chapter 22

• What is lattice enthalpy a measure of?

  The strength of ionic bonding in a giant ionic lattice.
• What is the definition of lattice enthalpy?

  The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
• Is lattice enthalpy endothermic or exothermic?

  Exothermic

  
  

  The value for enthalpy change will always be negative
• What is the energy cycle called in order to indirectly measure lattice enthalpy?

  The Born-Haber cycle

  
  

  Shows two routes for changing elements in their standard states into an ionic lattice
• What is the first route?

  Formation of gaseous atoms:

  Changing the elements in their standard states into gaseous atoms.

  This change is endothermic as it involves bond breaking.

  
  

  Formation of gaseous ions:

  Changing the gaseous atoms into gaseous ions.

  Overall, the change is endothermic.

  
  

  Lattice formation:

  Changing the gaseous ions into the solid ionic lattice.

  This is the lattice enthalpy and is exothermic.
• What is the second route?

  Converts the elements in their standard states directly to the ionic lattice.

  
  

  There is just one enthalpy change, the enthalpy change of formation, and this is exothermic.
• What is the standard enthalpy change of formation?

  The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
• What is the stadard enthalpy change of atomisation?

  The enthalpy change of atomisation is the enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
• What type of process is the enthalpy change of atomisation?

  Endothermic

  
  

  Bonds are broken to form gaseous ions.
• What is the first ionisation energy?

  The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
• What type of process is the first ionisation energy?

  Endothermic

  
  

  Energy is required to overcome the attraction between a negative electron and the positive nucleus.
• What is the first electron affinity?

  The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous 1- ions.
• What does electron affinity do?

  Opposite of ionisation energy.

  
  

  Measures the energy to gain electrons.
• What type of process is electron affinity?

  Exothermic.

  
  

  The electron being added is attracted in towards the nucleus.
• When are successive electron affinities required for a Born-Haber cycle?

  When an anion has a greater charge than 1-, such as O²⁻.
• What type of process are second electron affinities?

  Endothermic.

  
  

  A second electron is being gained by a negative ion, whcih repels the electron away.

  
  

  So energy must be put in to force the negatively-charged electron onto the negative ion.

  
  

  A Born-Haber cycle including O²⁻(g) would contain the two energy changes.
• What is the standard enthalpy change of solution?

  The enthalpy change that takes place when one mole of a solute dissolves in a solvent.
• What type of process is the enthalpy change of solution?

  Can be endothermic or exothermic, depending on the relative sizes of the lattice enthalpy and the enthalpy changes of hydration.
• Which two process take place when a solid ionic compound dissolves in water?

  The ionic lattice breaks up

  
  

  Water molecules are attracted to, and surround, the ions.
• Which two types of energy change are involved when a solid ionic compound dissolves in water?

  The ionic lattice is broken up forming separate gaseous ions. This is the opposite energy change from lattice energy, which forms the ionic lattice from gaseous ions.

  
  

  The separate gaseous ions interact with polar water molecules to form hydrated aqueous ions. The energy change involved is called the enthalpy change of hydration.
• What is the enthalpy change of hydration?

  The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.
• What are the factors affecting lattice enthalpy?

  Ionic size

  
  

  Ionic charge
• What is the effect of ionic size on lattice enthalpy?

  Increasing size means:

  
  

  larger ionic radius

  
  

  attraction between ions decreses

  
  

  lattice energy less negative

  
  

  melting point decreases
• What is the effect of ionic charge on lattice enthalpy?

  Increasing charge means:

  
  

  attraction between ions increases

  
  

  lattice enery becomes more negative

  
  

  melting point increases
• What is the overall effect of ionic size and charge moving right from the start of period three?

  Two supporting effects:

  
  

  Increasing charge gives more attraction.

  Decreasing size gives more attraction.
• What is the overall effect of ionic size and charge moving left from the end of period three?


  Two opposing effects:

  
  

  Increasing charge gives more attraction.

  Increasing size gives less attraction.
• What does the magnitude of lattice energy give a good incdication of?

  The melting point of an ionic compound.

  
  

  The more negative, the higher the melting point.
• How does ionic size effect hydration enthalpies?

  Ionic radius increases:

  
  

  Attraction between ion and water molecules decreases.

  
  

  Hydration energy less negative.
• How does ionic charge affect hydration enthalpy?


  Ionic charge increases:

  
  

  Attraction with water molecules increases.

  
  

  Hydration energy becomes more negative.
• How do you predict solubility using hyration and lattice enthalpies?

  To break an ionic compound, the energy supplied must equal the lattice enthalpy.

  
  

  Water molecules are attracted to the ions, surrounding them and releasing energy equal to hydration enethalpy.

  
  

  If the sum of the hydration enthalpies is larger than the magnitude of the lattice enthalpy, the overall enthalpy change will be exothermic and the compound should dissolve.
• What are the limitations of this?

  Many compounds with endothermic enthalpy chnages of solution are soluble so this does not provide the total picture.

  
  

  The reasons for solubility also depends on temperature and another variable called entropy.
• What is entropy?


  The dispersal of energy within the chemicals making up the chemical system.

  
  

  The greater the entropy value, the greater that energy is spread out per Kelvin per mole.
• What are the units for entropy?

  JK⁻¹mol⁻¹
• What happens to entropy when any substance changes from solid to liquid to gas?

  Melting and boiling increase the randomness of particles.

  
  

  Energy is more spread out and entropy change is positive.
• What is standard entropy?

  The entropy of one mole of a substance, under standard conditions.

  
  

  Always positive.
• What does feasibility mean?

  Used to describe whether a reaction is able to happen and is energetically feasible.
• What is the overall energy change called during a chemical reaction?

  The free energy change.
• What is the free energy change made up of?

  The enthalpy change - heat transfer between the chemical system and the surroundings.

  
  

  The entropy change at the temperature of the reaction T∆S - dispersal of energy within the chemical system itself.
• What is the Gibbs' equation?

  Shows the relationship between the two types of energy.

  
  

  ∆G = ∆H - T∆S

  
  

  Temperature is in Kelvin.

  
  

  All units must be changed to kJ
• What does the value of ∆G have to be for a reaction to be feasible?

  There must be a decrease in free energy.

  
  

  ∆G < 0

  
  

  The value for ∆H is usually much larger.