Lattice enthalpy

• Define lattice enthalpy


  The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions (under standard conditions)

  -> Exothermic

  
• Define standard enthalpy of formation

  The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions

  -> Exothermic

  
• Define standard enthalpy change of atomisation

  The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions

  -> Endothermic

  --> 2 atomisation energies are required for the separate energy changes

  
• Define first ionisation energy

  The energy required to remove one electron from each atom in one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions

  -> Endothermic

  
• Define second ionisation energy

  The energy to required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.

  -> Endothermic

  
• Define first electron affinity

  The enthalpy change when one electron is gained by each atom in one mole of gaseous atoms (to form one mole of gaseous 1- ions)

  -> Exothermic

  
• Define second electron affinity

  The enthalpy change when one electron is gained by each ion in one mole of gaseous 1- ions (to form one mole of gaseous 2- ions)

  -> Endothermic

  
• Lattice enthalpies are:


  - always exothermic

  - cannot be measured directly (calculated indirectly using energy changes in an energy cycle)

  - only used for ionic compounds

  - magnitude indicates ionic bond strength

  
• Born-Haber Cycles


  Energy cycles for the formation of ionic compounds.

  
• How many routes are there for changing elements in their standard states into an ionic lattice?

  2 routes

  Route 1 and Route 2
• Route 1 (3 proceses)

  - Formation of gaseous atoms: Changing elements in their standard states into gaseous atoms (endothermic - bond breaking process)

  - Formation of gaseous ions: Changing gaseous atoms into positive and negative gaseous ions (endothermic)

  - Lattice formation: Changing the gaseous ions into a solid ionic lattice (lattice enthalpy and exothermic)
• Route 2


  Converts elements in their standard states directly to the ionic lattice (enthalpy change of formation and is exothermic)
• Route 1 + Route 2 (x-1) = Lattice enthalpy

  
• Factors affecting lattice enthalpy

  Ionic radius --> When ionic radius increases, attraction between ions decreases, lattice energy is less negative and melting point decreases

  Ionic charge --> When ionic charge increases, attraction between ions increases, lattice energy becomes more negative and melting point increases

  
• Effect of ionic charge and size across Period 3

  
• Define standard enthalpy change of solution

  The enthalpy change that takes place when one mole of a solute dissolves in a solvent/water

  The enthalpy change can be exothermic or endothermic

  
• Experimental procedure to determine the enthalpy change of solution (KCL)

  1) Weigh out a sample of KCL(s)

  2) Use a measuring cylinder, pour 25.0cm^3 of distilled water into a plastic cup in the beaker. Measure the temperature of the water, to the nearest 0.5°C.

  3) Quickly tip all of the KCL(s) into the water in the plastic cup. Stir the mixture with the thermometer until all of the KCL(s) has dissolved and the temperature no longer changes

  Record this value to the nearest 0.5°C.
• Mass of solution = mass of water + mass of ionic compound
• Define enthalpy change of hydration

  The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions

  
• Factors affecting hydration
• Enthalpy changes involved in the dissolving of an ionic compound in water

  Ionic radius --> When ionic radius increases, attraction between ion and water molecules decreases, hydration energy is less negative

  Ionic charge --> When ionic charge increases, attraction with water molecules increases and hydration energy becomes more negative