Entropy and Second Law
Spontaneity and entropy change in processes. (Chemistry › Thermodynamics, NEET UG syllabus.)
What is Entropy and Second Law?
A thermodynamic property that measures the ° of randomness or disorder in a system.
Key formula / rule: Entropy Change for Reversible Process
Key points
- Define entropy and explain its significance as a measure of disorder.
- State the Second Law of Thermodynamics and its implications for spontaneity.
- Calculate entropy changes for various processes, including phase transitions and chemical reactions.
- Relate ΔSsystem, ΔSsurroundings, and ΔStotal to determine spontaneity.
Common exam trap
Confusing ΔSsystem with ΔStotal; a negative ΔSsystem does not necessarily mean a non-spontaneous process.
Definitions
- Term
Entropy (S)
- Meaning
A thermodynamic property that measures the ° of randomness or disorder in a system.
- Term
Second Law of Thermodynamics
- Meaning
States that for any spontaneous process, the total entropy of the universe (system + surroundings) must increase (ΔStotal > 0).
- Term
Spontaneous Process
- Meaning
A process that occurs naturally without continuous external intervention once initiated.
- Term
Reversible Process
- Meaning
An idealized process that can be reversed by an infinitesimal change in conditions, leaving no net change in the system or surroundings.
- Term
Irreversible Process
- Meaning
A process that cannot be reversed without leaving a permanent change in the surroundings; all real, natural processes are irreversible.
Learning objectives
Define entropy and explain its significance as a measure of disorder.
State the Second Law of Thermodynamics and its implications for spontaneity.
Calculate entropy changes for various processes, including phase transitions and chemical reactions.
Relate ΔSsystem, ΔSsurroundings, and ΔStotal to determine spontaneity.
Understand the connection between entropy, enthalpy, and Gibbs free energy.
Predict the sign of entropy change for given physical and chemical processes.
Formulae
- Name
Entropy Change for Reversible Process
- Note
qrev is heat absorbed reversibly, T is absolute temperature in Kelvin.
- Expression
ΔS = qrev / T
- Name
Entropy Change of Surroundings
- Note
ΔHsystem is the enthalpy change of the system, T is absolute temperature.
- Expression
ΔSsurroundings = -ΔHsystem / T
- Name
Total Entropy Change (Universe)
- Note
For spontaneous processes, ΔStotal > 0.
- Expression
ΔStotal = ΔSsystem + ΔSsurroundings
- Name
Entropy Change for Fusion (Melting)
- Note
Tf is the melting point in Kelvin.
- Expression
ΔSfusion = ΔHfusion / Tf
- Name
Entropy Change for Vaporization (Boiling)
- Note
Tb is the boiling point in Kelvin.
- Expression
ΔSvap = ΔHvap / Tb
- Name
Gibbs-Helmholtz Equation
- Note
Relates Gibbs free energy, enthalpy, and entropy. ΔG < 0 for spontaneity at constant T, P.
- Expression
ΔG = ΔH - TΔS
Prerequisites
Basic understanding of thermodynamics (system, surroundings, state functions).
First Law of Thermodynamics (conservation of energy, ΔU = q + w).
Concepts of enthalpy (ΔH) and heat capacity.
Knowledge of phase transitions (melting, boiling).
Basic algebra and unit conversions.
Common mistakes
Confusing ΔSsystem with ΔStotal; a negative ΔSsystem does not necessarily mean a non-spontaneous process.
Forgetting to use absolute temperature (Kelvin) in entropy calculations.
Incorrectly applying the sign conventions for qrev and ΔHsystem.
Assuming all processes that increase disorder are spontaneous without considering ΔSsurroundings.
Not understanding the relationship between ΔG and ΔStotal (ΔG = -TΔStotal).
Keywords
Entropy
Second Law
Spontaneity
Disorder
Randomness
ΔStotal
ΔSsystem
ΔSsurroundings
Gibbs Free Energy
Reversible
Irreversible
Thermodynamics
Practice preview
Which of the following statements best describes entropy?…
easy
When ice melts into water at 0 degrees Celsius and 1 atm pressure, what is the sign of the entropy change of the system (delta S_system)?…
easy
For a reaction to be spontaneous at all temperatures, which of the following conditions must be met?…
medium