Thermodynamics

• Lattice enthalpy

  measure of ionic bond strength

  used for ionic substances

  dissociation or formation
• enthalpy of lattice formation

  enthalpy change when one mole of a solid ionic substance is formed from its constituent ions in a gaseous state under standard conditions

  exothermic

  $Mg^{2+}(g)+$$2Cl^-(g)\rightarrow MgCl_2(s)$
• enthalpy of lattice dissociation

  enthalpy change when one mole of a solid ionic compound dissociates into its gaseous ions under standard conditions

  endothermic

  $MgCl_2(s)\rightarrow Mg^{2+}(g)+$$2Cl^-(g)$
• ionisation energy

  energy required to remove one mole of electrons from one mole of atoms in their gaseous state under standard conditions to form one mole of 1+ ions

  endothermic

  $Mg(g)\rightarrow Mg^+$$(g)+$$e^-$
• enthalpy of atomisation

  enthalpy change when one mole of gaseous atoms is formed form an element in its standard state under standard conditions

  endothermic

  $\frac{1}{2}I_{2\ (s)}\rightarrow\ I_{(g)}$
• bond dissociation enthalpy

  enthalpy change when one mole of covalent bonds is broken in the gaseous state

  endothermic

  $I_{2\ (g)}\rightarrow2I_{(s)}$
• enthalpy of electron affinity

  enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of 1- ions in their gaseous state under standard conditions

  first is exothermic, second is endothermic

  $Mg_{(g)}+$$e^-\rightarrow\ Mg_{(g)}^-$
• enthalpy change of hydration

  enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions under standard conditions

  exothermic

  $Mg_{(g)}^{2+}\rightarrow Mg_{(aq)}^{2+}$
• enthalpy change of solution

  enthalpy change when one mole of an ionic solid dissolves in enough water so that the dissolved ions no longer interact under standard conditions

  exo/endo varies

  $NaCl_{(s)}\rightarrow Na_{(aq)}^+$$+$$Cl_{(aq)}^-$
• Calculating lattice enthalpies

  
• cycle for group 2 elements

  
• cycle for when first and second electron affinity

  
• purely ionic model

  all ions are spherical

  charge evenly distributed

  100% ionic

  purely electrostatic

  theoretical
• experimental

  has covalent character

  polarisation

  non spherical
• experimental value different from theoretical

  compound has covalent character

  strength of bonding

  polarisation
• trends in lattice enthalpies

  larger ions = smaller lattice enthalpies due to reduced electrostatic force of attraction

  increases as size of charge increases due to more energy given out
• factors affecting polarisation

  positive ion = small size, high charge

  negative ion = large size (easy to distort), high charge
• 


  enthalpy change of solution

  
• entropy

  measure of disorder

  $J\ K^{-1}mol^{-1}$

  symbol S
• what affects entropy

  physical state ( solid<liquid<gas)

  dissolving solid (dissolution)

  number of particles (more = increase)
• entropy change

  
• feasible reaction

  reaction that takes place on its own accord, without energy being supplied to it
• when is reaction feasible

  when $\Delta G$ is negative

  not guaranteed as may have high $E_a$ or slow rate

  exothermic
• gibbs free energy change

  measure to predict feasibility

  
• when is $\Delta G$ always negative


  exothermic and positive $\Delta S$

  feasible at any temperature
• when is $\Delta G$ always positive


  endothermic and negative $\Delta S$

  not feasible at any temperature
• free energy graphs