Thermochemistry: Enthalpy and Standard Enthalpy Changes

Vocabulary flashcards covering enthalpy, heat capacities, state functions, standard enthalpy changes, and Hess's law concepts discussed in the thermochemistry lecture.

Enthalpy (H)

A state function representing the heat content of a system; defined as H = U + PV; changes in enthalpy relate to heat flow at constant pressure.

Internal energy (U)

Total microscopic energy of a system; related to enthalpy by H = U + PV.

Heat capacity at constant pressure (Cp)

The amount of heat required to raise the temperature of a system by 1 K at constant pressure.

Heat capacity at constant volume (Cv)

The amount of heat required to raise the temperature by 1 K at constant volume.

Cp − Cv (ideal gas relation)

For an ideal gas, Cp,molar − Cv,molar = R; reflects work done during expansion.

Standard enthalpy change (ΔH°)

The enthalpy change of a reaction when all reactants and products are in their standard states (1 bar, commonly 298 K).

Standard conditions

1 bar pressure with substances in their standard states; experimental data are typically reported at 298 K.

Standard enthalpy of vaporization (ΔHvap°)

Enthalpy change when 1 mole of liquid becomes gas at 1 bar (and 298 K).

Sublimation enthalpy (ΔHsub°)

Enthalpy change when a solid changes directly to a gas at 1 bar.

Thermochemical reaction

A chemical reaction for which an enthalpy change can be defined and linked to the reactants and products.

Thermochemical equation

A chemical equation that includes the standard enthalpy change (ΔHr°) for the reaction.

State function

A property that depends only on the current state of a system, not on the path taken; enthalpy is one.

Enthalpy of formation (ΔHf°)

Standard enthalpy change for forming 1 mole of a substance from its elements in their standard states.

Hess's law

The total enthalpy change for a process is the same regardless of the route taken; enthalpy is a state function.

Enthalpy change of transition (ΔHtrans°)

Enthalpy changes associated with physical phase transitions, such as ΔHfus°, ΔHvap°, and ΔHsub°.

Standard reaction enthalpy formula (ΔHr°)

ΔHr° = Σ (stoichiometric coefficient × ΔHf° of products) − Σ (stoichiometric coefficient × ΔHf° of reactants); equivalently sum over products minus reactants.