General Chemistry: Thermochemistry and the First Law of Thermodynamics

Vocabulary practice flashcards covering fundamental thermochemistry topics including the First Law of Thermodynamics, heat and work sign conventions, specific heat, phase changes, Hess's Law, and standard heats of formation.

First Law of Thermodynamics

A fundamental law stating that the change in internal energy ($\Delta E$) of a system is equal to the sum of the heat transferred into or out of the system ($q$) plus the work done on or by the system ($w$), expressed as $\Delta E = q + w$.

Work ($w$) Sign Convention

The thermodynamic convention where work is designated a positive value ($w > 0$) when work is done on the system by the surroundings, and a negative value ($w < 0$) when work is done by the system on the surroundings.

Heat ($q$) Sign Convention

The thermodynamic convention where heat transfer is positive ($q > 0$) when the system absorbs heat from the surroundings, and negative ($q < 0$) when the system releases heat to the surroundings.

Endothermic Process

A process or reaction in which thermal energy is absorbed by the system from the surroundings ($q > 0$), such as melting, evaporation, or the breaking of chemical bonds.

Exothermic Process

A process or reaction in which thermal energy is released by the system to the surroundings ($q < 0$), such as condensation, freezing, hydrocarbon combustion, or chemical bond formation.

Endothermic Phase Change Temperature Behavior

The phenomenon where the temperature of a substance remains constant during a phase change (such as melting or vaporization) despite heat being added, because the absorbed energy is used exclusively to overcome intermolecular forces between particles.

Specific Heat ($C_s$ or $C_p$)

The quantity of heat required to raise the temperature of $1\,\text{g}$ of a substance by $1\,\text{K}$ (or $1^{\circ}\text{C}$), calculated using the formula $C_s = \frac{q}{m \cdot \Delta T}$.

Molar Heat Capacity ($C_m$)

The physical property defined as the quantity of heat required to raise the temperature of $1\,\text{mol}$ of a substance by $1\,\text{K}$, related to specific heat capacity ($C_p$) and molar mass ($M_r$) by $C_m = C_p \times M_r$.

Heat of Fusion ($\Delta H_{fus}$)

The quantity of heat required to melt a unit mass or mole of a solid into liquid at its melting point without a change in temperature, calculated as $q = m \cdot \Delta H_{fus}$.

Heat of Vaporization ($\Delta H_{vap}$)

The quantity of heat required to convert a unit mass or mole of a liquid into gas at its boiling point without a change in temperature, calculated as $q = m \cdot \Delta H_{vap}$.

Chemical Bond Formation Energetics

A process that always releases energy (exothermic) when an attractive bond forms between atoms, yielding a negative enthalpy change.

Chemical Bond Cleavage Energetics

A process that requires an input of energy (endothermic) to overcome attractive electrostatic forces and separate bonded atoms, resulting in a positive enthalpy change.

Hess's Law

The principle stating that if a chemical reaction is carried out in a series of steps, the overall enthalpy change ($\Delta H_{rxn}$) is equal to the sum of the enthalpy changes of the component steps: $\Delta H_{rxn} = \Delta H_1 + \Delta H_2 + \Delta H_3 + \dots$.

Standard Enthalpy of Formation ($\Delta H^\circ_f$)

The enthalpy change accompanying the formation of one mole of a substance from its constituent elements in their standard reference states; defined as exactly $0\,\text{kJ}\,\text{mol}^{-1}$ for any pure element in its most stable form (e.g., $O_2(g)$, $Al(s)$).

Standard Enthalpy of Reaction ($\Delta H^\circ_{reaction}$)

The total heat of reaction calculated from tabulated heats of formation using the equation $\Delta H^\circ_{reaction} = \sum n\Delta H^\circ_f(\text{products}) - \sum m\Delta H^\circ_f(\text{reactants})$.

Melting (Fusion)

The endothermic phase transition in which a substance changes from a solid to a liquid upon absorbing thermal energy ($\Delta H > 0$).

Freezing (Solidification)

The exothermic phase transition in which a substance changes from a liquid to a solid upon releasing thermal energy ($\Delta H < 0$).

Vaporization

The endothermic phase transition in which a substance changes from a liquid to a gas upon absorbing thermal energy ($\Delta H > 0$).

Condensation

The exothermic phase transition in which a substance changes from a gas to a liquid upon releasing thermal energy ($\Delta H < 0$).

Sublimation

The endothermic phase transition in which a substance changes directly from a solid to a gas without passing through an intermediate liquid state ($\Delta H > 0$).

Deposition

The exothermic phase transition in which a substance changes directly from a gas to a solid without passing through an intermediate liquid state ($\Delta H < 0$).