topic test revision

• 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 under standard conditions.
• How is the enthalpy of sublimation related to the enthalpy of atomisation?

  It is numerically the same as the enthalpy of atomisation for a solid metal turning to gaseous atoms.
• What is bond dissociation enthalpy?

  It is the standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms.
• How is the bond dissociation enthalpy related to the enthalpy of atomisation for diatomic molecules?

  The bond dissociation enthalpy 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 gains 1 mole of electrons to form 1 mole of gaseous ions with a -1 charge.
• Why is the first electron affinity exothermic for atoms that normally form negative ions?

  Because the ion is more stable than the atom, leading to an attraction between the nucleus and the electron.
• 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 dissociation?

  It is the standard enthalpy change when 1 mole of an ionic crystal lattice is separated into its constituent ions in gaseous form.
• Why do the definitions of enthalpy changes have conflicting signs?

  Because the enthalpy of formation is exothermic while the lattice dissociation is endothermic.
• What is the enthalpy of hydration?

  It is the enthalpy change when one mole of gaseous ions become aqueous ions.
• 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 standard 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 Born-Haber cycle?

  • An indirect method to calculate lattice enthalpy.
  • Links various enthalpy changes together in a cycle.
  • 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.
• What is the lattice enthalpy for NaCl calculated 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: Higher charges lead to stronger lattices.
  3. Lattice enthalpies become less negative down a group.
• How does the size of ions affect lattice enthalpy?

  Larger ions result in less negative enthalpies of lattice formation due to weaker attractive forces.
• How does the charge of ions affect lattice enthalpy?

  Greater charges on ions lead to stronger lattice enthalpy, resulting in more negative values.
• What is the perfect ionic model?

  • Assumes ions are 100% ionic and spherical.
  • Attractions are purely electrostatic.
  • Real compounds may show covalent character under certain conditions.
• 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 lattice enthalpy is the experimental value.
  • Theoretical values assume perfect ionic character.
  • Differences increase with covalent character.
• What is the relationship between ionisation energy and lattice enthalpy?

  The enthalpy of formation is a balance between ionisation energy and lattice enthalpy.
• What is the significance of free-energy change (ΔG) in reactions?

  It indicates whether a reaction is spontaneous or not.
• How does entropy relate to spontaneous processes?

  Spontaneous processes tend to increase the disorder of a system, which is measured by entropy.
• What are the trends in entropy among different states of matter?

  • Solids have lower entropies than liquids.
  • Liquids have lower entropies than gases.
  • Compounds generally have higher entropies than elements.
• What is the relationship between exothermic reactions and thermodynamic stability of products?

  A reaction that is exothermic will result in products that are more thermodynamically stable than the reactants.
• What is a driving force behind many spontaneous reactions?

  The driving force is that exothermic reactions lead to products that are more thermodynamically stable.
• Can spontaneous reactions be endothermic, and how can this be explained?

  Yes, some spontaneous reactions are endothermic, which can be explained by considering entropy.
• What is entropy?

  Entropy, S, is a description of the number of ways atoms can share quanta of energy.
• What does a high number of ways of arranging energy indicate about a system's disorder and entropy?

  If the number of ways of arranging the energy (W) is high, then the system is disordered and entropy (S) is high.
• Which has lower entropies: elements or compounds?

  Elements tend to have lower entropies than compounds.
• How does the state of matter affect entropy?

  Solids have lower entropies than liquids, which are lower than gases.
• What happens to the entropy of a solid as its temperature increases?

  As the temperature of a solid increases, its entropy increases as the particles vibrate more.
• What is the effect of boiling compared to melting on entropy change?

  There is a bigger jump in entropy with boiling than that with melting.
• What generally leads to a significant increase in entropy?

  A significant increase in entropy will occur if there is a change of state from solid or liquid to gas or a significant increase in the number of molecules between products and reactants.
• What is the entropy change for the reaction: NH4Cl (s) → HCl (g) + NH3 (g)?

  ∆S˚ = +ve due to the change from solid reactant to gaseous products and an increase in the number of molecules.