2B7 Thermodynamics

Question 1

Define:

entropy

Answer 1

It is a measure of the disorder, or randomness, in a system.

Entropy is denoted as S and is a state function.

A high entropy indicates more disorder while a low entropy indicates less disorder.

Question 2

List three ways of finding entropy of a system.

Answer 2

1. Using a table of predetermined values.
2. Predicting the sign of the entropy.
3. Calculating directly by using thermodynamic quantities, such as heat and temperature (experiment).

Question 3

How is a change in entropy represented mathematically?

Answer 3

ΔS = Sfinal − Sinitial

A negative change indicates a decrease in the total entropy of the system while a positive change indicates an increase in the total entropy of the system.

Question 4

What is the equation for change in entropy at constant temperature?

Answer 4

ΔS = Q/T

Q is the heat exchanged and T is the temperature in Kelvin.

Question 5

What processes typically result in positive entropy?

Answer 5

• Melting
• Boiling
• Sublimation

Question 6

What is standard entropy?

Answer 6

A measure of the entropy of one mole of a substance at standard atmospheric pressure and temperature (298 K).

For example, the standard entropies of water at different phases are:

• Solid: 41 J/mol·K
• Liquid: 70 J/mol·K
• Gas: 186 J/mol·K

Question 7

True or false:

Solids have greater entropy than liquids.

Answer 7

False

Gases have greater entropy than liquids.

Question 8

List two factors that affect entropy.

Answer 8

1. Heating
2. Number of moles

Heating and increasing the number of moles increases entropy.

Question 9

What happens to the entropy when ice melts?

Answer 9

The entropy increases (positive change).

Entropy decreases when water condenses from a gas.

Question 10

Define:

spontaneity

in a chemical reaction

Answer 10

The ability of a reaction to take place without external influences.

Non-spontaneous reactions require an external force or catalyst.

Question 11

What is the role of temperature in chemical reactions?

Answer 11

It influences:

• spontaneity
• the state of matter

Temperature affects whether substances are in solid, liquid, or gas states.

Question 12

Define:

enthalpy

Answer 12

It is the total heat content of a system at constant pressure.

Represented as H = E + PV.

H is the enthalpy, E is internal energy, P is pressure, and V is volume.

Question 13

How is the enthalpy change of a reaction measured?

Answer 13

By adding up the enthalpies of products and subtracting the enthalpies of reactants.

Change in enthalpy is represented by the symbol Delta H (ΔH).

Delta H (ΔH) = Hfinal - Hinitial.

Hfinal is the enthalpy of the products and Hinitial is the enthalpy of the reactants.

Question 14

State Hess’s Law.

Answer 14

The total enthalpy change is the sum of the enthalpy changes of each step.

It allows for the indirect calculation of enthalpy changes.

Question 15

Fill in the blank:

Delta H (ΔH) is negative for _______ reactions.

Answer 15

exothermic

Negative ΔH indicates that the system releases energy to the surroundings as heat.

Question 16

Fill in the blank:

Delta H (ΔH) is positive for _______ reactions.

Answer 16

endothermic

Positive ΔH means the system gains energy as heat.

Question 17

What happens when you reverse a reaction in terms of enthalpy?

Answer 17

Multiply the enthalpy by -1.

This changes the direction of the reaction and the sign of Delta H.

Question 18

Define:

internal energy

Answer 18

It is the sum of the kinetic and potential energy of a system.

It represents the total stored energy in a system.

Question 19

How does the potential energy of a system vary in exothermic and endothermic reactions?

Answer 19

It is lowered in an exothermic reaction but raised in an endothermic reaction.

Burning a candle is an example of an exothermic reaction. Photosynthesis is an example of an endothermic reaction.

Question 20

Define:

Gibbs free energy

Answer 20

The energy available to do work in a thermodynamic system.

Gibbs free energy determines spontaneity.

Spontaneity is a measure of whether a reaction will occur naturally without external energy.

It is measured in joules or its multiples.

Question 21

What are the two main properties that Gibbs free energy (G) examines?

Answer 21

1. Enthalpy
2. Entropy

The two are related by the Gibbs free energy equation: ΔG = ΔH - TΔS
ΔG represents the change in Gibbs free energy, ΔH is the change in enthalpy, T is temperature, and ΔS is the change in entropy.

Question 22

What are the conditions for spontaneity based on enthalpy and entropy?

Answer 22

ΔH < 0 and ΔS > 0

ΔH is < 0 for exothermic reactions, but > 0 for endothermic reactions.

Exothermic reactions with increasing disorder are typically spontaneous.

Question 23

Fill in the blank:

A positive ΔG means the reaction is _______.

Answer 23

Non-spontaneous

Positive ΔG means the system requires energy input to proceed.

Negative ΔG indicates a spontaneous reaction (proceeds in the forward direction).

A spontaneous reaction signifies energy realease in a reaction.

Question 24

How is ΔG related to equilibrium constant (K)?

Answer 24

ΔG = -RTlnK

At equilibrium, ΔG = 0.

A system at equilibrium has no net energy change.

Question 25

What is **standard Gibbs free energy** change?

Answer 25

ΔG° = ΔH° - TΔS° ## Footnote Standard conditions are 1 atm pressure and 298 K temperature.

Question 26

How does temperature affect ΔG?

Answer 26

An increase in temperature makes **ΔG more negative**. ## Footnote The TΔS term becomes more significant at higher temperatures.

Question 27

# True or False: ΔG can predict **reaction speed**.

Answer 27

False ## Footnote ΔG determines spontaneity but not reaction kinetics.

Question 28

What is the relationship between enthalpy and entropy **at equilibrium**?

Answer 28

H = TS ## Footnote This relationship is important for understanding thermodynamic systems.

Question 29

# Define: bond energy

Answer 29

The amount of energy needed to **break a chemical bond**. ## Footnote Also referred to as bond enthalpy. It is a measure of stability of a chemical bond. Stability relates to how much energy is required to break the bond.

Question 30

# Define: average bond energy

Answer 30

An average value calculated from **similar types of bonds in different molecules**. ## Footnote Useful for predicting reactions in organic chemistry​.

Question 31

List *two* factors that **affect bond energy**.

Answer 31

1. Bond length 1. Bond polarity ## Footnote Bond energy decreases with increase in bond length but increases with increase in polarity. Bond length is measured in Picometers (pm).

Question 32

Which bond has the **highest bond energy** in hydrocarbons?

Answer 32

**C≡C** triple bond ## Footnote Triple bonds are stronger than double or single bonds​.

Question 33

What is a **practical use** of bond energy values?

Answer 33

To estimate **enthalpy changes** in reactions. ## Footnote Enthalpy calculations are crucial for predicting if a reaction will be endothermic or exothermic. In terms of enthalpy, bond energy can be defined as energy change for breaking a bond under constant pressure.

Question 34

What is the **bond dissociation energy**?

Answer 34

The energy required to **break a covalent bond by homolysis** to form fragments or radical species. ## Footnote Or, the amount of energy needed to break apart one mole of covalently bonded gases into a pair of radicals. It is an endothermic process.

Question 35

Which bond is stronger, **N≡N or N=N**?

Answer 35

N≡N ## Footnote Triple bonds are stronger than double bonds​.

Question 36

# Define: thermodynamics

Answer 36

The **study of energy, heat, work**, and their transformations. ## Footnote Basis for understanding physical and chemical processes​. Areas of applications of thermodynamics include power plants, engines, and refrigeration.

Question 37

Why are thermochemical equations **important**?

Answer 37

They provide insight into **heat changes and energy transfer** during physical and chemical reactions. ## Footnote Thermochemical equations combine stoichiometry and thermodynamics to predict reaction behavior.

Question 38

What does the **thermochemical equation** represent?

Answer 38

A balanced chemical equation with the **associated enthalpy change** (ΔH). ## Footnote Example: C+O2→CO2, ΔH= *-393.5* kJ/mol (exothermic).

Question 39

List the laws of **thermodynamics**.

Answer 39

1. Zeroth law 1. First law 1. Second law 1. Third law ## Footnote These govern energy transformations in all systems​.

Question 40

# Fill in the blank: The \_\_\_\_\_\_ law establishes **temperature equilibrium**.

Answer 40

zeroth ## Footnote It defines temperature and thermal equilibrium and is a basis for thermometers and temperature scales​.

Question 41

What does the **first law emphasize**?

Answer 41

Conservation of energy. ## Footnote The first law states that the total energy in an isolated system is conserved; energy cannot be created or destroyed.

Question 42

# Fill in the blank: The first law is **mathematically represented** as \_\_\_\_\_\_\_.

Answer 42

ΔU = ΔQ - ΔW ## Footnote ΔQ is the heat supplied, ΔU is the change in internal energy, and ΔW is the work done by the system.

Question 43

What does the **second law of thermodynamics** explain?

Answer 43

* The direction of natural processes. * The increase of entropy. ## Footnote It determines why certain processes are irreversible​. The law states that the disorder in a system increases with time, and not all energy can be used to perform useful work.

Question 44

# Fill in the blank: As **entropy increases**, the \_\_\_\_\_\_ \_\_\_\_\_\_ of a system decreases.

Answer 44

usable energy ## Footnote Entropy-associated energy is unavailable to do work since energy lost to disorder cannot perform work. High entropy reduces energy available for work.

Question 45

# True or False: The third law states that **entropy of a perfect crystal is zero** at 0 K.

Answer 45

True ## Footnote This applies to systems in a perfectly ordered state​.

Question 46

Define the concept of **reversibility** in thermodynamics.

Answer 46

A process that can **return to its original state** without changes in the surroundings. ## Footnote Only theoretical; no energy losses occur in a reversible process​. Entropy remains constant in a reversible process. True reversibility does not exist in real-world systems​.

Question 47

Which law limits **energy conversion efficiency**?

Answer 47

Second law ## Footnote Energy conversion is not always 100% efficient, some energy is lost as heat or friction​. For example, when pushing a car, a fraction of kinetic energy is wasted as thermal energy due to friction.

Question 48

List *three* **types of systems** in thermodynamics.

Answer 48

1. Closed system 1. Open system 1. Isolated system ## Footnote In a closed system, there is energy transfer but no matter transfer (matter is conserved). In an open system, there is matter and energy transfer in and out of the system. In an isolated system, neither matter nor energy transfers outside the system’s boundaries (energy is conserved).

Question 49

List *two* examples of **work done on a system**.

Answer 49

1. Compression of a gas. 1. Lifting a weight using a heat engine. ## Footnote Illustrates energy transformation. Work done is positive if work is done by the system and negative if work is done on it.

Question 50

# Define: an **isochoric** process

Answer 50

A process occurring at **constant volume**. ## Footnote No work is done because volume does not change​.

Question 51

What does a **cyclic process** imply for internal energy?

Answer 51

ΔU = 0 ## Footnote System returns to its initial state​.

Question 52

List *two* **limitations** of the first law of thermodynamics.

Answer 52

* Does not **predict direction of processes**. * It **ignores entropy**. ## Footnote Focuses only on energy conservation​.

Question 53

# Define: an **adiabatic** process

Answer 53

A process with **no heat exchange**. ## Footnote Changes in temperature are due to work alone​.

Question 54

List *two* examples of processes **governed by the second law**.

Answer 54

1. Melting of ice. 1. Expansion of gas. ## Footnote Both increase system entropy​.

Question 55

What is a **Carnot engine**?

Answer 55

A theoretical engine with **maximum possible efficiency**. ## Footnote Based on the second law; unattainable in practice. Engines lose energy due to the second law of thermodynamics, where some energy becomes unusable and is dissipated as waste heat, increasing the system's entropy.

Question 56

What does the second law predict about **energy distribution**?

Answer 56

Energy tends to **disperse uniformly**. ## Footnote Explains why systems move toward equilibrium​.