Thermochemistry & Thermodynamics — 60 Terms Defined

Adiabatic Process Thermochemistry & Thermodynamics J A process in which no heat is exchanged between the system and its surroundings. q = 0, ΔU = w

Bomb Calorimeter Thermochemistry & Thermodynamics J A rigid sealed vessel used to measure the heat released by combustion at constant volume. q_V = ΔU

Bond Dissociation Enthalpy Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of a specific bond is broken homolytically in the gas phase. A–B(g) → A·(g) + B·(g)

Calorific Value Thermochemistry & Thermodynamics kJ/g The heat released by the complete combustion of unit mass of a fuel. CV = ΔH_c / molar mass

Calorimetry Thermochemistry & Thermodynamics J The experimental measurement of heat exchanged during a physical or chemical change. q = mcΔT

Carnot Cycle Thermochemistry & Thermodynamics Dimensionless An idealised reversible cycle of two isothermal and two adiabatic steps that defines the maximum efficiency of a heat engine. η = 1 − T_cold / T_hot

Closed System Thermochemistry & Thermodynamics Dimensionless A system that can exchange energy but not matter with its surroundings.

Coupled Reactions Thermochemistry & Thermodynamics kJ/mol Two reactions linked through a shared intermediate so that a favourable one drives an unfavourable one. ΔG_total = ΔG₁ + ΔG₂ < 0

Cyclic Process Thermochemistry & Thermodynamics J A process in which a system undergoes a series of changes and returns to its exact initial state. ΔU = 0, q = −w

Endergonic Reaction Thermochemistry & Thermodynamics kJ/mol A reaction with a positive Gibbs free energy change, which requires an input of energy to proceed. ΔG > 0

Enthalpy Thermochemistry & Thermodynamics kJ/mol A thermodynamic state function equal to internal energy plus the product of pressure and volume. H = U + pV

Enthalpy Level Diagram Thermochemistry & Thermodynamics kJ/mol A diagram plotting the enthalpies of reactants and products to show the direction and size of the enthalpy change.

Enthalpy of Atomisation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of gaseous atoms is produced from an element in its standard state. ΔH_at°

Enthalpy of Dilution Thermochemistry & Thermodynamics kJ/mol The enthalpy change when a solution is diluted from one concentration to another by adding solvent. ΔH_dil

Enthalpy of Fusion Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of a solid melts to a liquid at its melting point. ΔH_fus

Enthalpy of Neutralisation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when an acid and a base react to form one mole of water under standard conditions. H⁺ + OH⁻ → H₂O, ΔH = −57.1 kJ

Enthalpy of Reaction Thermochemistry & Thermodynamics kJ/mol The heat absorbed or released when a reaction occurs at constant pressure in the molar amounts of the balanced equation. ΔH_rxn = ΣΔH_f(products) − ΣΔH_f(reactants)

Enthalpy of Sublimation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of a solid converts directly into vapour. ΔH_sub = ΔH_fus + ΔH_vap

Enthalpy of Vaporisation Thermochemistry & Thermodynamics kJ/mol The enthalpy change when one mole of a liquid becomes vapour at its boiling point. ΔH_vap

Entropy Thermochemistry & Thermodynamics J/(mol·K) A measure of the dispersal of energy and the number of microscopic arrangements available to a system. S = k ln W

Entropy Change of the Surroundings Thermochemistry & Thermodynamics J/K The entropy change experienced by the surroundings when they absorb or release heat from a system. ΔS_surr = −ΔH_sys / T

Exergonic Reaction Thermochemistry & Thermodynamics kJ/mol A reaction with a negative Gibbs free energy change, which therefore releases usable energy and proceeds spontaneously. ΔG < 0

First Law of Thermodynamics Thermochemistry & Thermodynamics J The energy of an isolated system is constant; energy can be converted between forms but never created or destroyed. ΔU = q + w

Free Energy and Equilibrium Constant Thermochemistry & Thermodynamics kJ/mol The relation connecting the standard Gibbs energy change of a reaction to its equilibrium constant. ΔG° = −RT ln K