Ch 22 Enthalpy And Entropy Flashcards
Why are solid ionic compounds very stable
Strength of ionic bonds, electrostatic attractions creates a substantial energy barrier that must be overcome
The Born-Haber cycle
Lattice enthalpy (ionic bond formation from separate gaseous ions) can’t be measured directly so this cycle used See diagram 1 Route 2 converts elements directly to ionic lattice, this is enthalpy change of formation and is exothermic
In its solid lattice a compound will always be…
Ionic
Standard enthalpy change of atomisation
See diagram 2
It’s always endothermic because bonds broken to form gaseous atoms
Born Haber cycle examples
See diagram 3 and 4
Successive electron affinities
See diagram 5
Enthalpy of solution is exo or endo
Can be either
In q=mcT mass is
Mass of solution not mass of water
Determination of enthalpy change of solution
See diagram 6
Enthalpy change of solution diagram
See diagram 7
Enthalpy change of solution examples
See diagram 8. And 9
Properties of ionic compounds
High melting and boiling points
Soluble in polar solvents
Conduct electricity when molten or in aqueous solution
Indication of m.p. Of ionic compound
Magnitude of lattice energy
Effect of ionic charge and size across period 3
See diagram 10
To predict solubility
To dissolve ionic compound in water quantity of energy must equal lattice enthalpy
H2O molecules attracted to ions surrounding them and releasing energy equal to hydration enthalpy
If sum of hydration enthalpies is larger then magnitude of lattice enthalpy overall enthalpy change will be exothermic and compound should dissolve but solubility also depends on temp and entropy
Effect of entropy on dispersal of energy
Greater entropy greater dispersal of energy and greater disorder
At 0k
No energy and all substances would have an entropy val of zero
Above this temp energy becomes dispersed amongst particles and substances have positive entropy
If a System changes to become more random
Energy spreads out more and entropy change will be more positive
Production of a gas
Increases disorder of particles
Energy spread out more
Entropy change positive
Decrease in number of gas molecules
Decrease in randomness of particles
Energy spread out less
Entropy change is negative
Standard entropies
Units JK^-1 mol^-1
Always positive
Free energy change
Change in energy during reaction
In Gibbs equation all units must be changed to
kJK^-1mol^-1
At room temp delta H
Much larger than TDeltaS
