thermodynamics

• hess’s law states that the enthalpy change for a reaction is independent of the route taken
• the standard enthalpy of formation is


  the enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions, with all reactants and products in their standard states
• the standard enthalpy of an element is 0
• the standard enthalpy of combustion is

  the enthalpy change when one mole of a substance is completely burnt in (excess) oxygen
• the standard enthalpy of atomisation is


  enthalpy change when one mole of gaseous atoms is formed from a compound in its standard state in standard conditions
• first ionisation energy is

  enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
• 2nd ionisation energy

  enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
• first electron affinity is


  enthalpy change when one mole of gaseous atoms gains one mole of electrons to form one mole of gaseous 1- atoms
• second electron affinity

  enthalpy change when one mole of gaseous ions gains one mole of electrons to form one mole of gaseous 2- ions
• lattice enthalpy of formation is

  enthalpy change when one mole of solid ionic lattice is formed from its constituent gaseous ions
• lattice enthalpy of dissociation is

  enthalpy change when one mole of solid ionic lattice is dissociated (broken) into its gaseous ions
• enthalpy of hydration

  enthalpy change when one mole of gaseous ions become hydrated/dissolved in water to infinite dilution
• enthalpy of solution

  enthalpy change when one mole of solute dissolves completely in a solvent to infinite dilution
• mean bond dissociation enthalpy

  enthalpy change when one mole of covalent bonds is broken, with all species in the gaseous state
• equation of enthalpy of formation:

  Mg(s) + 1/2O2(g) -> MgO(s)
• enthalpy of combustion:

  CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)
• enthalpy of atomisation

  1/2 I2(g) -> I(g)
• first ionisation energy

  Li(g) -> Li+(g) + e-
• second ionisation energy

  Mg+(g) -> Mg2+(g) + e-
• first electron affinity

  Cl(g) + e- -> Cl-(g)
• second electron affinity

  O-(g) + e- -> O2-(g)
• lattice enthalpy of formation

  Na+(g) + Cl-(g) -> NaCl(s)
• lattice enthalpy of dissociation

  NaCl(s) -> Na+(g) + Cl-(g)
• enthalpy of hydration

  Na+(g) -> Na+(aq)
• enthalpy of solution NaCl(s) -> Na+(aq) + Cl-(aq)
• mean bond dissociation enthalpy

  Br2(g) -> 2Br(g)
• the born-haber cycle is a thermochemical cycle showing all of the enthalpy changes involved in the formation of an ionic compound. starts with elements in their standard states
• factors affecting the lattice enthalpy of an ionic compound:

  size of ions, charge on the ions
• how can you increase lattice enthalpy of a compound?


  smaller ions because the charge centres will be closer together.

  increased charge because there will be a greater electrostatic force of attraction between the oppositely charged ions.
• born haber cycles can be used to see if compounds theoretically exist because they use known data to predict certain values of theroetical compounds and then see if these compounds would be thermodynamically stable
• the perfect ionic model assumes that ions are perfectly spherical and that there is an even charge distribution
• the perfect ionic model is often not accurate because ions are not perfectly spherical. polarisation often occurs when small positive ions or large negative ions are involved, so the ionic bond gains covalent character. some lattices are not regular and the crystal structure can differ
• if a reaction is spontaneous and feasible, it will take place of its own accord; doesnt take account of rate of reaction
• a reaction with a negative enthalpy change is more likely to be spontaneous because it is exothermic
• entropy is the randomness/disorder of a system. the higher value for entropy = more disordered
• entropy is measure in..


  JK-1mol-1
• the second law of thermodynamics is that entropy (of an isolated system) always increases as it is overwhelmingly more likely for molecules to me disorder than ordered
• reactions with positive entropy change are more likely to be spontaneous because positive reactions always try and increase the amount of disorder
• how do you calculate entropy change

  entropy change = sum of products entropy - sum of reactants entropy
• gibbs free energy is

  △G = △H - T△S

  (G=gibbs free energy, H=enthalpy change, S=entropy change, T=temperature)