Thermodynamics

• enthalpy change of formation

  the standard enthalpy change of formation for a compound is the energy when 1 mole of the compound is formed from its element under standard condition, all reactant and products being in their standard states
• Enthalpy of atomisation 


  is the enthalpy change when 1 mole of gaseous atoms formed from the element in its standard state
• Na(s) + 1/2 Cl2(g) -> NaCl (s)

  enthalpy of formation
• Na(s) -> Na(g)

  1/2 O2 -> O (g)

  enthalpy of atomisation
• Enthalpy of sublimation 


  the enthalpy change for a solid metal turning to gaseous atoms
• Value of enthalpy of sublimation will always be the same as the enthalpy of atomisation
• Na(s) -> Na(g)


  enthalpy of sublimation
• Bond dissociation enthalpy (bond energy)

  the bond dissociation enthalpy is the standard molar enthalpy change when one mole of a covalent bon is broken into two gaseous atoms (or free radicals)
• Cl2(g) -> 2Cl(g)

  CH4(g) -> CH3(g) + H(g)

  Bond dissociation enthalpy
• First ionisation enthalpy 


  enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous ions with a 1+ charge
• Mg(g) -> Mg+ e-

  First ionisation enthalpy
• Second ionisation enthalpy


  is the enthalpy change to remove one mole of electrons from gaseous 1+ ions to produce one mole of gaseous 2+ ions
• First electron affinity


  the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous ions with a -1 charge
• Mg+(g) -> Mg2+(g) + e-

  second ionisation enthalpy
• O(g) + e- -> O-(g)

  first electron affinity
• O-(g) + e- -> O2-(g)

  second electron affinity
• Second electron affinity for oxygen is endothermic because it take energy to overcome repulsive force between the negative ion and the electron
• Enthalpy of lattice formation

  standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form
• Na+(g) + Cl-(g) -> NaCl(s)

  enthalpy of lattice formation
• Enthalpy of lattice dissociation

  the standard enthalpy change when 1 mole of an ionic crystal lattice dissociate into its gaseous atoms
• NaCl(s) -> Na+(g) +Cl-(g)

  enthalpy of lattice dissociation
• Enthalpy of hydration 


  enthalpy change when 1 mole of gaseous ions become aqueous ions
• X+(g) + aq -> X+(aq)

  X-(g) + aq -> X-(aq)

  enthalpy of hydration
• Enthalpy of hydration is always exothermic because the bonds made between the ions and the water molecules
• Enthalpy of solution 


  The standard enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact with one another
• NaCl (s) + aq -> Na+(aq) + Cl-(aq)

  enthalpy of solution
• things that affect lattice enthalpies

  • size of the ions
  • charges of the ions
• How does size of the ions affect lattice enthalpy?

  
  

  • larger the ions, the less negative enthalpies of lattice formation (weaker lattice)
  • As the ions are larger the charges become further apart and so have a weaker attractive force between them
• How does charge of ions affect lattice enthalpy?

  -the bigger the charge the greater the attraction between the ions so the stronger the lattice enthalpy

  -more negative, stronger lattice
• Perfect ionic model

  • theoretical lattice enthalpies assumes a perfect ionic model, where the ions are 100% ionic and spherical and the attraction are purely electrostatic
• Differences between theoretical and Born Haber lattice enthalpies

  The Born Haber lattice enthalpy is the real experimental value, when a compound shows covalent characters, the theoretical and the born haber lattice enthalpies differ. The more covalent character the bigger the difference between the values
• There is a tendency towards covalent character in ionic substances when

  the positive ion is

  • small
  • has multiple charges
• There is tendency towards covalent character in ionic substances when

  the negative ion is

  • large
  • has multiple negative charges
• When a compound has some covalent character...

  - it tends towards giant covalent so stronger lattice

  -Born-Haber values larger than theoretical value
• Why would the BH value be bigger than the theoretical value?

  When it has some covalent characters it tends towards giant covalent. So stronger lattice than if it was 100% ionic
• When the negative ion becomes distorted and more covalent we say

  it becomes polarised
• When 100% ionic, the ions are spherical. Theoretical and BH enthalpies will be the same
• Ionic with covalent characters

  Charge cloud distorted. The theoretical and experimental Born Haber enthalpies will differ
• Entropy (S')


  substances with more ways of arranging their atoms and energy have higher entropy
• A spontaneous process will proceed on its own without any external influence