3.1.8 Thermodynamics

Definitions

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 of one mole of a compound is formed from its elements under standard conditions and standard states

Standard enthalpy of combustion = The enthalpy change of one mole of a compound burned completely in oxygen in standard conditions when all reactants and products are in their standard states (EXOTHERMIC)

Standard enthalpy of Atomisation = The enthalpy change of one gaseous atom formed from its element in standard state (ENDOTHERMIC)

First Ionisation Enthalpy = The standard enthalpy change of 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 make a gaseous ion with 2+ charge (ENDOTHERMIC)

First Electron Affinity = The standard enthalpy change of one mole of gaseous ions is converted to a gaseous ion with -1 charge under standard conditions (EXOTHERMIC)

Second Electron Affinity = The standard enthalpy change of one mole of electrons is added to a gaseous ion with a single -1 charge to form an ion with 2- charge

Lattice Formation Enthalpy = The standard enthalpy of 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 of mole of gaseous ions converted into one mole of aqueous ions (EXOTHERMIC)

Standard Enthalpy of Solution = The standard enthalpy change of one mole of solute dissolves in solvent to form a solution where the ions are too far to interact

Mean Bond Enthalpy(Bond Dissociation Enthalpy) = The enthalpy change when one mole of gaseous molecules break a covalent bond to form two free radicals (ENDOTHERMIC)

Born-Haber Cycles

Rules:

• The direction of the arrows (^) = POSITIVE (v) = NEGATIVE

• Each step should be balanced

• Must include state symbols

Each Cycle starts with Enthalpy Change of Formation of the ionic compound

Like NaCl → INTO NA + CI

Then both elements → Standard enthalpy of Atomisation

Na(s) → Na(g)

Cl(s) → Cl(g)

Then the First Ionisation energy enthalpy

Na → Na + e

Then lastly the First Electron Affinity Ionisation of Cl

Cl gains the e- from Ionisation of the other element

Standard Enthalpy of Atomisation = Bond Dissociation / 2

More Negative Standard Enthalpy of Lattice Formation = Stronger Ionic Bonds

Trends

Hydration Enthalpies:

• The Size of the atoms (Ionic Radius)

• The Charge of the atoms

Charge is more important than size during comparisons

Stronger attraction of ions to water will result in a more exothermic hydration enthalpy

Charge : As charge increases attraction increases

Radius : As size decreases attraction increases

Perfect Ionic Model

“Perfect Ionic Model” refers to compounds which displays purely ionic bonding with no covalent character

• Ions act as point charges

• Ions are perfect spheres which cannot be distorted

• Ions shows purely ionic bonding with no covalent character

The theoretical Standard Enthalpy of Formation calculated by the perfect ionic model is equal to the Born-Haber Standard Enthalpy of Formation calculated by the experimental model.

If there is a covalent character in the ionic compound then :

The theoretical Standard enthalpy of formation calculated by the perfect ionic model is less exothermic than the born-Haber standard enthalpy of formation calculated by the experimental model

Entropy Change and Gibbs Free-Energy Change

Factors which determine whether a chemical reaction is feasible or not :

• Temperature

• Enthalpy - EXOTHERMIC REACTIONS ARE MORE FEASIBLE THAN ENDOTHERMIC

ENTROPY = IS THE AMOUNT OF DISORDER WITHIN A SYSTEM WHICH IS ‘S’ AND IS ALWAYS POSITIVE BUT ENTHALPY OF ENTROPY = POSITIVE AND NEGATIVE ( JK-1mol-1) and converted to (kJK-1mol-1) by /1000

Gases = Highest Entropies (more +)

Solids = Lowest Entropies (least +)

At 0K substances have 0 entropy and there is no disorder as particles are stationary

Enthalpy of G = Enthalpy of H - Temperature and Entropy change

A REACTION IS FEASIBLE IF THE ENTHALPY OF GIBBS FREE-ENERGY IS EQUAL OR LESS THAN 0