Chemical Energetics Flashcards
define lattice energy
the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under std conditions
is ^Hlatt exo or endo? explain why it is so
exo
because ions are combined to FORM BONDS in an ionic solid lattice
what does a large, negative value of ^Hlatt indicate
that the ionic compound is more stable than its gaseous
why is the ionic compound more stable than its gaseous ions
no electrosatic forces of attraction between them in gas phase so they are less stable
whereas strong electrostatic forces of attraction exist between +vely and -vely charged ions in lattice
how can ^Hlatt be determined
born haber cycle- type of energy cycle and experimental values
2 factors which affect lattice energy are …
charge and radiusof the ions which make up the crystalline lattice
lattice energy becomes _ as the ionic radius _. explain your answer too
less exo as radius increases.
this is bcos charge is more spread out over the ion as its larger.
as the ionic radius increases what happens to the lattice
the centres of the ions are further apart and so the electrostatic force of attraction between opp charged ions is weaker
as ionic charge _ lattice energy becomes _
increases
more exo
explain the connection between ionic charge and lattice energy
the greater the charge, the higher the charge density. this results in stronger electrostatic force of attraction between ions in the lattice. hence LE gets more exo
what is electron affinity
ability/strength of the nucleus to attract an incoming electron
what is the first electron affinity
the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of single negatively charged gaseous ion. this is ALWAYS exo
why are the successive electron affinities endo?
incoming electron is being added to an already negative ion; therefore, energy is required to overcome the repulsive forces
to use a born-haber cycle to calculate ^Hlatt what all must be known before hand
- ^Hf
- enthalpy changes when elements go from their std state to their gaseous ions ie: atomisation, ionisation for metals, electron affinities for non-metals
state hess’ law
enthalpy change in a chemical reaction is the same regardless of the route taken, as long as initial and final conditions of reactants and products are the same in each route
which 3 factors affect electron affinity
- nuclear charge: more the nuc charge the stronger attractive forces
- distance from nucleus: greater distance between outermost shell and nucleus the weaker the attraction
- shielding: more the no. of shells the weaker the force of attraction
state the degree of exo of electron affinity across a period and down a group
- more exo across pd
- less exo down grp
less effective the attraction of electrons to the nucleus the less _
exo
define ^H sol
enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form a very dilute solution
if an ionic compounds has exo? or endo? value of ^H sol it is likely to be soluble
^H sol should be either exo or only slight endo will it dissolve
if a compound has a large positive value of ^H sol it will be _
insoluble
define ^H hyd
enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form a very dilute solution
explain how ^H hyd compensates for the remaining lattice energy after ^H sol
- the ^H sol is used to dissolve the compound
- but more energy is needed to break the attractive forces between the ions
- this is compensated by the ^H hyd: ions form ion-dipole attractions with water molecules and in doing so release energy which can then be used to breal the lattice
how to find the total ^H hyd
^H hyd of both anions and cations
