OCR A-Level Chemistry: Chapter 22 - Enthalpy and Entropy

The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.

Exothermic

Enthalpy change is negative

A way of indirectly calculating lattice enthalpy from other known energy changes.

Changing elements in their standard states into gaseous atoms.
Endothermic as it involves bond breaking.

Changing gaseous atoms into positive and negative gaseous ions.
Endothermic.

Changing gaseous ions into the solid ionic lattice.

Exothermic.

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.

The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Energy is required to overcome the attraction between a negative electron and a positive nucleus.

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

Electron being added is attracted in towards the nucleus.

The negative ion repels the additional electron, so energy is required to force the electron onto it.

The enthalpy change that takes place when one mole of a solute dissolves in a solvent.

Ionic lattice breaks up

Water molecules are attracted to and surround the ions.

Ionic lattice is broken up forming separate gaseous ions.
Separate gaseous ions interact with polar water molecules to form hydrated aqueous ions.
Called enthalpy change of hydration.

The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.

Ionic radius increases
Attraction between ions decreases
Lattice enthalpy becomes less negative
Melting point decreases

Ionic charge increases
Attraction between ions increases
Lattice enthalpy becomes more negative
Melting point increases

Ionic radius increases
Attraction between ion and water molecules decreases
Hydration energy less negative

Ionic charge increases
Attraction with water molecules increases
Hydration energy becomes more negative

Many compounds with endothermic enthalpy changes of solution are soluble.

A measure of the dispersal of energy within the chemicals in a chemical system.

J/K/mol

In order of size:
Gases > liquids > solids
for entropy value

The entropy of one mole of a substance, under standard conditions.
(Always positive)

Change in entropy is the sum of the entropies of the products minus the sum of the entropies of the reactants.

Whether a reaction is able to happen and is energetically feasible (or spontaneous).

ΔG = ΔH - TΔS

ΔG < 0

| A decrease in free energy.

High activation energy resulting in a very slow rate.