Thermodynamics Explained: Core Concepts and Principles

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Basic Concepts of Thermodynamics

Thermodynamics studies the material world by selecting a portion of the universe on which attention is focused, known as a System. The remainder of the universe is considered the environment.

Classification of Thermodynamic Systems

Thermodynamic systems are categorized based on their interaction with the environment:

  • Open Systems: These systems can exchange both energy (in the form of work or heat) and matter with their surroundings.
    Example: An open glass of water.
  • Closed Systems: These systems can exchange energy with the surroundings but not matter.
    Example: A sealed jar of pickles.
  • Isolated Systems: These systems cannot exchange matter or energy with the environment. Consequently, the total amount of energy within an isolated system remains constant.
    Example: A well-insulated thermos.

State of a System

The state of a thermodynamic system is defined by its joint physical and chemical properties. For a thermodynamic system, we refer to its macroscopic properties, such as Pressure (P), Volume (V), Mass (M), Temperature (T), chemical concentration, and the concentration of each component.

Equilibrium States

Thermodynamics primarily deals with systems that are in a steady state—those whose state variables do not change over time. It studies how the variables of the system change between two equilibrium states: an initial state and a final state.

First Law of Thermodynamics

Every system possesses a thermodynamic property called internal energy (U). This property takes a defined value for each state and increases when the system absorbs heat or when work is done on it. The First Law of Thermodynamics is expressed as:

ΔU = Q + W

Where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done on the system.

Second Law of Thermodynamics

In any spontaneous process, the total entropy of the universe increases. This fundamental principle does not preclude the entropy of a system from decreasing; rather, it requires that the overall change in entropy for the universe is positive.

ΔSuniverse = ΔSsystem + ΔSenvironment > 0

Hess's Law

Hess's Law states that the enthalpy change of a reaction depends only on the initial and final states of the substances involved. Its value remains the same whether the process takes place in one or multiple stages. This law is crucial for calculating reaction enthalpies of processes that are not directly measurable, often by means of a thermodynamic cycle. Hess's Law ensures that the overall enthalpy change is independent of the path taken:

ΔHtotal = ΔH1 + ΔH2 + ΔH3 + ΔH4

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