5.2.1 (a-b, e) Flashcards Preview

OCR A Chemistry A Level - 5.2.1 (Lattice Enthalpy) > 5.2.1 (a-b, e) > Flashcards

Flashcards in 5.2.1 (a-b, e) Deck (8)
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Define lattice enthalpy

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


Define enthalpy change of formation

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) + ½Cl2(g) → Na+Cl-(s) 

(another way to form an ionic compound)

  • Exothermic for ionic compounds


Define enthalpy change of atomisation

(not a required definition)

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

e.g. Na(s) → Na(g)

½Cl2(g) → Cl(g)

(formation of gaseous atoms)

  • Endothermic as bonds are broken to form gaseous atoms


Define ionisation energy

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)

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


Define electron affinity

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

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


How are Born-Haber cycles constructed?

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


What are the factors affecting lattice enthalpy?

  • 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


What are the factors affecting hydration enthalpy?

  • 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