5.5- Enthalpy and Entropy

what are Born-Haber cycles used for

calculating lattice enthalpy of an ionic product

constructing Born Haber cycles

1. Atomisation

2. Ionisation

3. Electron Affinity

adapting Born-Haber cycles for different compounds

• group 2 compounds:

  • 2nd ionisation energy included

  • atomisation enthalpy of non metal double

• 2 group 1 compounds required:

  • atomisation enthalpy of metal doubled

  • second e^- affinity included

what is enthalpy chnage

heat energy transferred in a reaction at constant pressure

what is enthalpy change of formation

the enthalpy change when 1 mole of a compound is formed form its elements in their standard states under standard conditions

what is enthalpy change of atomisation

the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state under standard conditions

first ionisation energy

the enthalpy change when 1 mole of gaseous 1⁺ ions is formed from 1 mole gaseous atoms

second ionisation energy

the enthalpy change when 1 mole of gaseous 2⁺ ions is formed from 1 mole of gaseous 1⁺ ions

first electron affinity

the enthalpy change when 1 mole of gaseous 1^- ions is formed form 1 mole gaseous atoms

second electron affinity

the enthalpy change when 1 mole of gaseous 2^- ions is formed from 1 mole of gaseous 1^- ions

bond enthalpy

the enthalpy change when 1 mole of a particular covalent bond in the gaseous state is broken

what is lattice enthalpy

• the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions

• a measure of the strength of the electrostatic forces holding ions together in an ionic lattice

why is lattice enthalpy always negative

• energy is released when oppositely charged ions come together to form solid lattice→ bond forming

• more negative= stronger ionic bonding in the compound

factors affecting lattice enthalpy

• ionic charge:

  • ions with higher charges have stronger ES attractions than ions with lower charges

  • more energy released when lattice forms= more negative lattice enthalpy

• ionic radius:

  • smaller ions have a higher charge density and can pack more closely together in the lattice

  • increases the strength of the ES attraction between the ions

  • smaller ions= more negative lattice enthalpies

processes involved in dissolving ionic solids in water

1. breaking bonds in ionic lattice:

   • form gaseous ions

   • endothermic

   • equal to lattice enthalpy but +ive

2. forming bonds between ions and water:

   • bonds form between gaseous ions and water molecules to form hydrated ions

   • exothermic

   • known as enthalpy change of hydration

enthalpy change of hydration

• enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions

• always exothermic→ bond forming

enthalpy change of solution

• enthalpy change when 1 mole of solute is dissolved in sufficient water to form a very dilute solution

• can be either endo or exothermic depending on balance between bond breaking and bond forming energy

calculating enthalpy change of solution

factors affecting enthalpy of hydration

• ionic charge:

  • greater charge= stronger ES attractions to polar water molecules

  • more energy releases when bonds form= more exothermic

• ionic radius:

  • smaller ions= stronger charge density= can attract water molecules more strongly compared to larger ions

  • smaller ions= more exothermic

what is entropy

• a measure of disorder/ randomness

• S

entropy values

• entropy is always positive- increases as disorder increases

• high entropy= high level of disorder

• systems naturally tend toward states of higher entropy

factors affecting entropy

• Physical state

• number of particles

how does physical state affect entropy

• solids have lowest entropy→ particles in fixed positions= highly ordered structure

• liquids have higher entropy as particles can move= some disorder

• gases have highest entropy- random motion and wide spacing of particles= most disordered

how does the number of particles affect entropy

• increases with number of particles in a system

• more particles= greater number of possible arrangements and energy distributions

entropy and reaction feasibility

• higher entropy= greater energetic stability, so particles tend toward more disordered states

• drive for disorder makes certain reactions feasible even when endothermic

  • entropy increase overcomes endothermic enthalpy change= reaction can occur spontaneously at RTP

calculating entropy changes for reactions

ΔS = S_products − S_reactants

Gibbs free energy change

• represents overall change in energy during a chemical reaction

• used to determine if chemical reaction is thermodynamically feasible under certain conditions

the free energy equation

• ΔG = ΔH − TΔS

• ΔG- Gibbs free energy change (J mol^-1)

• ΔH- enthalpy change (Jmol^-1)

• T- temperature (K)

• ΔS- entropy change (JK^-1mol^-1)

feasibility when ΔH is negative and ΔS is positive

• ΔG always negative

• reaction feasible at any temperature

feasibility when ΔH is positive and ΔS is negative

• ΔG is always positive

• reaction is not feasible at any temperature

feasibility when ΔH and ΔS are positive

• reaction only feasible above specific temperature

• higher temperature means ΔS will be higher

feasibility when ΔH and ΔS are negative

• reaction is feasible below certain temp

calculating temp at which reaction becomes feasible

• ΔG=0

• T=ΔH/ΔS

limitations of using ΔG to predict reaction feasibility

• -ive ΔG doesn’t account for reactions rate or kinetic barriers

• some reactions with -ive ΔG may proceed very slowly or require significant activation energy→ unobservable under normal conditions