5.2.1 Lattice Enthalpy Flashcards
why are solid ionic compounds very stable
they contain very strong ionic bonds
- electrostatic attractions between oppositely charged ions in the ionic lattice structure
- provides a substantial barrier that must be overcome to break down
how can you measure the strength of ionic bonds in a lattice
lattice enthalpy
what is lattice enthalpy
- the enthalpy change
- that accompanies the formation of one mole of
- an ionic compound from
- its gaseous ions
- under standard conditions
what would be an equation of lattice enthalpy
gaseous ions ==> solid ionic compound
- e.g. K+(g) + Cl-(g) ===> KCl(s)
what is the sign for lattice enthalpy
normal symbol with LE in middle
is lattice enthalpy always endo or exo
- always exothermic
- as you are forming ionic bonds
- so always negative
how can you indirectly measure lattice enthalpy
Born-Haber cycle
- as you cannot measure lattice enthalpy any other way
what are the two routes of a lattice enthalpy cycle
- always start with elements in gaseous state
1) lattice formation
2) form gaseous atoms, then ions and then lattice enthalpy
what is the order of lattice enthalpy, longer route
1) standard enthalpy change of atomisation
2) first ionisation energy
3) first electron affinity
4) LATTICE ENTHALPY
what is standard enthalpy change of formation
enthalpy change that takes place when 1 moles of a compound is formed from its element under standard conditions, with all reactants and products in their standard conditions
- sign = f
what would be the equation of standard enthalpy change of formation
elements in their standard state ===> one mole of ionic compound in solid lattice
- e.g. Na(g) + 1/2 Cl2(g) ===> NaCl(s)
what is the standard enthalpy change of atomisation
enthalpy change that takes place for the formation of one mole of gaseous atoms from the elements in their standard states and conditions
- at
what is the equation for atomisation
1/2Cl2(g) ===> Cl(g)
- related to moles, so if we have 1/2 moles, then 1/2 the bond enthalpy
is atomisation endo or exo
always ENDO as bonds are being broken to form gaseous atoms
- POSITIVE
what is first ionisation energy
the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to one mole of gaseous 1+ ions
-IE
what is the equation of first ionisation energy
gaseous atom ===> gaseous 1+ ion
- e.g. Na(g) ===> Na+(g) + e-
- can have multiple following dependant on the ion needed
is ionisation energy always endo or exo
ENDO
- energy is required to overcome the attraction between the negative electron and the positive nucleus
- POSITIVE
what is first electron affinity
the enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
- EA
what is the equation of electron affinity
Cl(g) + e- ===> Cl- (g)
is electron affinity ENDO or EXO
FIRST: exothermic as electron is being added is attracted to the nucleus, so negative
AFTER: endothermic as you are adding a negative electron into a negative ion, which repels, so energy must be put into system for them to react
what is always important to remember with a Born-Haber cycle
ALWAYS TAKE INTO ACCOUNT THE MOLES
enthalpy change of solution
enthalpy change that takes place when one mole of solute dissolves into a solvent
- overall energy energy change associated with the dissolving process
what is the equation for enthalpy of solution
solid ionic lattice + aq ===> aqueous ions
- e.g. NaCl(s) + aq ===> Na+(aq) +Cl-(aq)
- aq represents excess of water molecules
is enthalpy change of solution endo or exo
could be either
how can you determine the value of enthalpy change of solution
- using an experiment
- using q=mCΔT formula
- where mass used is the mass of solution, so WATER and IONS
what are the two steps of the dissolving process
1) the ionic lattice breaks up and forms the separate gaseous ions
2) the separate gaseous ions interact with the polar water molecules and form hydrated aqueous ions
what is enthalpy change of hydration
the enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
what is the equation of enthalpy change of hydration
Na+(g) + aq ===> Na+(aq)
is enthalpy change of hydration endo or exo
can be either, depending of the relative sizes of lattice enthalpy and enthalpy changes of hydration
how do you draw a diagram for enthalpy change of solution and hydration
1) gaseous ions going down to ionic lattice via lattice enthalpy
2) gaseous ions going down to aqueous ions
3) ionic lattice going up to gaseous ions
what are the general characteristics of ionic compounds
- having high melting and boiling point
- soluble in polar solvents
- conduct electricity when polar or aqueous
how does ionic size affect lattice enthalpy
- ionic radius increases as you go down group
- so attraction between ions decreases
- lattice enthalpy becomes less negative
- so melting point decreases
how does ionic charge affect lattice enthalpy
- ionic charge increases
- attraction between ions increases
- lattice energy becomes more negative
- melting point increases
how does ionic size effect enthalpy of hydration
- ionic radius increases
- attraction between H2O molecules and ion decreases
- hydration energy becomes less negative
how does ionic charge affect enthalpy of hydration
- increase in ionic charge
- attraction with H2O molecules increases
- more negative hydration energy
explain the energy changes that take place when an ionic compound dissolves
- energy equal to the lattice enthalpy is required to be overcome to break the attraction between ions in the lattice
- water molecules are then attracted, releasing energy equal to hydration energy
when does an ionic compound dissolve
- if sum of hyd>lat
- the overall energy change is exo
- should dissolve
- some endo can also dissolve
