Adiabatic process: A process in which no heat exchange occurs between the system and surroundings ($q = 0$). The system is perfectly insulated.

Bond dissociation enthalpy: Energy required to break 1 mole of a specific bond in gaseous molecules. Always positive (endothermic).

Cyclic process: A process where the system returns to its original state. All state function changes are zero ($\Delta U = \Delta H = \Delta S = \Delta G = 0$).

Endothermic reaction: Reaction that absorbs heat from surroundings ($\Delta H > 0$).

Enthalpy ($H$): Thermodynamic quantity $H = U + PV$. Change at constant pressure equals heat exchanged ($\Delta H = q_p$).

Entropy ($S$): Measure of disorder or randomness. $\Delta S = q_{rev}/T$. State function measured in J/(mol·K).

Exothermic reaction: Reaction that releases heat to surroundings ($\Delta H < 0$).

Free expansion: Expansion of gas into vacuum ($P_{ext} = 0$). No work done ($w = 0$).

Gibbs free energy ($G$): $G = H - TS$. Determines spontaneity at constant $T$ and $P$. $\Delta G < 0$ for spontaneous processes.

Hess's Law: The enthalpy change of a reaction is path-independent (because $H$ is a state function).

Internal energy ($U$): Total energy stored in a system (kinetic + potential energy of molecules). State function.

Isobaric process: Process at constant pressure. Heat exchanged equals $\Delta H$.

Isochoric process: Process at constant volume. No PV work done ($w = 0$).

Isothermal process: Process at constant temperature. For ideal gases: $\Delta U = 0$, $\Delta H = 0$.

Path function: A quantity that depends on the path taken (not just initial and final states). Examples: heat ($q$) and work ($w$).

Reversible process: An idealized process conducted infinitesimally slowly such that the system is always in equilibrium. Gives maximum work.

Spontaneous process: A process that proceeds without continuous external assistance. Characterized by $\Delta G < 0$ at constant $T$ and $P$.

Standard enthalpy of formation ($\Delta H_f^\circ$): Enthalpy change when 1 mole of a compound is formed from its elements in standard states at 298 K, 1 bar.

Standard state: Pure substance at 1 bar and specified temperature (usually 298 K). $\Delta H_f^\circ$ of elements in standard state = 0.

State function: A property that depends only on the current state, not on how the state was reached. Examples: $U$, $H$, $S$, $G$, $P$, $V$, $T$.