1.2 Thermodynamics Fundamentals
2 min read•july 22, 2024
Thermodynamic properties and laws form the backbone of chemical engineering. They help us understand how energy flows and changes in systems, from simple reactions to complex industrial processes.
Enthalpy, , and free energy changes are key concepts for predicting reaction feasibility. By calculating these values, engineers can determine if a reaction will occur spontaneously or require external input, crucial for designing efficient chemical processes.
Thermodynamic Properties and Laws
Thermodynamic properties and equilibrium
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- Thermodynamic properties measure or calculate a system's characteristics (temperature, pressure, volume, internal energy, enthalpy, entropy)
- State variables depend only on the current state, not the path taken to reach it (pressure, temperature, volume) independent of the system's history
- Equilibrium occurs when a system's properties remain constant over time with no net change in macroscopic properties
- : no net heat transfer between system and surroundings
- : no net change in pressure or volume
- : no net change in the system's composition
First and second laws in chemistry
- states energy cannot be created or destroyed, only converted between forms
- Change in internal energy () equals heat added () minus work done by the system ():
- states entropy of an isolated system always increases or remains constant
- Heat flows spontaneously from hot to cold bodies
- Impossible to completely convert heat into work in a cyclic process
- Apply these laws to analyze heat transfer and work in chemical reactions and phase changes, determine process efficiency, and assess reaction feasibility and spontaneity
Thermodynamic Calculations and Reaction Feasibility
Enthalpy, entropy, and free energy changes
- Enthalpy () measures a system's total heat content
- Change in enthalpy () equals heat absorbed or released at constant pressure:
- Entropy () measures a system's disorder or randomness
- Change in entropy () equals heat absorbed or released divided by absolute temperature for reversible processes:
- () measures maximum useful work obtainable from a system
- Change in Gibbs free energy () equals change in enthalpy minus the product of temperature and change in entropy:
Reaction feasibility and spontaneity
- Spontaneous reactions occur without external intervention and have negative change in Gibbs free energy ()
- Feasible reactions can occur under given conditions and have negative or zero change in Gibbs free energy ()
- Relationship between , , and determines reaction spontaneity:
- and : always spontaneous
- and : never spontaneous
- and or and : spontaneity depends on temperature
Key Terms to Review (18)
Adiabatic Process: An adiabatic process is a thermodynamic process in which no heat is exchanged with the surroundings. This means that all the energy transfer happens as work, leading to changes in temperature and pressure without heat transfer. Adiabatic processes are crucial in understanding how systems behave under different conditions, particularly in engines and natural phenomena like atmospheric changes.
Carnot Cycle: The Carnot Cycle is an idealized thermodynamic cycle that provides a standard of comparison for the efficiency of heat engines. It operates between two heat reservoirs and consists of four reversible processes: two isothermal processes and two adiabatic processes. Understanding the Carnot Cycle helps illustrate the fundamental principles of thermodynamics and lays the groundwork for exploring molecular interactions in thermal systems.
Chemical Equilibrium: Chemical equilibrium is the state in a chemical reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentration of reactants and products over time. This dynamic balance is essential for understanding how reactions proceed and how various factors, like temperature and pressure, can influence the position of equilibrium.
Closed System: A closed system is a physical system that does not allow matter to enter or leave, though energy can be exchanged with its surroundings. This concept is essential in understanding how energy transformations occur within a defined boundary, which is crucial for analyzing processes such as heat transfer and work done on or by the system. Closed systems simplify the analysis of mass and energy balances by isolating the system from external influences while allowing for energy interactions.
Cop (coefficient of performance): The coefficient of performance (COP) is a measure of the efficiency of a heating or cooling system, defined as the ratio of useful heating or cooling provided to the work input required. A higher COP indicates a more efficient system, meaning it can provide more heating or cooling per unit of energy consumed. This metric is crucial in evaluating energy systems, helping to determine their effectiveness in transferring heat in thermodynamic cycles.
Entropy: Entropy is a measure of the disorder or randomness in a system, reflecting the number of ways a system can be arranged. It is a central concept in thermodynamics, illustrating the direction of spontaneous processes and the tendency of systems to move towards greater disorder. Entropy connects to energy dispersal and equilibrium, highlighting how energy transformations are often accompanied by an increase in entropy, which ultimately drives the behavior of both macroscopic and microscopic systems.
First Law of Thermodynamics: The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another, which is a fundamental principle in understanding energy interactions within systems. This law connects the concepts of heat transfer, work done by or on a system, and internal energy changes, establishing the foundation for analyzing energy balances and molecular behavior in thermodynamic processes.
Gibbs Free Energy: Gibbs Free Energy is a thermodynamic potential that measures the maximum reversible work obtainable from a closed system at constant temperature and pressure. This concept helps determine whether a process will occur spontaneously, as it combines enthalpy and entropy to predict system stability and phase behavior.
Isothermal Process: An isothermal process is a thermodynamic process that occurs at a constant temperature, meaning that the internal energy of the system remains unchanged throughout the process. In such a process, any heat added to the system is entirely converted into work done by the system, making it an essential concept in understanding both classical and molecular thermodynamics.
Lord Kelvin: Lord Kelvin, born William Thomson, was a prominent physicist and engineer known for his foundational work in thermodynamics and the establishment of the absolute temperature scale. His contributions have significantly influenced the understanding of energy transformations and the behavior of physical systems, laying the groundwork for future advancements in thermal physics.
Mechanical Equilibrium: Mechanical equilibrium refers to the state in which the net force acting on a system is zero, leading to no acceleration and a stable condition for the system. In this state, all the forces and moments acting on the system are balanced, meaning that the system can either be at rest or move with a constant velocity. Understanding mechanical equilibrium is crucial in thermodynamics because it helps explain how forces interact within systems, which is fundamental for analyzing energy transfer and system stability.
Open System: An open system is a physical system that exchanges both matter and energy with its surroundings. This concept is essential for understanding processes where substances enter and leave a defined system boundary, impacting mass and energy calculations in various applications. In many real-world scenarios, like chemical reactions or biological processes, systems are seldom isolated; hence, recognizing their open nature is crucial for accurate analysis.
Rankine Cycle: The Rankine cycle is a thermodynamic cycle that converts heat into mechanical work, commonly used in steam power plants. It operates through a series of processes including the phase change of water from liquid to vapor and back, making it an essential concept in energy conversion and thermodynamic analysis.
Sadi Carnot: Sadi Carnot was a French physicist and engineer known as the father of thermodynamics, who introduced the concept of the Carnot cycle in 1824. His work laid the foundation for understanding heat engines and the principles governing their efficiency, highlighting the relationship between heat and work. Carnot's insights help define the second law of thermodynamics and are critical in analyzing energy conversion processes.
Second Law of Thermodynamics: The Second Law of Thermodynamics states that in any energy transfer or transformation, the total entropy of an isolated system can never decrease over time. This principle highlights the directionality of processes and establishes that natural processes tend to move towards a state of greater disorder or entropy. It connects various aspects of thermodynamics, including the efficiency of energy conversion, the behavior of systems far from equilibrium, and molecular interactions at a microscopic level.
Thermal Efficiency: Thermal efficiency is a measure of how effectively a system converts heat energy into useful work output. It is defined as the ratio of the useful work output to the total heat energy input, often expressed as a percentage. A higher thermal efficiency indicates a more efficient system, minimizing waste energy and optimizing performance in processes involving heat transfer and conversion.
Thermal Equilibrium: Thermal equilibrium occurs when two or more bodies in thermal contact reach the same temperature and, therefore, do not exchange heat with each other. This state signifies that the energy transfer due to heat has ceased, resulting in a stable condition where no net heat flow occurs. Understanding thermal equilibrium is essential for analyzing heat transfer processes and energy balance in thermodynamics.
Triple Point: The triple point of a substance is the unique set of conditions at which the three phases of that substance (solid, liquid, and gas) coexist in thermodynamic equilibrium. This concept is crucial because it defines the specific temperature and pressure at which all three states are stable and can exist together, making it a fundamental point in thermodynamics for understanding phase transitions.