Thermodynamics: Systems, Energy, and Entropy Explained

Thermodynamic Systems and Their Interactions

A system is an isolated part of the universe under study, while the rest is considered the environment. Systems can be:

• Open: Exchange matter and energy with the environment.
• Closed: Exchange energy but not matter with the environment.
• Isolated: No exchange of matter or energy with the environment.

Chemical Systems and Thermodynamics

In chemical reactions:

• System: A group of chemicals (reactants and products).
• Thermodynamics: The study of energy exchange between a chemical system and its surroundings.

Chemical systems evolve from reactants to products, either releasing energy (exothermic reactions) or requiring energy (endothermic reactions).

Variables and Functions of State

Variables of State: Magnitudes that vary during a process (e.g., the progress of a chemical reaction).

Functions of State: Variables with a unique value for each system. Their variation depends only on the initial and final states, not the path taken. Heat and work are not functions of state.

First Law of Thermodynamics: Internal Energy

Internal energy (U) is the total energy of a system, including kinetic energy of vibration and all molecules. While impossible to measure directly, changes in internal energy can be measured. Energy supplied to the system is considered positive, while energy extracted is negative. Thus, Q and W > 0 indicate energy added to the system.

In a closed volume, the exchange of energy does not change the total energy.

Heat, Pressure, and Enthalpy

Most chemical processes occur at constant pressure, usually atmospheric. In such cases, W = -PΔV. If ΔV > 0, the system performs work on the environment and loses energy.

Standard Enthalpy of Reaction: The change in enthalpy for a reaction where both reactants and products are in standard conditions, measured in J or kJ.

Standard Enthalpy of Formation: The enthalpy change (ΔH) when one mole of a compound is formed from its elements in their standard states. This is a molar heat, measured in J/mol.

Hess's Law: The enthalpy change in a chemical reaction is constant, regardless of whether the reaction occurs in one or multiple steps.

Entropy and the Second Law of Thermodynamics

Entropy: A measure of the disorder of a system, which can be measured and tabulated.

Second Law of Thermodynamics: In any spontaneous process, the entropy of the universe tends to increase. While a system may lose entropy (become more ordered) spontaneously, the environment will become more disordered.

Third Law of Thermodynamics

Third Law of Thermodynamics: The entropy of any substance at 0K is equal to 0 (highest order).