Fiveable
Fiveable

or

Log in

Find what you need to study


Light

Find what you need to study

9.1 Introduction to Entropy

5 min readjanuary 8, 2023

Dylan Black

Dylan Black

Dylan Black

Dylan Black

What is Entropy?

In unit 5 you were introduced to the world of thermochemistry. That is, applying the rules of thermodynamics to our study of chemistry! During this unit, you learned about one key measure of energy: heat or enthalpy. In unit 9, our view of thermodynamics will be expanded past simply heat to measure reactions in terms of two other measures: and Gibbs’ Free Energy. In doing so, we’ll explore a reaction’s spontaneity. In simple terms, a will occur without an outside intervention like adding energy to the system. For example, a ball rolling down a hill is a because it occurs without needing anything to happen. However, the reverse process, rolling the ball back up, is non-spontaneous. It takes work to roll a ball up a hill. In this section, we’ll dive into the idea of .

can be thought of as a measure of “chaos” in a system. Essentially, answers the question of “how ordered is this system?” is also described as the number of possible arrangements in a system. Essentially, the more spread out and chaotic the system is, the higher the .

is important to chemists because it helps them understand energy flows in terms of creating order or disorder, especially when discussing reversible processes. To understand this, you can think about a practical example: your bedroom can get messy pretty easily. You can ruin the sheets, punch holes in the wall, throw clothes everywhere, get crazy (Fiveable does not endorse dirty rooms. In fact, go clean your room). This process is a process in which increases. The room went from ordered to disordered meaning increased. You can also use your energy to clean your room. This would mean your room would go from disordered to ordered and the of your room would decrease. This situation explains how systems tend towards chaos—it didn’t take energy to ruin your room but it did take energy to clean it. This is part of the which we’ll discuss in more detail later in this guide. 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-DsxTrpywMmLd.png?alt=media&token=f02baba6-13cb-4332-9a7e-15454cfacf2f

Entropy and States of Matter

There are meaningful connections between and as well. In general, have the least amount of disorder, followed by , and finally, have the most . This makes sense when you think about the properties of , , and gasses. are tightly packed in and have little movement between the pieces of the solid (either atoms or molecules). Therefore there is the least amount of . Moving to , we see more movement in the molecules and more disorder. Finally, are extremely chaotic. Remember that in a gas the molecules theoretically are wildly flying around. Compare this to a solid where there is not much movement at all. 

We can represent state changes in the following way:

X (s) ⇌ X (l) ⇌ X (g)

Moving forward along this chain increases and moving backward decreases .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-CZ94obc6F3nE.jpg?alt=media&token=1e4ba579-bce3-46e3-a7ad-4cac56e64893

Image From ProProfs

The Three Laws of Thermodynamics

For unit 9, there are three fundamental laws of thermodynamics that you will want to know. You will not have to memorize them verbatim for the AP exam but can be applied to explanations for problems and will give a greater conceptual understanding of the material that will be invaluable.

Law #1: The Law of Conservation of Energy

You’re probably already familiar with the First Law of Thermodynamics. This law is also known by a more common name: the . This tells us that energy can never be created or destroyed but rather can only change form. For example, energy can change from potential energy to kinetic energy, but the energy between the system and the surroundings is constant. Anything lost to the system is gained by the surroundings and vice versa. From this we also know that the total energy of an isolated system remains constant.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-etI2EgHmdNzI.gif?alt=media&token=989774f8-10da-4a30-9d17-c73ba571ff88

Image From NASA

Law #2: Energy Quality and Entropy Increases In Isolated Systems

As far as our discussion of goes, the is perhaps the most important. There are two main parts of this law.

The first part of the refers to . It tells us that as energy is converted from form to form and/or transfers from body to body, some of it is lost to the surroundings as heat. For example, suppose a power plant runs a turbine to create electricity. In that case, some of the mechanical energy of the turbine is converted to electrical energy while some of it is lost as heat energy via friction. This same idea is applicable to any energy usage. 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-lGiJtQoRF7ju.png?alt=media&token=ef83b2e9-487f-43fe-b39a-b5b74fab5110

Image From SlidePlayer

The second part of the refers to changes in an isolated system. The tells us that in an isolated system, will tend to either increase or stay constant. The states that for any , the overall ΔS must be greater than or equal to zero.  

Law #3: Absolute Zero

The third law of thermodynamics is the least important for unit 9 but is still incredibly important. This law tells us that at (0K = -273.15°C), is a constant zero. This is because at , all molecular motion stops and there is no disorder. The formal definition of this law is that “the of a system approaches a constant value as its temperature approaches .”

Key Terms to Review (12)

Absolute Zero

: Absolute zero is the lowest possible temperature where all molecular motion stops (except quantum fluctuations). It's equal to 0 Kelvin, -273.15 degrees Celsius, or -459.67 degrees Fahrenheit.

Energy Quality

: Energy quality refers to the ability of a form of energy to do useful work. High-quality energy is organized and can perform significant work, like electricity, while low-quality energy is disorganized and less capable of doing work, such as heat.

Entropy

: Entropy refers to the measure of disorder or randomness in a system. In chemistry, higher entropy means higher disorder and less predictability.

Gases

: Gases are one of the four fundamental states of matter. They have neither a defined shape nor volume, and they expand to fill any space available to them.

Isolated Systems

: An isolated system is one that does not exchange matter or energy with its surroundings. In reality, perfectly isolated systems don't exist but they are useful theoretical models in thermodynamics.

Law of Conservation of Energy

: The law of conservation of energy states that total amount of energy in an isolated system remains constant; it cannot be created nor destroyed, only transformed from one form to another.

Liquids

: Liquids are another state of matter where molecules flow freely around each other but remain close together due to intermolecular forces. They have an indefinite shape but definite volume.

Non-spontaneous Process

: A non-spontaneous process is one that does not occur naturally under a set of conditions and requires an external influence or energy input to proceed.

Second Law of Thermodynamics

: The Second Law of Thermodynamics states that the total entropy, or disorder, in an isolated system can only increase over time. It also states that heat cannot spontaneously flow from a colder location to a hotter area.

Solids

: Solids are one of the states of matter where molecules are closely packed together and vibrate around a fixed position. They have a definite shape and volume.

Spontaneous Process

: A spontaneous process is one that occurs naturally under certain conditions without any external intervention.

States of Matter

: The states of matter refer to the distinct forms that different phases of matter take on. The four most common states are solid, liquid, gas, and plasma.

9.1 Introduction to Entropy

5 min readjanuary 8, 2023

Dylan Black

Dylan Black

Dylan Black

Dylan Black

What is Entropy?

In unit 5 you were introduced to the world of thermochemistry. That is, applying the rules of thermodynamics to our study of chemistry! During this unit, you learned about one key measure of energy: heat or enthalpy. In unit 9, our view of thermodynamics will be expanded past simply heat to measure reactions in terms of two other measures: and Gibbs’ Free Energy. In doing so, we’ll explore a reaction’s spontaneity. In simple terms, a will occur without an outside intervention like adding energy to the system. For example, a ball rolling down a hill is a because it occurs without needing anything to happen. However, the reverse process, rolling the ball back up, is non-spontaneous. It takes work to roll a ball up a hill. In this section, we’ll dive into the idea of .

can be thought of as a measure of “chaos” in a system. Essentially, answers the question of “how ordered is this system?” is also described as the number of possible arrangements in a system. Essentially, the more spread out and chaotic the system is, the higher the .

is important to chemists because it helps them understand energy flows in terms of creating order or disorder, especially when discussing reversible processes. To understand this, you can think about a practical example: your bedroom can get messy pretty easily. You can ruin the sheets, punch holes in the wall, throw clothes everywhere, get crazy (Fiveable does not endorse dirty rooms. In fact, go clean your room). This process is a process in which increases. The room went from ordered to disordered meaning increased. You can also use your energy to clean your room. This would mean your room would go from disordered to ordered and the of your room would decrease. This situation explains how systems tend towards chaos—it didn’t take energy to ruin your room but it did take energy to clean it. This is part of the which we’ll discuss in more detail later in this guide. 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-DsxTrpywMmLd.png?alt=media&token=f02baba6-13cb-4332-9a7e-15454cfacf2f

Entropy and States of Matter

There are meaningful connections between and as well. In general, have the least amount of disorder, followed by , and finally, have the most . This makes sense when you think about the properties of , , and gasses. are tightly packed in and have little movement between the pieces of the solid (either atoms or molecules). Therefore there is the least amount of . Moving to , we see more movement in the molecules and more disorder. Finally, are extremely chaotic. Remember that in a gas the molecules theoretically are wildly flying around. Compare this to a solid where there is not much movement at all. 

We can represent state changes in the following way:

X (s) ⇌ X (l) ⇌ X (g)

Moving forward along this chain increases and moving backward decreases .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-CZ94obc6F3nE.jpg?alt=media&token=1e4ba579-bce3-46e3-a7ad-4cac56e64893

Image From ProProfs

The Three Laws of Thermodynamics

For unit 9, there are three fundamental laws of thermodynamics that you will want to know. You will not have to memorize them verbatim for the AP exam but can be applied to explanations for problems and will give a greater conceptual understanding of the material that will be invaluable.

Law #1: The Law of Conservation of Energy

You’re probably already familiar with the First Law of Thermodynamics. This law is also known by a more common name: the . This tells us that energy can never be created or destroyed but rather can only change form. For example, energy can change from potential energy to kinetic energy, but the energy between the system and the surroundings is constant. Anything lost to the system is gained by the surroundings and vice versa. From this we also know that the total energy of an isolated system remains constant.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-etI2EgHmdNzI.gif?alt=media&token=989774f8-10da-4a30-9d17-c73ba571ff88

Image From NASA

Law #2: Energy Quality and Entropy Increases In Isolated Systems

As far as our discussion of goes, the is perhaps the most important. There are two main parts of this law.

The first part of the refers to . It tells us that as energy is converted from form to form and/or transfers from body to body, some of it is lost to the surroundings as heat. For example, suppose a power plant runs a turbine to create electricity. In that case, some of the mechanical energy of the turbine is converted to electrical energy while some of it is lost as heat energy via friction. This same idea is applicable to any energy usage. 

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-lGiJtQoRF7ju.png?alt=media&token=ef83b2e9-487f-43fe-b39a-b5b74fab5110

Image From SlidePlayer

The second part of the refers to changes in an isolated system. The tells us that in an isolated system, will tend to either increase or stay constant. The states that for any , the overall ΔS must be greater than or equal to zero.  

Law #3: Absolute Zero

The third law of thermodynamics is the least important for unit 9 but is still incredibly important. This law tells us that at (0K = -273.15°C), is a constant zero. This is because at , all molecular motion stops and there is no disorder. The formal definition of this law is that “the of a system approaches a constant value as its temperature approaches .”

Key Terms to Review (12)

Absolute Zero

: Absolute zero is the lowest possible temperature where all molecular motion stops (except quantum fluctuations). It's equal to 0 Kelvin, -273.15 degrees Celsius, or -459.67 degrees Fahrenheit.

Energy Quality

: Energy quality refers to the ability of a form of energy to do useful work. High-quality energy is organized and can perform significant work, like electricity, while low-quality energy is disorganized and less capable of doing work, such as heat.

Entropy

: Entropy refers to the measure of disorder or randomness in a system. In chemistry, higher entropy means higher disorder and less predictability.

Gases

: Gases are one of the four fundamental states of matter. They have neither a defined shape nor volume, and they expand to fill any space available to them.

Isolated Systems

: An isolated system is one that does not exchange matter or energy with its surroundings. In reality, perfectly isolated systems don't exist but they are useful theoretical models in thermodynamics.

Law of Conservation of Energy

: The law of conservation of energy states that total amount of energy in an isolated system remains constant; it cannot be created nor destroyed, only transformed from one form to another.

Liquids

: Liquids are another state of matter where molecules flow freely around each other but remain close together due to intermolecular forces. They have an indefinite shape but definite volume.

Non-spontaneous Process

: A non-spontaneous process is one that does not occur naturally under a set of conditions and requires an external influence or energy input to proceed.

Second Law of Thermodynamics

: The Second Law of Thermodynamics states that the total entropy, or disorder, in an isolated system can only increase over time. It also states that heat cannot spontaneously flow from a colder location to a hotter area.

Solids

: Solids are one of the states of matter where molecules are closely packed together and vibrate around a fixed position. They have a definite shape and volume.

Spontaneous Process

: A spontaneous process is one that occurs naturally under certain conditions without any external intervention.

States of Matter

: The states of matter refer to the distinct forms that different phases of matter take on. The four most common states are solid, liquid, gas, and plasma.


© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.