Thermodynamics Definitions

• Enthalpy Change of Formation


  ∆H, is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
• Bond Dissociation Enthalpy

  ∆H(diss), is the enthalpy change when all the bonds of the same type in one mole of gaseous molecules are broken
• Enthalpy Change of Atomisation of an Element 


  ∆H(at), is the enthlpy change when one mole of gaseous atoms is formed from and element in its standard state
• Enthalpy Change of Atomisation of a Compound 


  ∆H(at), is the enthalpy change when one mole of compound in its standard state is converted to gaseous atoms
• The First Ionisation Energy 


  ∆H(ie1), is the enthalpy change when one mole of gaseous 1+ ions is formed from one mole of gaseous atoms
• Second Ionisation Energy


  ∆H(ie2), is the enthalpy change when one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions
• First Electron Affinity 


  ∆H(ea1), is the enthalpy change when one mole of gaseous 1- ions is made from 1 mole of gaseous atoms
• Second Electron Affinity 


  ∆H(ea2), is the enthalpy change when one mole of gaseous 2- ions is made from one mole of gaseous 1- ions
• Enthalpy Change of Hydration

  ∆H(hyd), is the enthalpy change when one mole of aqueous ions is formed from gaseous ions
• Enthalpy Change of Solution

  ∆H(solution), is the enthalpy change when one mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution
• Enthalpy Change of Combustion

  Enthalpy change when one mole of a substance is burned in oxygen under standard conditions with all reactants and products in their standard states
• Enthalpy of neutralisation


  Enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions (exo)
• Assumptions made when lattice enthalpy is calculated using theory only

  • Spherical ions, purely ionic bonding (perfect charge separation), no covalent character, ions are point charges (charge concentrated at centre of ion)

  
• Theoretical and experimental (from Born Haber cycle) values for lattice enthalpy differ

  The lattice must have some covalent character (the theory value tends to under estimate lattice enthalpy)
• Factors affecting the size of lattice enthalpy


  • The size of the ions/ionic radii and the charges on the ions – higher charge density (small ion – high charge) means higher enthalpy
• When are you most likely to see covalent character in a lattice?
• Sequence in a Born Haber cycle

  Reverse sign if lattice enthalpy of dissociation. Direction of arrows follows from defs.
• How to work out lattice enthalpy from hydration enthalpy and enthalpy of solution


  Reverse sign if lattice enthalpy of dissociation.
• Lattice enthalpy

  A strong ionic bond
• Comparing three lattices with different enthalpies of formation


  The one with the largest value (more energy released on formation – most stable) is most likely to form
• How covalent character affects the properties of a lattice

  • It makes it less soluble/insoluble, may have lower conductivity
• Why is enthalpy of 2nd electron affinity usually negative?
• Entropy

  The disorder of a system ('simplification') the number of ways energy can be distributed among the particles of a system (more accurate)
• Units of entropy


  J K-1 mol-1
• How entropy changes with state of matter

  • Solids < liquids < gases entropy increases as there are more ways for liquids and gases to be arranged.
• How entropy tends to change
• How entropy changes with temperature


  • It increases with temperature; it is 0 at 0K
• Sketch the graph for entropy and temperature
• How to work out the entropy change of a reaction

  S(products) – S(reactants) - positive if entropy increases
• Is a reaction more likely if it has a positive or negative entropy change
• Gibbs Free Energy

  G = H - TS
• What value must delta G be for a reaction to be feasible (able to take place)
• What is the value for delta G at a state change
• Why would a reaction with positive enthalpy and negative entropy not be feasible?