the form of heat or work.

Closed System and Equilibrium

In a closed system, since no particles can enter or leave, reactants and products are confined within the system. Over time, the chemical reactions taking place in the closed system will reach a dynamic equilibrium. Dynamic equilibrium is a state where the rate of the forward reaction (from reactants to products) is equal to the rate of the reverse reaction (from products to reactants). This results in a constant concentration of reactants and products since the amount of each is neither increasing nor decreasing.

Definition of an Open System

In contrast to a closed system, an open system is a physical system that can exchange both matter and energy with its surroundings. This means that particles can enter or leave the system, and energy (heat and work) can also be exchanged between the system and its surroundings.

Open System and Equilibrium

In an open system, equilibrium is generally not reached because reactants and products are constantly entering and leaving the system. This constant exchange of particles prevents the system from achieving a stable state where the rates of forward and reverse reactions are equal. As a result, the concentrations of reactants and products do not remain constant over time, and dynamic equilibrium cannot be established.

Key concepts

Closed System

A closed system is fundamental to understanding chemical equilibria since it defines the boundaries in which reactions occur. In the realm of chemistry and physics, a closed system is essentially a sealed box - matter cannot get in or out, but energy can freely exchange in the form of heat or work.

This isolation of matter ensures that any reactions within the closed system are only influenced by the reactants and products present at the start. Picture a soda bottle, tightly capped; the carbon dioxide can't escape, so the system is closed. Over time, the rate of the carbon dioxide dissolving into the soda equals the rate at which it escapes back into the bottle's space – this is the dynamic equilibrium we observe in a closed system.

Dynamic Equilibrium

When we talk about a dynamic equilibrium, we touch upon the dance that occurs at the molecular level. It is not a still life but a bustling city square - reactions are in constant motion.

In a closed system, this state reflects a balance between the forward and reverse reactions — the transformation of reactants to products and vice versa happens at an equal pace. Contrary to what might be inferred from the term 'equilibrium', it's not a static state - it is incredibly active. Yet, for an observer, it appears tranquil because the amounts of reactants and products stay steady over time, their fluctuating quantities in a kind of molecular harmony. This is akin to a room where two doors open and close at the same rate, with people constantly entering and leaving, yet the total count remains unchanged.

Open System

An open system is the antithesis of a closed one; it's a system without walls. It's like a birdcage with an open door - birds (matter) can fly in and out, and the wind (energy) can sweep through. In such a system, both matter and energy exchange freely with the surroundings.

This openness is what disrupts the possibility of reaching a chemical equilibrium. Since new reactants might be introduced or products removed, we can't achieve that fine balance where everything within the system is constant. It's akin to trying to keep water level in a pool steady when it's raining and there's a hole in the bottom – the dynamics of the system are constantly altered by external factors.

Forward and Reverse Reactions

The heart of chemical equilibrium lies in understanding forward and reverse reactions. In any chemical process, we can often go both ways - from reactants to products (forward) and from products back to reactants (reverse). At the outset of a reaction in a closed system, the forward reaction is typically predominant because the reactants are plentiful.

As time marches on and products accumulate, the reverse reaction starts to increase because there's more 'fuel' for the journey back. Eventually, these two rates will become equal if the system is closed, and that's when dynamic equilibrium is achieved. It's like two runners on a track, one starting after the other, but eventually settling into the same pace - they remain in constant motion, but their relative positions stabilize.