5.2.1 (a-b, e)

Question 1

Define lattice enthalpy

Answer 1

The lattice enthalpy of an ionic compound is the enthalpy change accompanying the formation of 1 mol of an ionic compound from its constituent gaseous ions under standard conditions

e. g. Na⁺_(g) + Cl^-_(g) → Na⁺Cl^-_(s)
* Always exothermic

Question 2

Define enthalpy change of formation

Answer 2

Enthalpy change accompanying the formation of 1 mol of a substance from its elements in their natural states under standard conditions

e.g. Na_(s) + ½Cl_2(g) → Na⁺Cl^-_(s)

(another way to form an ionic compound)

• Exothermic for ionic compounds

Question 3

Define enthalpy change of atomisation

(not a required definition)

Answer 3

Enthalpy change accompanying the formaiton of 1 mol of gaseous atoms from thte element in its standard states

e.g. Na_(s) → Na_(g)

½Cl_2(g) → Cl_(g)

(formation of gaseous atoms)

• Endothermic as bonds are broken to form gaseous atoms

Question 4

Define ionisation energy

Answer 4

Enthalpy change accompanying the removal of one electron from each atom in 1 mol of gaseous atoms to form 1 mol of gaseous 1+ ions

e.g. Na_(g) → Na⁺_(g) + e^-

(formation of positive ions)

• 1^stIE is always endothermic (energy needed to overcome attraction between electron and nucleus)

Question 5

Define electron affinity

Answer 5

Enthalpy change accompanying the addition of an electron to each atom in 1 mol of gaseous atoms to form 1 mol of gaseous 1^- ions

• 1^stEA is always exothermic (e.g. Cl_(g) + e^- → Cl_(g)^- )
  
  • (formation of negative ions)
• 2^ndEA always endothermic
  
  • A second electron is being added to an already negative ion which will repel the electron, so energy must be put in

Question 6

How are Born-Haber cycles constructed?

Answer 6

2 routes to go from constituent elements in standard states to the ionic lattice

Route 1: atomisation + ionisation + electron affinity + lattice enthalpy

Route 2: enthalpy change of formation

Hess’s Law states R1=R2

Question 7

What are the factors affecting lattice enthalpy?

Answer 7

• As ionic radius increases, the charge density decreases, the attraction between ions decreases, the lattice enthalpy becomes less exothermic
• As ionic charge increases, the charge density increases, the attraction between ions increases, the lattice enthalpy becomes more exothermic

Question 8

What are the factors affecting hydration enthalpy?

Answer 8

• As ionic radius increases, the attraction between water molecules and ions decreases, the hydration enthalpy becomes less exothermic
• As ionic charge increases, the attraction between water molecules and ions increases, the hydration enthalpy becomes more exothermic