Concepts of entropy, enthalpy, and internal energy

Entropy

1. Entropy is a measure of the disorder or randomness of a system.
2. The symbol for entropy is S, and its SI unit is joules per kelvin (J/K).
3. It quantifies the unavailability of a system's energy to perform useful work.
4. The Second Law of Thermodynamics states that entropy always increases in a spontaneous process.
5. ΔS = Q/T, where:
   • ΔS is the change in entropy.
   • Q is the heat exchanged.
   • T is the absolute temperature.
6. In an isolated system, the total entropy either increases or remains constant.
7. Reversible processes occur with no change in entropy, while irreversible processes increase entropy.
8. Entropy explains the direction of natural processes, such as heat flow from hot to cold.
9. It is a key factor in determining the efficiency of thermodynamic systems.
10. Entropy changes are significant in chemical reactions, phase changes, and heat engines.

Enthalpy

1. Enthalpy is the total heat content of a system at constant pressure.
2. The symbol for enthalpy is H, and its SI unit is joules (J).
3. It is defined as H = U + PV, where:
   • U is the internal energy.
   • P is the pressure of the system.
   • V is the volume of the system.
4. Enthalpy changes are used to measure heat transfer during chemical reactions and phase changes.
5. The change in enthalpy (ΔH) for a process is given by:
   • ΔH = Q at constant pressure.
6. For exothermic reactions, ΔH is negative, as heat is released.
7. For endothermic reactions, ΔH is positive, as heat is absorbed.
8. Enthalpy is a key concept in understanding thermodynamic cycles, such as the Rankine and Brayton cycles.
9. Enthalpy is used to calculate heat changes in enthalpy of formation and enthalpy of combustion.

Internal Energy

1. Internal energy is the total energy of a system, including kinetic and potential energy at the molecular level.
2. The symbol for internal energy is U, and its SI unit is joules (J).
3. It includes the energy due to molecular motion (kinetic) and interactions (potential).
4. Internal energy is a state function, meaning it depends only on the state of the system, not the process path.
5. The change in internal energy (ΔU) is given by the First Law of Thermodynamics:
   • ΔU = Q - W
   • Q is the heat added to the system.
   • W is the work done by the system.
6. Internal energy is crucial in understanding heat and work interactions in thermodynamic systems.
7. In an ideal gas, internal energy depends only on temperature.
8. It is used to describe energy changes in processes such as heating, cooling, and compression.
9. Internal energy plays a central role in determining the properties of thermodynamic systems.

Applications of Entropy, Enthalpy, and Internal Energy

1. These concepts are essential in designing and analyzing engines, refrigerators, and power plants.
2. Entropy helps determine the direction and feasibility of natural processes.
3. Enthalpy is used to calculate heat changes in chemical reactions and phase transitions.
4. Internal energy is fundamental in energy conservation and thermodynamic cycles.
5. These principles are applied in studying the Earth's atmosphere, weather systems, and climate.
6. They are crucial in chemical engineering, particularly in reaction thermodynamics.
7. Advanced technologies like fuel cells, heat pumps, and solar panels rely on these thermodynamic concepts.