Thermodynamics: The Physics of Energy Transfer

Introduction to Thermochemistry

Thermodynamics governs the relationships between heat, work, and energy. Thermochemistry specifically focuses on measuring and quantifying these changes within chemical systems. Understanding these principles is fundamental to many areas of science and engineering.

mysimulator teamUpdated June 2026≈ 5 min read▶ Open the simulation

System, Surroundings, and State

A thermodynamic system is defined by its boundaries, which separate it from its surroundings. The surroundings encompass everything outside the system.

The state of a system describes its condition at a specific point in time – defined by variables like temperature (T), pressure (P), and volume (V). Changes in these variables drive thermodynamic processes.

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Heat, Work, and Energy Transfer

Heat (Q) is the transfer of energy due to a temperature difference. It's measured in Joules (J).

Work (W) is done when a force causes displacement. In thermodynamics, this often refers to pressure-volume work: W = -PΔV.

W = -PΔV

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The First Law of Thermodynamics

The first law states that energy is conserved. The change in internal energy (ΔU) of a system equals the heat added to it minus the work done by it: ΔU = Q - W.

ΔU = Q - W

Enthalpy and Heat Changes

Enthalpy (H) is a thermodynamic property defined as H = U + PV. It’s useful for calculating heat changes during reactions at constant pressure.

The change in enthalpy (ΔH) represents the heat absorbed or released during a process at constant pressure. Exothermic processes have ΔH < 0, while endothermic processes have ΔH > 0.

ΔH = ΔU + PΔV

Frequently asked questions

What is the difference between heat and temperature?

Heat is a form of energy transfer due to a temperature difference, while temperature is a measure of the average kinetic energy of molecules.

Why is pressure-volume work important in thermodynamics?

Pressure-volume work occurs when a gas expands or contracts against an external pressure – a common process in many chemical reactions.

How does the first law relate to conservation of energy?

The first law demonstrates that energy cannot be created or destroyed; it can only change forms, like heat and work.

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