System, state functions and the first law

Thermodynamics studies energy changes. First, some words:

• The system is the part of the universe we are studying, such as the contents of a beaker.
• Everything else is the surroundings.
• The boundary separates them.

Systems are of three types:

• Open: exchanges both matter and energy with the surroundings (an open beaker of hot water: steam escapes, heat escapes).
• Closed: exchanges energy only, not matter (a sealed metal flask of hot water: it cools, but nothing escapes).
• Isolated: exchanges neither (a perfect thermos flask, with a lid).

The state of a system is described by properties such as pressure, volume, temperature and amount. A state function depends only on the present state, not on how the system got there: U, H, p, V, T. Think of the height of a mountain top: it is the same whichever path you climbed. Heat and work, on the other hand, depend on the path taken; they are path functions.

Internal energy, U, is the total energy stored in a system: chemical, electrical, the energy of moving molecules, and so on. It changes when heat flows in or out, or when work is done on or by the system. A warm bottle placed in a bucket of cold water loses heat until both reach the same temperature.

The IUPAC sign convention looks from the system's point of view: q and w are positive when energy enters the system, and negative when energy leaves it. A wall that lets no heat through is adiabatic; for a change in such a system, q = 0.

The first law of thermodynamics: energy can be neither created nor destroyed; it can only change form. For a system:

∆U = q + w.