2B7 Thermodynamics Flashcards
Describe entropy, Gibb's free energy, bond energy, and the laws of thermodynamics and their application to physical and chemical properties.
Define:
entropy
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.
List three ways of finding entropy of a system.
- Using a table of predetermined values.
- Predicting the sign of the entropy.
- Calculating directly by using thermodynamic quantities, such as heat and temperature (experiment).
How is a change in entropy represented mathematically?
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.
What is the equation for change in entropy at constant temperature?
ΔS = Q/T
Q is the heat exchanged and T is the temperature in Kelvin.
What processes typically result in positive entropy?
- Melting
- Boiling
- Sublimation
What is standard entropy?
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
True or false:
Solids have greater entropy than liquids.
False
Gases have greater entropy than liquids.
List two factors that affect entropy.
- Heating
- Number of moles
Heating and increasing the number of moles increases entropy.
What happens to the entropy when ice melts?
The entropy increases (positive change).
Entropy decreases when water condenses from a gas.
Define:
spontaneity
in a chemical reaction
The ability of a reaction to take place without external influences.
Non-spontaneous reactions require an external force or catalyst.
What is the role of temperature in chemical reactions?
It influences:
- spontaneity
- the state of matter
Temperature affects whether substances are in solid, liquid, or gas states.
Define:
enthalpy
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.
How is the enthalpy change of a reaction measured?
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.
State Hess’s Law.
The total enthalpy change is the sum of the enthalpy changes of each step.
It allows for the indirect calculation of enthalpy changes.
Fill in the blank:
Delta H (ΔH) is negative for _______ reactions.
exothermic
Negative ΔH indicates that the system releases energy to the surroundings as heat.
Fill in the blank:
Delta H (ΔH) is positive for _______ reactions.
endothermic
Positive ΔH means the system gains energy as heat.
What happens when you reverse a reaction in terms of enthalpy?
Multiply the enthalpy by -1.
This changes the direction of the reaction and the sign of Delta H.
Define:
internal energy
It is the sum of the kinetic and potential energy of a system.
It represents the total stored energy in a system.
How does the potential energy of a system vary in exothermic and endothermic reactions?
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.
Define:
Gibbs free energy
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.
What are the two main properties that Gibbs free energy (G) examines?
- Enthalpy
- 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.
What are the conditions for spontaneity based on enthalpy and entropy?
ΔH < 0 and ΔS > 0
ΔH is < 0 for exothermic reactions, but > 0 for endothermic reactions.
Exothermic reactions with increasing disorder are typically spontaneous.
Fill in the blank:
A positive ΔG means the reaction is _______.
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.
How is ΔG related to equilibrium constant (K)?
ΔG = -RTlnK
At equilibrium, ΔG = 0.
A system at equilibrium has no net energy change.
What is standard Gibbs free energy change?
ΔG° = ΔH° - TΔS°
Standard conditions are 1 atm pressure and 298 K temperature.
How does temperature affect ΔG?
An increase in temperature makes ΔG more negative.
The TΔS term becomes more significant at higher temperatures.
True or False:
ΔG can predict reaction speed.
False
ΔG determines spontaneity but not reaction kinetics.
What is the relationship between enthalpy and entropy at equilibrium?
H = TS
This relationship is important for understanding thermodynamic systems.
Define:
bond energy
The amount of energy needed to break a chemical bond.
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.
Define:
average bond energy
An average value calculated from similar types of bonds in different molecules.
Useful for predicting reactions in organic chemistry.
List two factors that affect bond energy.
- Bond length
- Bond polarity
Bond energy decreases with increase in bond length but increases with increase in polarity.
Bond length is measured in Picometers (pm).
Which bond has the highest bond energy in hydrocarbons?
C≡C triple bond
Triple bonds are stronger than double or single bonds.
What is a practical use of bond energy values?
To estimate enthalpy changes in reactions.
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.
What is the bond dissociation energy?
The energy required to break a covalent bond by homolysis to form fragments or radical species.
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.
Which bond is stronger, N≡N or N=N?
N≡N
Triple bonds are stronger than double bonds.
Define:
thermodynamics
The study of energy, heat, work, and their transformations.
Basis for understanding physical and chemical processes.
Areas of applications of thermodynamics include power plants, engines, and refrigeration.
Why are thermochemical equations important?
They provide insight into heat changes and energy transfer during physical and chemical reactions.
Thermochemical equations combine stoichiometry and thermodynamics to predict reaction behavior.
What does the thermochemical equation represent?
A balanced chemical equation with the associated enthalpy change (ΔH).
Example: C+O2→CO2, ΔH= -393.5 kJ/mol (exothermic).
List the laws of thermodynamics.
- Zeroth law
- First law
- Second law
- Third law
These govern energy transformations in all systems.
Fill in the blank:
The ______ law establishes temperature equilibrium.
zeroth
It defines temperature and thermal equilibrium and is a basis for thermometers and temperature scales.
What does the first law emphasize?
Conservation of energy.
The first law states that the total energy in an isolated system is conserved; energy cannot be created or destroyed.
Fill in the blank:
The first law is mathematically represented as _______.
ΔU = ΔQ - ΔW
ΔQ is the heat supplied, ΔU is the change in internal energy, and ΔW is the work done by the system.
What does the second law of thermodynamics explain?
- The direction of natural processes.
- The increase of entropy.
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.
Fill in the blank:
As entropy increases, the ______ ______ of a system decreases.
usable energy
Entropy-associated energy is unavailable to do work since energy lost to disorder cannot perform work.
High entropy reduces energy available for work.
True or False:
The third law states that entropy of a perfect crystal is zero at 0 K.
True
This applies to systems in a perfectly ordered state.
Define the concept of reversibility in thermodynamics.
A process that can return to its original state without changes in the surroundings.
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.
Which law limits energy conversion efficiency?
Second law
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.
List three types of systems in thermodynamics.
- Closed system
- Open system
- Isolated system
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).
List two examples of work done on a system.
- Compression of a gas.
- Lifting a weight using a heat engine.
Illustrates energy transformation.
Work done is positive if work is done by the system and negative if work is done on it.
Define:
an isochoric process
A process occurring at constant volume.
No work is done because volume does not change.
What does a cyclic process imply for internal energy?
ΔU = 0
System returns to its initial state.
List two limitations of the first law of thermodynamics.
- Does not predict direction of processes.
- It ignores entropy.
Focuses only on energy conservation.
Define:
an adiabatic process
A process with no heat exchange.
Changes in temperature are due to work alone.
List two examples of processes governed by the second law.
- Melting of ice.
- Expansion of gas.
Both increase system entropy.
What is a Carnot engine?
A theoretical engine with maximum possible efficiency.
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.
What does the second law predict about energy distribution?
Energy tends to disperse uniformly.
Explains why systems move toward equilibrium.
