UC San Diego Chemistry 6B - Thermodynamics Lecture Notes

Vocabulary flashcards covering the core principles of thermodynamics including laws, state functions, work, heat, enthalpy, entropy, and Gibbs free energy based on Chem 6B lecture notes.

Thermodynamics

The study of the transformations of energy from one form into another.

System

The region of interest; for example, a chemical reaction.

Surroundings

Everything besides the system; in aqueous reactions, the water is considered part of this.

Universe

The combination of the system and the surroundings.

Open System

A system that can exchange both matter and energy with the surroundings, such as the human body.

Closed System

A system with a fixed amount of matter that can still exchange energy with the surroundings, like a hot or cold pack.

Isolated System

A system that has no contact with its surroundings and can exchange neither matter nor heat, such as liquid inside a sealed thermally insulated vacuum flask.

Work ($w$)

The process of achieving motion against an opposing force, calculated as $w = F \times d$.

Energy

The capacity of a system to do work.

Internal Energy ($U$)

The total store of energy in a system.

Expansion Work

Work arising from a change in the volume of a system.

Nonexpansion Work

Work that does not involve a change in volume, such as a chemical reaction in a battery causing electrical current to flow.

Free Expansion

Expansion against zero pressure ($P_{ex} = 0$), which occurs in a vacuum and produces no work ($w = 0$).

Heat ($q$)

Energy transferred between a system and surroundings as a result of a temperature difference.

Exothermic Process

A process that releases heat into the surroundings, denoted by a negative value ($q < 0$).

Endothermic Process

A process that absorbs heat from the surroundings, denoted by a positive value ($q > 0$).

Heat Capacity ($C$)

The amount of heat needed to cause a specific $\triangle T$ in a given amount of substance; the ratio of heat supplied to the temperature rise produced ($C = \frac{q}{\triangle T}$).

Specific Heat Capacity ($C_s$)

The amount of heat needed to raise the temperature of $1.0\text{ g}$ of a substance by $1\text{ degree}$; units are $J/g\text{ °C}$ or $J/g\text{ K}$.

Molar Heat Capacity ($C_m$)

The amount of heat needed to raise the temperature of $1.0\text{ mol}$ of a substance by $1\text{ degree}$; units are $J/mol\text{ K}$.

Calorimeter

An instrument used to measure the transfers of energy as heat.

First Law of Thermodynamics

States that the internal energy of an isolated system is constant ($\triangle U = q + w$).

State Function

A property that depends only on the current state of the system and is independent of the path taken to achieve that state, such as $P$, $V$, $T$, and internal energy ($U$).

Enthalpy ($H$)

A state function that tracks energy changes at constant pressure, defined as $H = U + PV$. At constant pressure, $\triangle H = q_p$.

Enthalpy of Vaporization ($\triangle H_{vap}$)

The difference in molar enthalpy between the vapor and liquid states ($\triangle H_{vap} = H_{m(vapor)} - H_{m(liquid)}$).

Enthalpy of Fusion ($\triangle H_{fus}$)

The enthalpy change accompanying the melting or fusion of a substance ($\triangle H_{fus} = H_{m(liquid)} - H_{m(solid)}$).

Enthalpy of Sublimation ($\triangle H_{sub}$)

The molar enthalpy change when a substance changes directly from a solid to a gas ($\triangle H_{sub} = \triangle H_{fus} + \triangle H_{vap}$).

Heating Curve

A graph showing the variation in the temperature of a sample as it is heated at a constant rate and constant pressure.

Thermochemical Equation

A chemical equation that includes a statement of the corresponding enthalpy change for the reaction ($\triangle H_{rxn}$).

Standard State

The pure form of a substance at exactly $1\text{ bar}$ of pressure.

Standard Reaction Enthalpy ($\triangle H^°$)

The reaction enthalpy when all reactants and products are in their standard states.

Hess’s Law

States that the overall reaction enthalpy is the sum of the reaction enthalpies of the individual steps into which the reaction can be divided.

Standard Molar Enthalpy of Formation ($\triangle H_f^°$)

The enthalpy change for a reaction in which one mole of a substance is formed from its elements in their most stable forms.

Bond Enthalpy ($\triangle H_B$)

The heat required to break a specific type of covalent bond at constant pressure in the gas phase; always a positive value.

Spontaneous Change

A change or reaction that has a tendency to occur without needing an external influence.

Entropy ($S$)

A measure of disorder; energy and matter tend to disperse in a disorderly fashion.

Second Law of Thermodynamics

States that the entropy of an isolated system increases in the course of any spontaneous change.

Positional Disorder

Disorder related to the locations occupied by molecules, primarily affected by volume changes.

Thermal Disorder

Disorder related to the thermal motion of molecules, primarily affected by temperature changes.

Transition Temperature

The temperature at which a substance undergoes a phase change, such as the normal boiling point ($T_b$) or normal melting point ($T_f$).

Microstate

Each possible arrangement of the molecules in a sample representing a specific state.

Boltzmann Formula

Relates the statistical entropy ($S$) to the number of microstates ($W$) as $S = k \text{ ln } W$, where $k = 1.381 \times 10^{-23}\text{ J/K}$.

Third Law of Thermodynamics

States that the entropies of all perfect crystals approach zero as the absolute temperature approaches zero.

Standard Molar Entropy ($S_m^°$)

The molar entropy of a pure substance at $1\text{ bar}$ of pressure.

Dynamic Equilibrium

A state where forward and reverse processes continue at matching rates, resulting in no net macroscopic change ($\triangle S_{total} = 0$).

Gibbs Free Energy ($G$)

A state function defined as $G = H - TS$, used to predict spontaneity at constant temperature and pressure.

Thermodynamically Stable Compound

A compound with a negative standard Gibbs free energy of formation ($\triangle G_f^° < 0$).

Thermodynamically Unstable Compound

A compound with a positive standard Gibbs free energy of formation ($\triangle G_f^° > 0$), showing a tendency to decompose into its elements.

Nonlabile (Inert)

Describes thermodynamically unstable substances that survive for long periods without decomposing.

Labile

Describes thermodynamically unstable substances that decompose readily or react rapidly.