16.3 The Second and Third Laws of Thermodynamics
3 min read•june 25, 2024
The explains why ice melts and coffee cools. It states that entropy, or disorder, always increases in isolated systems. This law helps us understand spontaneous processes and why they're without external energy.
We can calculate entropy changes in chemical reactions using standard molar entropies. This helps predict if a reaction will happen on its own. The third law sets a reference point for entropy, stating it's zero for perfect crystals at .
The Second Law of Thermodynamics
Explain the second law of thermodynamics and its implications for spontaneous processes
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- States that the total entropy of an isolated system always increases over time
- Entropy measures disorder or randomness in a system
- Spontaneous processes occur in the direction of increasing entropy (ice melting at room temperature)
- Implies that spontaneous processes are irreversible
- Cannot be reversed without input of energy from an external source
- Examples of spontaneous processes:
- Heat flowing from a hot object to a cold object (coffee cooling down)
- Gas molecules spreading out to fill a container uniformly (perfume diffusing through a room)
- The concept of a , which converts thermal energy into mechanical work, is based on the
Calculate entropy changes for chemical reactions using standard molar entropies
- Entropy changes () for a chemical reaction can be calculated using the standard molar entropies () of reactants and products
- is the entropy of one mole of a substance at 1 atm pressure and usually 298 K
- The entropy change of a reaction is given by:
- is the sum of the standard molar entropies of the products multiplied by their stoichiometric coefficients
- is the sum of the standard molar entropies of the reactants multiplied by their stoichiometric coefficients
- If , the reaction leads to an increase in entropy and is more likely to be spontaneous (decomposition of hydrogen peroxide)
The Third Law of Thermodynamics
Third law and absolute zero
- States that the entropy of a perfect crystal at absolute zero (0 K) is zero
- A perfect crystal has a regular, repeating structure with no defects (diamond)
- As the temperature of a substance approaches absolute zero, its entropy approaches a constant minimum value
- This minimum value is zero for perfect crystals
- Provides a reference point for measuring absolute entropies of substances
- Absolute entropy values are used to calculate standard molar entropies ()
- Implies that it is impossible to reach absolute zero temperature through any finite number of cooling steps
- As temperature approaches absolute zero, it becomes increasingly difficult to remove heat from a system (liquid helium cooling)
Statistical Mechanics and Entropy
- , developed by , provides a microscopic interpretation of entropy
- The (k) relates the microscopic properties of particles to the macroscopic properties of materials
- The , an ideal thermodynamic cycle, demonstrates the maximum efficiency possible for converting thermal energy into work
Key Terms to Review (19)
Absolute Zero: Absolute zero is the lowest possible temperature, representing the complete absence of thermal energy. It is the point at which the motion of atoms and molecules reaches its minimum, and no further heat can be extracted from a system.
Boltzmann Constant: The Boltzmann constant is a fundamental physical constant that relates the average kinetic energy of particles in a gas to the absolute temperature of the gas. It is a crucial parameter in the study of thermodynamics and the behavior of systems at the molecular level.
Carnot Cycle: The Carnot cycle is an idealized thermodynamic cycle that describes the maximum theoretical efficiency of a heat engine operating between two thermal reservoirs. It is named after the French engineer and physicist Sadi Carnot, who developed the concept in the 1820s as a way to understand the limitations of steam engines and other heat-based power sources.
Heat Engine: A heat engine is a device that converts the heat energy from a high-temperature source into mechanical work or electricity. It operates based on the principles of thermodynamics, specifically the Second and Third Laws, to harness the flow of thermal energy and generate useful power.
Irreversible: Irreversible refers to a process or change that cannot be reversed or undone. It describes a situation where the original state or condition cannot be restored, and the system or reaction proceeds in a single direction without the possibility of going back to the starting point.
Ludwig Boltzmann: Ludwig Boltzmann was an Austrian physicist who made significant contributions to the field of statistical mechanics, particularly in the understanding of entropy and the second law of thermodynamics. His work laid the foundation for the modern understanding of the behavior of gases and the relationship between microscopic and macroscopic properties of systems.
Nonspontaneous process: A nonspontaneous process is a chemical or physical change that requires external energy to proceed. These processes have a positive change in Gibbs free energy ($\Delta G > 0$).
S°: S° is the standard entropy, which is a measure of the disorder or randomness of a system at standard temperature and pressure conditions. It is a fundamental concept in the Second and Third Laws of Thermodynamics, describing the spontaneous tendency of systems to move towards greater entropy and disorder over time.
Second law of thermodynamics: The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. It implies that natural processes increase the overall disorder or randomness of a system.
Second Law of Thermodynamics: The second law of thermodynamics states that the total entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. It is a fundamental principle that describes the direction of spontaneous processes in the universe.
Spontaneous Process: A spontaneous process is a natural, self-driven change that occurs in a system without the need for external intervention. It is a fundamental concept in thermodynamics that describes the natural tendency of a system to evolve towards a more stable or lower energy state over time.
Standard entropies (S°): Standard entropies ($S^\circ$) are the absolute entropy values of substances in their standard states at a specified temperature, typically 298.15 K (25°C). These values provide a reference for calculating changes in entropy during chemical reactions.
Standard entropy change (ΔS°): Standard entropy change ($\Delta S^\circ$) is the change in entropy for a reaction under standard conditions, typically 1 bar pressure and 298.15 K (25°C). It provides insight into the disorder or randomness added to the system during a reaction.
Standard Molar Entropy: Standard molar entropy is a thermodynamic property that quantifies the degree of disorder or randomness associated with a substance in its standard state. It represents the amount of energy dispersed as heat per unit of temperature during a reversible process at standard conditions.
Statistical Mechanics: Statistical mechanics is a branch of physics that applies the principles of probability and statistics to the study of the behavior of large systems of particles, such as gases, liquids, and solids. It provides a fundamental understanding of how the microscopic properties of individual particles give rise to the macroscopic properties of a system.
Third law of thermodynamics: The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero temperature is exactly zero. This implies that it is impossible to reach absolute zero in a finite number of steps.
Third Law of Thermodynamics: The Third Law of Thermodynamics states that as a system approaches absolute zero, all processes cease and the entropy of the system approaches a constant, usually zero, value. It establishes a fundamental limit to the ability to reduce the entropy of a system to zero by processes in a finite number of steps.
ΔS: ΔS, or change in entropy, is a fundamental concept in thermodynamics that describes the measure of disorder or randomness in a system. It is a key factor in understanding the spontaneity and direction of natural processes, as well as the efficiency of energy conversions.
ΔS°reaction: ΔS°reaction, or the standard change in entropy for a reaction, is a thermodynamic quantity that describes the change in the disorder or randomness of a system during a chemical reaction at standard conditions. It is a crucial factor in determining the spontaneity and feasibility of a reaction.