Thermodynamics

• Enthalpy of atomisation


  The enthalpy of atomisation of an element is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state
• Bond dissociation enthalpy (bond energy)


  The bond dissociation enthalpy is the standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals)
• First ionisation enthalpy


  The enthalpy change required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a +1 charge
• Second ionisation enthalpy

  The enthalpy change to remove 1 mole of electrons from one mole of 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
• Second electron affinity


  The enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions
• Enthalpy of lattice formation


  The standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form
• Enthalpy of lattice dissociation

  The standard enthalpy change when 1 mole of an ionic crystal lattice form is separated into its constituent ions in gaseous form
• Enthalpy of hydration hydH


  Enthalpy change when one mole of gaseous ions become aqueous ions
• 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
• By applying Hess's law, the heat of formation equals to the sum of everything else
• Factors affecting the strength of an enthalpy of lattice formation

  • The sizes of the ions
  • The charges on the ion
• The larger the ions


  The less negative the enthalpies of lattice formation (i.e. a weaker lattice)
• The bigger the charge of the ion

  The greater the attraction between the ions so the stronger the lattice enthalpy (more negative values)
• Polarisation


  When the negative ion becomes distorted and more covalent
• Tendencies towards covalent character in ionic substances

  • The positive ion is small
  • The positive ion has multiple charges
  • The negative ion is large
  • The negative ion has multiple negative charge
• Spontaneous process


  A process that will proceed on its own without any external influence
• An exothermic reaction


  Results in products that are more thermodynamically stable than the reactants
• Entropy


  A description of the number of ways atoms can share energy
• Substances with more ways of arranging their atoms and energy (more disordered)

  Have a higher entropy
• An increase in disorder and entropy

  Will lead to a positive entropy change
• Situations where a significant increase in entropy will occur

  • There is a change of state from solid or liquid to gas
  • There is a significant increase in number of molecules between products and reactants
• Gibbs Free Energy Change, ∆G


  The balance between entropy and enthalpy determines the feasibility of a reaction. This is given by the relationship: ∆G = ∆H - T∆S
• For any spontaneous change

  ∆G will be negative
• Increasing temperature


  Makes it more likely that ∆G is negative and more likely that the reaction will occur if the reaction involves an increase in entropy (∆S is +ive)
• Increasing temperature


  Makes it less likely that ∆G is negative and less likely that the reaction will occur if the reaction involves a decrease in entropy (∆S is - ive)
• If the reaction has a ∆S close to zero

  Temperature will not have a large effect on the feasibility of the reaction as - T∆S will be small and ∆G will not change much
• The gradient of the graph of ∆G = ∆H - T∆S

  Is equal to -∆S
• The positive gradient of the graph of ∆G = ∆H - T∆S


  Means ∆S is negative which corresponds to the equation above showing increasing order
• The higher the charge density


  The greater the hydration enthalpy (e.g. smaller ions or ions with larger charges) as the ions attract the water molecules more strongly
• Enthalpy of fromation 


  enthalpy change when 1 mole of compund formed from its elements