Thermodynamics

• Why are theoretical lattice enthalpies often different from experimental values?

  Purely ionic model of lattice (theoretical enthalpies based on) assumes all ions are spherical & have their charge evenly distributed around them. Experimental values different as ionic compounds have some covalent character. Positive & negative ions aren't spherical, & positive ions polarise neighbouring negative ions to different extents. The more polarisation, the more covalent bonding.
• Enthalpy change of formation:

  Enthalpy change when 1 mole of a compound is formed from its elements in their standard states.
• Lattice formation enthalpy:


  Enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions.
• Lattice dissociation enthalpy:


  Enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions.
• Enthalpy change of hydration:

  Enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.
• Enthalpy change of solution:

  Enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.
• Second ionisation energy:


  Enthalpy change when 1 mole of gaseous 2+ ions is formed from 1 mole of gaseous 1+ ions.
• First electron affinity:


  Enthalpy change when 1 mole of gaseous 1- ions is made from 1 mole of gaseous atoms.
• Enthalpy change of atomisation of a compound:

  Enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms.
• First ionisation energy:


  Enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms.
• Enthalpy change of atomisation of an element:

  Enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.
• Bond dissociation enthalpy:


  Enthalpy change when all bonds of the same type in 1 mole of gaseous molecules are broken.
• What is enthalpy change?

  Heat energy transferred in a reaction at constant pressure.
• What happens when solid ionic lattices are dissolved in water?

  Bonds between ions break to give gaseous ions (endothermic process). This is lattice enthalpy of dissociation. Bonds between ions & water are formed (exothermic process). This is enthalpy change of hydration.
• What is entropy?


  Measure of disorder. Number of ways particles can be arranged & number of ways energy can be shared out between the particles.
• How does physical state affect entropy?

  Solid particles have lowest entropy & gas particles have highest entropy.
• How does dissolution affect entropy?

  Dissolving solid increases its entropy.
• How do you calculate entropy changes?


  ^S = S products - S reactants (JK-1 mol-1)
• What is standard entropy?

  Entropy of 1 mole of a substance measured under standard conditions.
• What is free-energy change? (^G)

  Measure used to predict whether a reaction is feasible (once started, will carry on to completion, without any energy being supplied to it). If negative or 0, reaction is feasible.
• Free-energy change equation:


  ^G = ^H-T^S T= ^H * ^S