Enthalpy and Entropy

• solid ionic compound tend to be very stable - the stability arises from the strength of the ionic bonds, electrostatic attractions between oppositely-charged ions in the ionic lattice structure. The creates a substantial energy barrier that must be overcome to break down the lattice, reflected in high MP of many ionic compounds.
• Lattice enthalpy is a measure of the strength of ionic bonding in a giant ionic lattice
• Lattice enthalpy is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
• Lattice enthalpy involves ionic bond formation from separate gaseous ions

  • it is an exothermic change
  • value for enthalpy change will always be negative
• Lattice enthalpy can not be measured directly and must be calculated indirectly using known energy changes in an energy cycle
• formation of gaseous atoms:

  • changing the elements in their standard states into gaseous atoms
  • this change is endothermic as it involves bond breaking
• formation of gaseous ions:

  • changing the gaseous atoms into positive and negative gaseous ions
  • overall, this change is endothermic
• lattice formation:

  • changing the gaseous ions into the solid ionic lattice
  • this is the lattice enthalpy and is exothermic
• the standard enthalpy change of formation:

  • is the enthalpy change that takes place when one mole of a compound is formed from its elements under std conditions , with all reactants and products in their std states
  • the compound will always be an ionic compound and in its solid lattice
• the standard enthalpy of atomisation:

  • is the enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state, under std conditions
  • always endo - bonds are broken to form gaseous atoms
  • when the element is a gas in its std state - this value is related to the bond enthalpy of the bond being broken
• 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
  • always endo - energy required to overcome attraction between negative electron and positive nucleus
• electron affinity is the opposite of ionsation energy:

  • measures the energy to gain electrons
  • ionisation energy measures the energy to lose electrons
• 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 - electron being added is attracted in towards the nucleus
• successive electron affinities:

  • when an anion has a greater charge than 1-, successive electron affinities are required
  • dealt with in a similar way to successive ionisation energies