States and State Functions

The state of a system is described by the macroscopic properties of the system. Examples of
macroscopic properties include temperature (T), pressure (P), and volume (V). When the state of a
system changes, the values of the properties also change. If the change in the value of a property
depends only on the initial and final states of the system, and not on the path of the change (how
the change was accomplished), that property is known as a state function. Pressure, temperature,
and volume are important state functions. Other examples are enthalpy (H), entropy (S), Gibbs free
energy (G) (all discussed below), and internal energy (E or U).

DON’T MIX THESE UP ON TEST DAY

Do  not confuse standard    conditions  with    standard    temperature and pressure    (STP)   used    in

gas law calculations.

A set of standard conditions (25°C and 1 atm) is normally used for measuring the enthalpy, entropy,
and Gibbs free energy of a reaction. A substance in its most stable form under standard conditions is
said to be in its standard state. Examples of substances in their standard states include hydrogen as
H 2 (g), water as H 2 O (l), and salt as NaCl (s). The changes in enthalpy, entropy, and Gibbs free energy
that occur when a reaction takes place under standard conditions are symbolized by ΔH°, ΔS°, and
ΔG°, and are known as the standard change in enthalpy (or standard enthalpy change), et cetera.

ENTHALPY

Most reactions in the lab occur under constant pressure (at 1 atm, in open containers). (Reactions
carried out under a constant pressure are said to be isobaric.) To express heat changes at constant
pressure, chemists use the term enthalpy (H), often thought of as the “heat content” of a system.
The change in enthalpy (∆H) of a process is equal to the heat absorbed or evolved by the system at
constant pressure. Since enthalpy is a state function, the enthalpy change of a process depends only