3.1.8 Thermodynamics

Hess’ Law

Hess’ Law

The enthalpy change for a chemical reaction is the same regardless of the route from reactants to products.

Standard enthalpy of formation

The enthalpy change when one mole of a compound is formed from its elements under standard conditions and standard states.

Standard enthalpy of combustion

The enthalpy change when one mole of a compound is burned completely in oxygen under standard conditions (EXOTHERMIC).

Standard enthalpy of atomisation

The enthalpy change when one gaseous atom is formed from its element in standard state (ENDOTHERMIC).

First ionisation enthalpy

The standard enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give the atom a positive charge (ENDOTHERMIC).

Second ionisation enthalpy

The standard enthalpy change when one mole of electrons is removed from a +1 gaseous ion to form a gaseous ion with a 2+ charge (ENDOTHERMIC).

First electron affinity

The standard enthalpy change when one mole of gaseous ions is converted to a gaseous ion with a -1 charge under standard conditions (EXOTHERMIC).

Second electron affinity

The standard enthalpy change when one mole of electrons is added to a gaseous ion with a -1 charge to form an ion with a 2- charge.

Lattice formation enthalpy

The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions (EXOTHERMIC).

Lattice dissociation enthalpy

The standard enthalpy change when one mole of solid ionic compound dissociates into gaseous ions (ENDOTHERMIC).

Standard enthalpy of hydration

The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions (EXOTHERMIC).

Standard enthalpy of solution

The standard enthalpy change when one mole of solute dissolves in solvent to form a solution where the ions are too far to interact.

Mean bond enthalpy

The enthalpy change when one mole of gaseous molecules breaks a covalent bond to form two free radicals (ENDOTHERMIC).

Born-Haber cycle

A thermodynamic cycle that relates the enthalpy changes involved in the formation of an ionic compound from its elements.

Perfect ionic model

A theoretical model where ions are considered as point charges and perfect spheres with no covalent character.

Entropy

The amount of disorder within a system, always positive, and measured in JK⁻¹mol⁻¹.

Gibbs free-energy change

A measure that determines whether a chemical reaction is feasible, calculated as ΔG = ΔH - TΔS.

Feasibility of a reaction

A reaction is feasible if the Gibbs free-energy change is equal to or less than 0.