Enthalpy and Entropy

Lattice enthalpy

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

Formation of gaseous atoms

• changing the elements in their standard states into gaseous atoms

• endothermic as it involves bond breaking

Formation of gaseous ions

• changing the gaseous atoms into positive and negative gaseous ions

• endothermic

Lattice formation

• changing the gaseous ions into the solid ionic lattice

• exothermic

Standard enthalpy change of formation

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

Standard enthalpy change of atomisation

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

First ionisation energy

• 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

• endothermic because energy is required to overcome the attraction between a negative electron and the positive nucleus

First electron affinity

• 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

• exothermic because the electron being added is attracted in towards the nucleus

• second electron affinity is endothermic

Standard enthalpy change of solution

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

Enthalpy change of hydration

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

Dissolving process of solid ionic compound

1. the ionic lattice is broken up forming separate gaseous ions. this is the opposite energy change from lattice energy, which forms the ionic lattice from gaseous ions

2. the separate gaseous ions interact with polar water molecules to form hydrated aqueous ions. the energy involved is called the enthalpy change of hydration

Enthalpy Change of Solution

• the enthalpy change when one mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution

• can be exothermic or endothermic depending on the relative sizes of the lattice enthalpy and the enthalpy changes of hydration

Factors affecting lattice enthalpy - ionic size

As lattice enthalpy decreases -

• ionic radius decreases

• attraction between ions decreases

• lattice energy is less negative

• melting point decreases

Factors affecting lattice enthalpy - ionic charge

As lattice enthalpy increases -

• ionic charge increases

• attraction between ions increases

• lattice energy becomes more negative

• melting point increases

Factors affecting hydration - ionic size

As hydration enthalpy decreases -

• ionic radius increases

• attraction between ion and water molecules decreases

• hydration energy less negative

Factors affecting hydration - ionic charge

As lattice enthalpy increases -

• ionic charge increases

• attraction with water molecules increases

• hydration energy becomes more negative

Entropy

• the dispersal of energy within the chemicals making up the chemical system

• the greater the entropy, the greater the dispersal of energy and the greater the disorder

• solids have the smallest, liquids have greater, gases have the greatest entropies

Predicting entropy changes

• if a system changes to become more random, energy can be spread out more - there will be an entropy change which will be positive

• if a system changes to become less random, energy becomes more concentrated - the entropy change will be negative

Feasibility

• a reaction can happen if the products have a lower overall energy than the reactants

• feasibility is used to describe whether a reaction is able to happen and is energetically feasible

• spontaneous may also be used for energetically feasible

Free energy

• overall change in energy during a chemical reaction

• made up of 2 types - enthalpy and entropy

• enthalpy - heat transfer between the chemical system and the surroundings

• entropy - dispersal of energy within the chemical system itself