Closed Systems

Closed Systems Equilibrium

To understand what closed systems are, we need to review the basics of Chemical Equilibrium.

Recall that many Chemical Reactions are reversible, and therefore proceed in the forward and reverse direction. When the Reaction Rates of the forward and the rates of the reverse reactions are equal, the reaction has achieved Chemical Equilibrium!

• \( \text{Forward reaction: A + B}\longrightarrow \text{C + D} \)

• \( \text{Reverse reaction: A + B} \longleftarrow \text{C + D} \)

• \( \text{Equilibrium reaction: A + B} \rightleftharpoons \text{C + D} \)

When there is a higher Concentration of products than reactants, we say that the equilibrium lies to the right. When there are more reactants than products, the equilibrium lies on the left.

Chemical equilibrium can be used by chemists to predict how reactants behave in a closed system! A system is simply the environment being studied, and the types of transfers that can occur in a system are matter transfer and energy transfer as work and heat.

• Heat is the energy that causes the temperature of an object to increase.
• Work is the energy that causes an object that has mass to move.

The reactants and products make up the system when dealing with Chemical Reactions in a laboratory, while the container and everything else are the environment.

The image below shows the basic diagram of a system and its surroundings.

Types of Closed Systems

Systems can be classified into three types based on the energy and matter exchanges happening between the system and the surroundings:

• Open system.

• Isolated system.

• Closed system.

Let's start by talking about open systems.

For example, if you boil a pot of water and leave the pot uncovered, you are are in the presence of an open system! In this case, heat in getting into the system from the stove, while water vapor is being released to the surroundings.

Next, we have isolated systems. Isolated systems are the opposite of an open system because no exchange is allowed in this system.

A common example of an insulated system is an insulated thermos containing hot tea. However, since the tea gets cold eventually, we cannot consider this system as being perfectly isolated!

When dealing with an isolated system, we need to take into account the law of conservation of mass. This law tells us that for any system that is isolated, matter cannot be created or destroyed. Matter can only change form, but it is conserved.

Isolated systems also need to follow the first law of thermodynamics. According to the first law of thermodynamics, energy cannot be created or destroyed within an isolated system. As a result, chemists can figure out where energy comes from or where it goes. It is also true that an isolated system has a constant total energy, known as the law of conservation of energy.

Closed Systems definition

Now, let's look at the definition of a closed system. In a closed system, no reactants or products can be added or removed, only energy can be transferred in or out.

As an example, think about the chemical reaction between hydrogen gas (H₂) and oxygen gas (O₂) to form water (H₂O) happening in a closed system (figure 4). In this case, the system is hydrogen and oxygen, while the cylinder and everything else (including us) are the surroundings.

Now, when this reaction happens, energy is released to the surroundings in the form of work and heat. However, the mass of the system remains the same because it has not gained or lost mass: the only thing happening is that the chemical form of H₂ and O₂ gets converted into H₂O by the reaction.

$$ \text{2 H}_{2} (g) \text{ + O}_{2} (g)\text{ }\longrightarrow \text{2 H} _{2}\text{O}(g) \text{ + Energy} $$

Closed Systems in Thermodynamics

In thermodynamics, the focus is on the interactions between the system and the environment, and closed systems are of particular interest. So, let's dive into closed systems and chemical reactions in more detail.

Reversible chemical reactions that happen in closed systems are able to reach equilibrium because substances cannot get in or out of the system. However, energy does get transferred to or from the surroundings.

• If the forward reaction is exothermic in a reversible chemical reaction, then the reverse reaction will be endothermic (and vice versa!)

For example, if calcium carbonate (CaCO₃) is heated in a closed system (for example, inside a closed container) the reaction will eventually reach equilibrium as no CO₂ gas will be lost into the surroundings. Heat will be the only thing transferred from the surroundings into the system.

$$ \text{CaCO}_{3} (s) \rightleftharpoons \text{CaO} (s)\text{ + CO}_{2} (g) $$

Now, if this chemical reaction was done in an open system, then Carbon dioxide (CO₂) would be lost to the surroundings.

Closed Systems Examples

Now that you know what closed systems are, let's look at another example. Suppose that you are in a laboratory, and you are asked to perform a chemical reaction between a solution of calcium chloride (CaCl₂) and a solution of sodium sulfate (Na₂SO₄) in a closed system. If you have 180 grams of reactants, what would you expect to be the mass of the products?

This chemical reaction forms calcium sulfate (CaSO₄) and sodium chloride (NaCl) as products. And, since the chemical reaction happened in a closed system, then the mass of the products should equal the mass of the reactants. Remember: whenever a closed system is present, it means that only energy can be transferred between the closed system and its surroundings.

$$ \text{CaCl}_{2}\text{ + Na}_{2}\text{SO}_{4} \longrightarrow \text{CaSO}_{4} \text{ + 2 NaCl} $$

Closed Systems vs. Open Systems

Lastly, let's review the difference between an open system, an isolated system, and a closed system. In an open system, transfer of both matter and energy can happen between the system and the surroundings, whereas in an isolated system neither energy not matter can be exchanged with the surrounding. In a closed system, only energy exchange can happen between the system and the surroundings.

Now, I hope that you were able to understand what closed systems are!