Thermodynamics

• What does ΔΗ°_f represent in thermodynamics?

  Standard enthalpy of formation
• What is the standard enthalpy of formation?

  It is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 298K and 101kPa
• Write the equation for the standard enthalpy of formation of MX.

  M(s) + X2(g) → MX(s)
• What does ΔΗ°_c represent?

  Standard enthalpy of combustion
• What is the standard enthalpy of combustion?

  It is the enthalpy change when 1 mole of a compound burns completely in oxygen in their standard states at 298K and 101kPa.
• Write the equation for the standard enthalpy of combustion of methane.

  CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
• What does ΔΗ°r represent?

  Standard enthalpy of a reaction
• What is the standard enthalpy of a reaction?

  It is the enthalpy change when reactants react in stoichiometry shown in the equation at 298K and 101kPa.
• Write the equation for the standard enthalpy of a reaction involving ammonia and hydrochloric acid.

  NH3(g) + HCl(g) → NH4Cl(s)
• What does ΔΗ°sol represent?

  Standard enthalpy of solution
• What is the standard enthalpy of solution?

  It is the enthalpy change when 1 mole of a substance dissolves completely at 298K and 101kPa.
• Write the equation for the standard enthalpy of solution of sodium chloride.

  NaCl(s) → Na+(aq) + Cl-(aq)
• What does ΔΗ°hyd represent?

  Standard enthalpy of hydration
• What is the standard enthalpy of hydration?

  It is the enthalpy change when 1 mole of gaseous ions are surrounded by water molecules at standard conditions.
• Write the equation for the standard enthalpy of hydration of fluoride ions.


  F-(g) → F-(aq)
• What does ΔΗ°latt represent?

  Lattice enthalpy of association
• What is the lattice enthalpy of association?

  It is the enthalpy change when 1 mole of a crystalline solid ionic compound is formed from its gaseous ions at standard conditions.
• Write the equation for the lattice enthalpy of association of calcium chloride.


  Ca2+(g) + 2Cl-(g) → CaCl2(s)
• What does ΔΗ°_diss represent?

  Lattice enthalpy of dissociation
• What is the lattice enthalpy of dissociation?

  It is the enthalpy change when one mole of a crystalline ionic solid dissociates into gaseous ions.
• Write the equation for the lattice enthalpy of dissociation of calcium chloride.


  CaCl2(s) → Ca2(g) + 2Cl(g)
• What is the relationship between lattice enthalpy of association and lattice enthalpy of dissociation?

  They are equal and opposite values.
• What does ΔΗ°_bond represent?

  Bond dissociation enthalpy
• What is bond dissociation enthalpy?

  It is the enthalpy change when 1 mole of a covalent bond is broken by homolytic fission at standard conditions.
• Write the equation for the bond dissociation enthalpy of chlorine.

  Cl2(g) → 2Cl(g)
• What does ΔΗ°at represent?

  Standard enthalpy of atomisation
• What is the standard enthalpy of atomisation?

  It is the enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state at standard conditions.
• Write the equation for the standard enthalpy of atomisation of sodium.

  Na(s) → Na(g)
• What is the first ionisation energy?

  It is the enthalpy change where one mole of gaseous atoms loses an electron.
• What is the second ionisation energy?

  It is the enthalpy change where one mole of gaseous +1 ions loses an electron.
• What is the first electron affinity?

  It is the enthalpy change where one mole of atoms gains an electron.
• What is the second electron affinity?

  It is the enthalpy change where one mole of gaseous -1 ions gains an electron.
• What is the nature of the first electron affinity?

  It is exothermic.
• What is the nature of subsequent electron affinities?

  They are endothermic.
• What is the process that occurs when the standard enthalpy of atomisation of iodine is measured?

  It involves the formation of gaseous atoms from solid iodine.
• A larger magnitude of lattice enthalpy is indicative of stronger ionic bonding taking place between ions with greater charge density
• Enthalpy change of solution can be exothermic of endothermic depending on the balance between the magnitude of the lattice enthalpy and the sum of hydration enthalpies for each of its ions.

  
  

  If the sum of the hydration enthalpies exceeds the sum of the lattice enthalpies then it is exothermic, and vice versa.
• Hydration enthalpies are always exothermic because of the forces of attraction between the charged ions and either the delta H or O atoms in the water molecules.
• What do hydration enthalpies depend on?


  • Charge density - directly proportional
  • charge of the ion
  • size of the ion
  • there is greater attraction between small highly charged ions and water molecules
• Steps of a Born-Haber Cycle 


  • Enthalpy change of atomisation
  • Bond dissociation enthalpy (remember always one mole even for diatomic)
  • FIrst ionisation energy
  • Subsequence ionisation energy(es)
  • First electron affinity(ies)
  • Lattice enthalpy
  • Enthalpy change of formation