Physical Chemistry Thermodynamics: Enthalpy, Heat Capacity, and Reaction Energetics

Practice vocabulary flashcards covering thermodynamic system definitions, work and heat molecular definitions, internal energy, enthalpy, heat capacities, Hess's law, transition enthalpies, and standard formation enthalpies.

Open System

A thermodynamic system that can exchange both matter and energy with its surroundings.

Closed System

A thermodynamic system that can exchange energy with its surroundings, but cannot exchange matter.

Isolated System

A thermodynamic system that exchanges neither matter nor energy with its surroundings.

Energy Transfer as Work

Transfer of energy that gives rise to uniform motion of molecules in the surroundings.

Energy Transfer as Heat

Transfer of energy that gives rise to random motion of molecules in the surroundings.

Thermodynamic Sign Convention for Heat ($q$)

$q > 0$ when heat enters the system, increasing system energy, and $q < 0$ when heat leaves the system, decreasing system energy.

Thermodynamic Sign Convention for Work ($w$)

$w > 0$ when work is performed on the system, increasing system energy, and $w < 0$ when the system performs work on the surroundings, decreasing system energy.

Irreversible Expansion Work

Work performed when a system expands against a fixed external pressure ($p_{\text{ex}}$), calculated as $w = -p_{\text{ex}} \cdot \Delta V$.

Reversible Isothermal Expansion Work

Work performed during a reversible isothermal expansion, equal to the area under the $pV = nRT$ isotherm and calculated as $w = -nRT \ln\left(\frac{V_f}{V_i}\right)$.

Heat Capacity ($C$)

The ratio of heat supplied to a system to the resulting temperature change ($C = \frac{q}{\Delta T}$), having units of $J\,K^{-1}$ or $J\,^\circ\text{C}^{-1}$.

Internal Energy ($U$)

A state function representing the total energy reserves of a system, consisting of all kinetic and potential energies associated with its constituent atoms, ions, and molecules.

First Law of Thermodynamics (Mathematical Form)

The equation $\Delta U = q + w$, stating that the change in internal energy equals the heat added to the system plus the work performed on the system.

Heat Capacity at Constant Volume ($C_V$)

The thermodynamic quantity defining how the internal energy of a system varies with temperature at constant volume, given by $C_V = \frac{\Delta U}{\Delta T} = \frac{dU}{dT}$.

Enthalpy ($H$)

A thermodynamic state function defined as $H = U + pV$, equal to the heat transferred to or lost from a system at constant pressure ($\Delta H = q_p$).

Heat Capacity at Constant Pressure ($C_p$)

The thermodynamic quantity defining how the enthalpy of a system varies with temperature at constant pressure, given by $C_p = \frac{\Delta H}{\Delta T} = \frac{dH}{dT}$.

Comparison of $C_p$ and $C_V$

Because expansion work is performed during heating at constant pressure, $C_p > C_V$ and $C_{p,m} > C_{V,m}$ always; for an ideal gas, $C_p = C_V + nR$.

Standard State

The reference condition defined as a pure substance at a pressure of $p = 1\,\text{bar} = 10^5\,\text{Pa}$ and a specified temperature, typically $25.00\,^\circ\text{C} = 298.15\,\text{K}$.

Hess's Law

The principle stating that the standard enthalpy change for an overall process is equal to the sum of the enthalpy changes of its individual intermediate steps, which holds because enthalpy is a state function.

Enthalpy of Fusion ($\Delta_{\text{fus}}H$)

The standard enthalpy change accompanying the phase change from solid to liquid ($s \rightarrow l$) at the transition temperature.

Enthalpy of Vaporization ($\Delta_{\text{vap}}H$)

The standard enthalpy change accompanying the phase change from liquid to gas ($l \rightarrow g$) at the transition temperature.

Enthalpy of Sublimation ($\Delta_{\text{sub}}H$)

The standard enthalpy change accompanying the direct transition from solid to gas ($s \rightarrow g$), satisfying the relationship $\Delta_{\text{sub}}H = \Delta_{\text{fus}}H + \Delta_{\text{vap}}H$.

Thermochemical Equation

A balanced stoichiometric chemical equation that explicitly includes the standard enthalpy change ($\Delta H^\theta$) for the reaction as written.

Standard Reaction Enthalpy ($\Delta_r H^\theta$)

The enthalpy change per mole of reaction under standard conditions, calculated via $\Delta_r H^\theta = \sum \nu \Delta_f H^\theta(\text{products}) - \sum \nu \Delta_f H^\theta(\text{reactants})$.

Bond Dissociation Enthalpy ($\Delta_{(A-B)}H$)

The enthalpy change accompanying the breaking of a specific $A-B$ chemical bond in the gas phase.

Mean Bond Enthalpy ($\Delta_B H$)

The average bond dissociation enthalpy for a specific chemical bond measured across a variety of polyatomic gas-phase molecules.

Standard Enthalpy of Formation ($\Delta_f H^\theta$)

The standard molar enthalpy change for forming $1\,\text{mol}$ of a substance from its constituent elements in their reference states.

Reference State of an Element

The most stable physical form of an element at $p = 1\,\text{bar}$ and a designated temperature (usually $298.15\,\text{K}$), defined to have $\Delta_f H^\theta = 0$.

Exothermic Compound

A compound with a negative standard enthalpy of formation ($\Delta_f H^\theta < 0$), indicating that it is thermodynamically stable relative to its constituent elements.

Endothermic Compound

A compound with a positive standard enthalpy of formation ($\Delta_f H^\theta > 0$), indicating that it is thermodynamically unstable relative to its constituent elements.