energetics 2

• What is lattice enthalpy (∆​le​H) used for?

  Lattice enthalpy is used for ionic substances.
• What is lattice dissociation enthalpy?

  Lattice dissociation enthalpy is the enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous constituent ions under standard conditions.
• Is lattice dissociation enthalpy an endothermic or exothermic process?

  Lattice dissociation enthalpy is an endothermic process.
• What is lattice formation enthalpy?

  Lattice formation enthalpy is the enthalpy change when one mole of a solid ionic compound is formed from its gaseous constituent ions under standard conditions.
• Is lattice formation enthalpy an endothermic or exothermic process?

  Lattice formation enthalpy is an exothermic process.
• What is atomisation enthalpy (∆​at​H)?

  Atomisation enthalpy is the energy required for the formation of one mole of gaseous atoms from an element under standard conditions.
• What is the enthalpy of electron affinity (∆​ea​H)?

  The enthalpy of electron affinity is the enthalpy change when one mole of electrons is added to a mole of gaseous atoms to form one mole of gaseous 1- ions under standard conditions.
• What are Born-Haber cycles used for?

  • Born-Haber cycles allow enthalpy changes to be determined which cannot be measured directly.
  • They are similar to the cycles used with Hess’s Law.
  • Arrows in the cycle represent reactions and are treated like vectors.
  • If the cycle counters the direction of the arrow, the sign of the enthalpy change is reversed.

  
• How is lattice enthalpy of formation (∆​le​H) calculated in a Born-Haber cycle?

  It is calculated by following anticlockwise around the cycle: ∆​le​H = [-(-355)-(121)-(502)-107+(-411)] = -786 kJ mol​-1.
• What does the perfect ionic model assume about ions?

  The perfect ionic model assumes that all the ions are perfectly spherical, charge is distributed evenly throughout the ion, and the ions display no covalent character.

  
• What is covalent character in ions?

  Covalent character occurs in ions when two joined ions have varying sizes or charges, leading to uneven charge distribution.
• What are the definitions of enthalpy of solution (∆​sol​H) and enthalpy of hydration (∆​hyd​H)?

  • Enthalpy of solution (∆​sol​H): The enthalpy change when one mole of ionic solid is dissolved in water to infinite dilution.
  • Enthalpy of hydration (∆​hyd​H): The enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions.

  
• Why is the enthalpy of hydration nearly always negative?

  The enthalpy of hydration is nearly always negative because water molecules have δ+ regions that naturally attract negative ions.

  
• What factors affect enthalpy of hydration and lattice enthalpy?

  • Enthalpy of hydration is influenced by the size and charge of the ions.
  • Smaller ions and ions with greater charge lead to stronger attractions.
  • Hydration enthalpies decrease as you move down a group.
  • Lattice dissociation enthalpy increases with decreasing ionic radius and increasing charge.

  
• What is entropy (∆S) in chemistry?

  Entropy is a measure of disorder, with greater disorder corresponding to a greater entropy value.

  
• How does temperature affect entropy?

  Entropy increases as temperature increases because particles gain energy and move faster and further apart, becoming less ordered.

  
• Which state of matter has the greatest entropy?

  Gases have the greatest entropy compared to solids and liquids.
• What happens to entropy when a substance melts or evaporates?

  There is a sudden increase in entropy when a substance melts or evaporates.

  
• Why is the entropy change of vaporisation greater than that of fusion?

  The entropy change of vaporisation is much greater than that of fusion because a gas is much more disordered than a liquid or solid.
• When does a reaction show an increase in entropy?

  A reaction shows an increase in entropy when there are more moles of product than reactants.
• What happens to entropy when a lattice is dissolved in solution?

  When a lattice is dissolved in solution, there is an increase in entropy because the ions can move freely in the solution.

  
• How is the overall entropy change for a reaction calculated?

  The overall entropy change for a reaction is calculated by finding the difference between the standard entropies of the products and the reactants.

  
• What does a positive entropy change indicate about the products and reactants?

  A positive entropy change indicates that the products are more disordered than the reactants.

  
• What is the natural direction of change in terms of entropy?

  The natural direction of change is towards a more disorderly state, which is a positive change in entropy.

  
• What does Gibbs free-energy (∆G) quantify?

  Gibbs free-energy quantifies the balance between entropy and enthalpy in a system, indicating reaction feasibility.
• In what units is Gibbs free-energy measured?

  Gibbs free-energy is measured in kJ mol​-1.
• What is the relationship between ∆G, ∆H, and ∆S?

  ∆G is less than or equal to zero for all spontaneous reactions, indicating that both enthalpy and entropy influence reaction feasibility.

  
• How can the minimum temperature for a spontaneous reaction be found?

  The minimum temperature at which a reaction is spontaneous can be found by setting ∆G equal to zero.
• How can a positive ∆H reaction still occur spontaneously?

  A reaction with a positive ∆H can still occur spontaneously if the entropy change is positive and large enough to result in a negative ∆G.

  
• What does the second equation for ∆G relate to?

  The second equation for ∆G relates it to the equilibrium constant, indicating that reactions are feasible when the equilibrium constant is large.

  
• What are the kinetic factors that can affect a reaction's occurrence?

  • Activation energy may prevent a reaction from occurring even if it is thermodynamically feasible.
  • A reaction may have a negative ∆G but occur at a slow rate due to kinetic factors.
  • Kinetic factors can lead to reactions appearing not to occur at all despite being feasible.

  
• What is standard lattice energy?

  Energy change when 1 mole of an ionic solid is formed from its constituent gaseous ions under standard conditions.
• What is an example of standard lattice energy?

  Na⁺(g) + F⁻(g) → NaF(s)
• What is the standard enthalpy of atomisation?

  Enthalpy change when 1 mole of gaseous atoms is formed from the elements in their standard states.
• What is the nature of the standard enthalpy of atomisation?

  It is always endothermic.
• Provide an example of standard enthalpy of atomisation.


  Na(s) → Na(g)
• What is first electron affinity?

  The enthalpy change that occurs when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.
• Provide an example of first electron affinity.

  S(g) → S⁻(g)
• What is the enthalpy change of hydration?

  The enthalpy change when 1 mole of a gaseous ion is completely dissolved in water under standard conditions.
• Provide an example of enthalpy change of hydration.

  Na⁺(g) → Na⁺(aq)