Thermodynamics

• What is the enthalpy of atomisation of an element?

  It is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state.
• What is the enthalpy of atomisation for sodium (Na)?

  +148 kJ mol^-1
• What is the enthalpy of atomisation for oxygen (O)?

  +249 kJ mol^-1
• How is the enthalpy of sublimation related to the enthalpy of atomisation?

  The enthalpy of sublimation is numerically the same as the enthalpy of atomisation for solid metals turning to gaseous atoms.
• What is the bond dissociation enthalpy?

  It is the standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms.
• What is the bond dissociation enthalpy for Cl₂?

  +242 kJ mol^-1
• What is the bond dissociation enthalpy for CH₄?

  +435 kJ mol^-1
• How is the bond dissociation enthalpy related to the enthalpy of atomisation for diatomic molecules?

  The bond dissociation enthalpy of the molecule is the same as 2 times the enthalpy of atomisation of the element.
• What is the first ionisation enthalpy?

  It is 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.
• What is the second ionisation enthalpy?

  It is the enthalpy change to remove 1 mole of electrons from one mole of gaseous 1+ ions to produce one mole of gaseous 2+ ions.
• What is the first electron affinity?

  It is 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.
• What is the first electron affinity for oxygen (O)?

  • 141.1 kJ mol^-1
• Why is the first electron affinity exothermic for atoms that normally form negative ions?

  Because the ion is more stable than the atom, and there is an attraction between the nucleus and the electron.
• What is the second electron affinity?

  It is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions.
• What is the second electron affinity for oxygen (O)?

  +798 kJ mol^-1
• Why is the second electron affinity for oxygen endothermic?

  Because it takes energy to overcome the repulsive force between the negative ion and the electron.
• What is the enthalpy of lattice formation?

  It is the standard enthalpy change when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form.
• What is the enthalpy of lattice formation for NaCl?

  • 787 kJ mol^-1
• What is the enthalpy of lattice dissociation?

  It is the standard enthalpy change when 1 mole of an ionic crystal lattice is separated into its constituent ions in gaseous form.
• What is the enthalpy of lattice dissociation for NaCl?

  +787 kJ mol^-1
• What is the significance of the sign in the definitions of enthalpy changes?

  The sign indicates whether the process is exothermic (negative) or endothermic (positive).
• What is the enthalpy of hydration?

  It is the enthalpy change when one mole of gaseous ions become aqueous ions.
• What is the enthalpy of hydration for Li⁺?

  • 519 kJ mol^-1
• What is the enthalpy of hydration for F^-?

  • 506 kJ mol^-1
• Why is the enthalpy of hydration always exothermic?

  Because bonds are made between the ions and the water molecules, releasing energy.
• What is the enthalpy of solution?

  It is the standard enthalpy change when one mole of an ionic solid dissolves in water to ensure the dissolved ions are well separated.
• What is the enthalpy change of formation?

  It is the energy transferred when 1 mole of the compound is formed from its elements under standard conditions.
• What is the enthalpy change of formation for NaCl?

  • 411.2 kJ mol^-1
• What is the Born-Haber cycle?

  • An indirect method to calculate lattice enthalpy
  • Links enthalpy changes for which data are available
  • Uses Hess's law to relate different enthalpy changes
• How does Hess's law apply to the Born-Haber cycle?

  It states that the heat of formation equals the sum of all other enthalpy changes in the cycle.
• How do you calculate the lattice enthalpy using the Born-Haber cycle?

  Rearrange to give $\Delta LattH =$$\Delta fH - (\Delta atH + \Delta IEH + \Delta eaH)$
• What is the calculated lattice enthalpy for NaCl using the Born-Haber cycle?

  • 787 kJ mol^-1
• What factors affect the strength of lattice enthalpy?

  1. Sizes of the ions: Larger ions lead to weaker lattices.
  2. Charges on the ions: Greater charges lead to stronger lattices.
  3. Lattice enthalpies become less negative down a group.
• How does the size of ions affect lattice enthalpy?

  The larger the ions, the less negative the enthalpies of lattice formation, indicating a weaker lattice.
• How does the charge of ions affect lattice enthalpy?

  The bigger the charge of the ion, the greater the attraction between the ions, resulting in stronger lattice enthalpy.
• What is the perfect ionic model?

  • Assumes ions are 100% ionic and spherical
  • Attractions are purely electrostatic
  • Differences arise when covalent character is present
• What happens when a compound shows covalent character?

  The theoretical and Born-Haber lattice enthalpies will differ, with the Born-Haber value being larger.
• What are the differences between theoretical and Born-Haber lattice enthalpies?

  • Born-Haber is the experimental value.
  • Theoretical assumes perfect ionic character.
  • Differences increase with covalent character.
• Why does calcium chloride have the formula CaCl₂?

  Because the most exothermic enthalpy of formation corresponds to the most thermodynamically stable compound.
• What is the relationship between enthalpy of formation and lattice enthalpy?

  The enthalpy of formation is largely a balance of the ionisation energy and lattice enthalpy.