Why Does Equilibrium Need a Closed System?

A reversible reaction can reach dynamic equilibrium when the forward and reverse reactions occur at equal rates.

For example:

A + B ⇌ C + D

At equilibrium:

rate of forward reaction = rate of reverse reaction

For this balance to be maintained, the substances involved in the reaction must remain available to react.

If products escape from the system, there may no longer be enough of them to maintain the reverse reaction. If reactants are continually added, the balance is also disturbed.

For this reason, chemical dynamic equilibrium is established and maintained in a closed system.

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What Is a System?

In chemistry, a system is the particular part of the universe that we choose to study.

Everything outside the system is called the surroundings.

For example, if we are studying a chemical reaction inside a sealed flask:

• the chemicals inside the flask are the system
• the flask helps form the boundary
• the air and laboratory outside are the surroundings

Understanding exactly where the system begins and ends is important when discussing equilibrium.

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What Is a System Boundary?

A system boundary separates the system from its surroundings.

The boundary may be physical, such as:

• the walls of a flask
• the lid of a container
• the walls of a reaction vessel
• a sealed bottle

Or it may be an imaginary boundary chosen by a scientist when analyzing a process.

The important question is:

What can cross the boundary?

In particular, we need to consider matter and energy.

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Open Systems

An open system can exchange both matter and energy with its surroundings.

Matter can:

• enter the system
• leave the system

Energy can also be transferred between the system and surroundings.

Examples include:

• an uncovered beaker
• an open bottle
• a reaction vessel with gas escaping
• a cup of hot water without a lid
• many living organisms

Suppose a reaction produces a gas:

A ⇌ B(g)

If the container is open, B may escape.

Once B leaves, it is no longer available to undergo the reverse reaction.

This can prevent equilibrium from being maintained.

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Closed Systems

A closed system does not allow matter to freely enter or leave.

However, energy may still cross the system boundary.

For example, a sealed flask can still:

• gain thermal energy from a heater
• lose thermal energy to the surroundings
• absorb light
• transfer energy through its walls

But the reacting substances remain inside.

This allows products to remain available for the reverse reaction.

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Closed Does Not Mean Insulated

This is an important distinction.

A closed system prevents the transfer of matter across its boundary, but it can still exchange energy.

Imagine a sealed glass flask being heated.

The flask is closed because chemicals cannot freely escape.

However, thermal energy can enter through the glass.

Therefore:

closed system ≠ no energy transfer

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What Is an Isolated System?

A third type of system is sometimes considered.

An isolated system ideally exchanges neither matter nor energy with its surroundings.

Perfectly isolated systems are difficult to create in reality, but the concept is useful when studying energy and thermodynamics.

For chemical equilibrium, we normally focus on closed systems rather than requiring perfect isolation.

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Why Does Matter Need to Stay Inside?

Consider:

A ⇌ B

Suppose we begin with A.

The forward reaction produces B:

A → B

As B accumulates, the reverse reaction becomes increasingly important:

B → A

Eventually:

forward rate = reverse rate

and equilibrium is established.

Now imagine that B continuously escapes from the system.

There are now fewer B particles available for the reverse reaction.

Removing B therefore interferes with the balance between the forward and reverse reactions.

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Particle View of a Closed System

Consider:

A₂(g) + B₂(g) ⇌ 2AB(g)

Inside a sealed container:

• A₂ and B₂ particles collide
• some successful collisions produce AB
• AB particles also collide and undergo the reverse reaction
• all substances remain inside the container

Eventually:

rate of A₂ + B₂ → 2AB

equals:

rate of 2AB → A₂ + B₂

Because the substances remain inside the system, both reactions can continue.

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Conditions Needed for Dynamic Equilibrium

Several conditions are needed for chemical dynamic equilibrium.

1. The reaction must be reversible

There must be both a forward and reverse reaction:

reactants ⇌ products

2. The system must be closed

Reactants and products must not freely escape.

3. The reaction must be given time

The forward and reverse rates may initially be different.

They need time to become equal.

4. Conditions must remain constant

Once equilibrium has been established, changing conditions such as:

• temperature
• pressure
• concentration

can disturb the existing equilibrium state.

5. Both reactions must continue

At dynamic equilibrium:

forward rate = reverse rate ≠ 0

The reactions have not stopped.

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Establishing Equilibrium Step by Step

Consider:

A + B ⇌ C

Suppose the system initially contains mostly A and B.

Stage 1

There are many reactant particles.

The forward reaction is relatively fast.

There is little or no C, so the reverse reaction is initially very slow.

Stage 2

C begins to accumulate.

The reverse reaction becomes faster because more C particles are available.

Stage 3

Eventually:

forward rate = reverse rate

Dynamic equilibrium has been established.

The concentrations of A, B, and C then remain constant as long as the equilibrium conditions are maintained.

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What Happens in an Open System?

Now imagine that C is a gas and the container is open.

A + B ⇌ C(g)

As C forms, it escapes into the surroundings.

The concentration of C inside the system remains lower than it would in a closed container.

That reduces the opportunity for:

C → A + B

to occur.

The system therefore cannot maintain the same dynamic equilibrium that would exist if all substances were retained.

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Example: Carbonated Drinks

A sealed bottle of carbonated drink provides a useful example of why system boundaries matter.

Carbon dioxide can move between the gas above the liquid and the dissolved state:

CO₂(g) ⇌ CO₂(aq)

Bottle closed

Carbon dioxide remains within the bottle.

Opposing processes can establish a dynamic balance.

Bottle opened

The boundary changes.

Carbon dioxide gas can now escape into the surroundings.

As CO₂ leaves the system, more dissolved CO₂ can leave the liquid.

Eventually, the drink becomes less fizzy.

Opening the bottle has changed the system from effectively closed to open with respect to carbon dioxide.

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Example: Water in a Sealed Container

Consider liquid water in a sealed container.

Some molecules evaporate:

H₂O(l) → H₂O(g)

Some water-vapour molecules condense:

H₂O(g) → H₂O(l)

Together:

H₂O(l) ⇌ H₂O(g)

Eventually:

rate of evaporation = rate of condensation

The amount of liquid and vapour remains constant.

This is a physical dynamic equilibrium.

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What If the Container Is Open?

Now remove the lid.

Water molecules can evaporate and escape into the surrounding air.

Many of those molecules never return to the liquid.

The system therefore does not maintain the same balance between evaporation and condensation that exists in the sealed container.

Over time, the amount of liquid can decrease.

This demonstrates how changing the system boundary changes the behaviour of the system.

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Example: Hydrogen Iodide Equilibrium

Consider:

H₂(g) + I₂(g) ⇌ 2HI(g)

In a sealed reaction vessel, all three gases remain available.

Hydrogen and iodine form hydrogen iodide.

Hydrogen iodide can also decompose into hydrogen and iodine.

At equilibrium:

forward rate = reverse rate

If one of the gases were continuously removed, the original equilibrium could not be maintained.

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Example: The Haber Process

The Haber process uses the reversible reaction:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Nitrogen and hydrogen react to produce ammonia.

Ammonia can also undergo the reverse reaction.

Industrial systems carefully control:

• temperature
• pressure
• gas flow
• reactant supply
• product removal

This demonstrates that real industrial equilibrium systems can be more complicated than a simple sealed classroom flask. Engineers control what enters and leaves different parts of the process to obtain useful production while managing the reversible reaction.

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Equilibrium and Constant Conditions

A closed system alone is not enough to guarantee that an existing equilibrium remains unchanged.

Conditions also matter.

Important factors include:

• temperature
• pressure
• concentration

Suppose a system has reached equilibrium.

If its temperature changes, the rates of the forward and reverse reactions may change differently.

If pressure changes in a gaseous system, the equilibrium composition may change.

If the concentration of a substance changes, the system may respond.

The system may then establish a new equilibrium under the new conditions.

This idea leads into Le Chatelier's principle.

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Predicting Whether Equilibrium Can Be Maintained

When given a situation, ask the following questions.

Is the reaction reversible?

If not, dynamic chemical equilibrium cannot be established between the forward and reverse reactions.

Is matter able to escape?

If important reactants or products continuously leave, equilibrium may not be maintained.

Is matter being added?

Continually adding substances changes the composition and can disturb the existing equilibrium.

Are the conditions constant?

Changes in temperature, pressure, or concentration can disturb the equilibrium.

Are both reactions still occurring?

Dynamic equilibrium requires both directions to continue.

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Worked Example 1: Sealed Flask

A reversible gaseous reaction occurs in a sealed flask:

A(g) ⇌ B(g)

After some time, the forward and reverse rates become equal.

Can equilibrium be maintained?

Analysis

• The reaction is reversible.
• The flask is sealed.
• A and B cannot freely escape.
• Both reactions can continue.

Answer

Yes. Dynamic equilibrium can be maintained if the conditions remain constant.

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Worked Example 2: Open Flask

Consider the same reaction:

A(g) ⇌ B(g)

but the flask is open and B escapes continuously.

Can the original equilibrium be maintained?

Analysis

B is being removed from the system.

There are therefore fewer B particles available for the reverse reaction.

Answer

No. The original equilibrium cannot be maintained while B continuously escapes.

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Worked Example 3: Heating a Closed System

A reversible reaction has reached equilibrium in a sealed flask.

The flask is then heated.

Is the system still closed?

Yes.

Matter still cannot freely enter or leave.

But has a condition changed?

Yes. Temperature has changed.

Therefore, although the system remains closed, the original equilibrium may be disturbed and the system may establish a new equilibrium.

This shows that:

closed system ≠ equilibrium can never change

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Worked Example 4: Removing Product

Consider:

A ⇌ B

The system is initially at equilibrium.

A device continuously removes B.

What happens?

Removing B means the system is no longer closed with respect to B.

The original equilibrium cannot simply remain unchanged.

This demonstrates why the system boundary matters.

If material crosses the boundary, the chemical behaviour of the system can change.

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System Boundaries in Real Situations

A boundary does not always have to be the wall of a glass flask.

Scientists define boundaries depending on what they are studying.

For example:

Aquarium

If the entire aquarium is the system, matter may enter through:

• food
• added water
• gases exchanged with air

and leave through:

• evaporation
• waste removal
• gas exchange

Therefore, it is an open system.

Sealed reaction vessel

Matter is contained inside, but thermal energy may pass through the walls.

This is a closed system.

Open cup of water

Water can evaporate and energy can enter or leave.

This is an open system.

Understanding the boundary tells us what interactions with the surroundings are possible.

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Open and Closed Systems Compared

The key difference is the movement of matter across the boundary.

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Macroscopic and Particle Views

At equilibrium, the system can look unchanged.

Macroscopic view

We may observe:

• constant colour
• constant pressure
• constant concentration
• constant amounts of substances

Particle view

Particles continue:

• moving
• colliding
• reacting
• forming products
• reforming reactants

The closed boundary keeps the particles within the system so these opposing reactions can continue.

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Why System Boundaries Matter in Chemistry

Defining the system boundary helps chemists determine:

• what substances are included in the system
• whether matter can enter
• whether matter can leave
• whether energy can be transferred
• whether equilibrium can be established
• whether equilibrium can be maintained

Without clearly defining the boundary, statements such as "the system is closed" or "the amount of matter remains constant" may be unclear.

The boundary tells us exactly what we are tracking.

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Common Mistakes

Mistake 1: Thinking a closed system exchanges nothing

A closed system can exchange energy with its surroundings.

It does not freely exchange matter.

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Mistake 2: Confusing closed and isolated systems

Closed:

no matter transfer, but energy transfer is possible

Isolated:

ideally no matter or energy transfer

They are not the same.

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Mistake 3: Thinking a lid automatically guarantees equilibrium

A closed container allows equilibrium to be established, but other requirements still matter.

The reaction must be reversible and enough time must pass for the forward and reverse rates to become equal.

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Mistake 4: Thinking equilibrium cannot change in a closed system

Changing temperature, pressure, or concentration can disturb an equilibrium even if the container remains closed.

The system may then establish a new equilibrium.

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Mistake 5: Ignoring escaping gases

If a gaseous product escapes from an open container, it is no longer available for the reverse reaction.

This can prevent equilibrium from being maintained.

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Mistake 6: Thinking equilibrium means equal amounts

A closed system may reach equilibrium with very different concentrations of reactants and products.

The important equality is:

forward rate = reverse rate

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Key Terms

System — The part of the universe selected for study.

Surroundings — Everything outside the system.

System boundary — The real or imaginary boundary separating a system from its surroundings.

Open system — A system that can exchange both matter and energy with its surroundings.

Closed system — A system that does not freely exchange matter with its surroundings but can exchange energy.

Isolated system — An idealized system that exchanges neither matter nor energy with its surroundings.

Dynamic equilibrium — A state in a closed reversible system where the forward and reverse reactions occur at equal rates.

Reversible reaction — A reaction that can proceed in both directions.

Forward reaction — The reaction converting reactants into products as written.

Reverse reaction — The reaction converting products back into reactants.

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Check Your Understanding

1. Define a closed system.

2. Define an open system.

3. What is a system boundary?

4. Explain the difference between a closed system and an isolated system.

5. Why is a closed system important for dynamic equilibrium?

6. Consider:

A(g) ⇌ B(g)

Explain what could happen if B escapes from the container.

7. Give two examples of open systems.

8. Give two examples of closed systems.

9. Can energy enter or leave a closed system? Explain.

10. A reversible reaction occurs in a sealed flask. Is this enough to prove that the system is at equilibrium? Explain.

11. A reaction has reached equilibrium in a sealed container. The temperature is then changed. Can the original equilibrium be disturbed even though the system is still closed? Explain.

12. Explain why a carbonated drink behaves differently before and after the bottle is opened.

13. Describe what happens to water molecules in a sealed container when evaporation and condensation reach dynamic equilibrium.

14. A student says, "A closed system is the same as an isolated system." Explain the student's mistake.

15. A reversible reaction is at equilibrium. One product is then continuously removed. Predict whether the original equilibrium can be maintained and explain why.

16. Explain why identifying the system boundary is important when analyzing a chemical process.

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Key Takeaways

• Dynamic equilibrium is established in a closed reversible system.
• A system is the part of the universe being studied.
• The surroundings are everything outside that system.
• A system boundary separates the system from its surroundings.
• An open system can exchange matter and energy with its surroundings.
• A closed system does not freely exchange matter, although energy can still be transferred.
• An isolated system ideally exchanges neither matter nor energy.
• Keeping reactants and products inside the system allows both the forward and reverse reactions to continue.
• If a reacting substance continuously escapes, the original equilibrium cannot be maintained.
• A closed system alone does not guarantee equilibrium; the reaction must also be reversible and have time to reach equilibrium.
• At equilibrium, the forward and reverse reaction rates are equal.
• Changing conditions can disturb an equilibrium even when the system remains closed.
• Clearly defining the system boundary allows chemists to determine what matter and energy can enter or leave.

Did You Know?

A sealed container is not necessarily completely inactive or cut off from its surroundings. A sealed glass flask can absorb heat, release heat, or even allow light to pass through while still being considered a closed system. In chemistry, the key feature of a closed system is not that nothing crosses the boundary—it is that matter is retained within the defined system.