topic 13 Flashcards
what is the enthalpy chnage of 1st electron afinity
the enthalpy change when 1 mole of gaseous 1- ions are made from 1 mole of gaseous atoms
O(g) –> O- (g)
what is the lattice enthalpy of formation
the enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions under standard condition
Ca 2+ (g) + 2Cl- (g) –> CaCl2 (s)
what is the enthalpy change of second electron affinity
the enthalpy change when 1 mole of gaseous 2- ions are made form 1 mole of gaseous 1- ions
O- (g) –> O 2- (g)
what is the enthalpy change of atomisation
the enthalpy change when 1 mole of gaseous atoms is made from an element in its standard state
!/2 F2 (g) —> F (g)
what affects the strenght of an ionic bond
size of the charge
- bigger chrges = stronger electrostatic attraction
- more energy required so increased mp
size of ion
- smaller ion = stronger electrostatic attraction
- smaller ions can pack closer and more energy required to overcome so increased mp
draw a born haber cycle for lithium chloride
see flash card
draw a born haber cycle for magnesium oxide
see flash card
what is theoretical lattice enthalpies
calculated from data assuming a perfectly ionic
- ions are perfectly spherical
- charge is evenly distributed
what is experimental lattice enthalpies
can be different to theoretical as ionic comound has covalent chaacterisitcs, more polarisation = more covalent
positive ion polarised negative ion
what causes higher polarisation
smaller cations more polarising = higher charge density so pulls electrons towards itself more readily
large anions with large charge are polarised more easily = electrons further away from nucleus and more repulsion between electrons in ion
small ions
larger charge
what does more polarisation do to covalent character of and ionic compound
increases covalent character
what is electronegativity
the ability for an atom to attract electrons towards itself in a covalent bond
where are the most elctronegative elements in the periodic table
further up and right
what is electronegativity measured in
pauling scale
what does a big difference in electronegativity suggest
more ionic the substance
what is enthalpy change of solution
the enthalpy change when 1 mole of ionic substance is dissolved in the minimum amount of solvent to ensure no further enthalpy change is observed upon further dilution
what is hydration
when the ionic lattice in solid form is mixed with water
substance bonds broken to create free moving ions
bonds formed between ions and water
what must happen for a substance to dissolve
the new bonds formed must be same strength or greater than those broken
tend to have exothermic enthalpies of solution
how to calculate enthalpy of solution
know lattice enthalpy of dissociation and hydration
see flash card for cycle
what 2 things affect enthalpy of hydration and why
charge
- high charge attract water molecules more strongly as electrostatic attraction stronger, more energy released when bond is made so more exothermic enthalpy of hydration
- larger charge more -ve ethalpy of hydration
size
- smaller ions have higher charge density so can attract water molecules more strongly so more exothermic enthalpy of hyration
- smaller ion more -ve enthalpy of hydration
what is entropy
measure of disorder in a system
what does more disorder in a system mean in terms of enropy
higher entropy
which out of solid liquid and gas has highest entropy
gas
does more or less moles have higher entropy
more moles
how can a enthalpically unfavourable (endothermic) still be feasible
if changes in enropy overcome changes in enthalpy
how to work out entopy of system and what are the units
S products - S reactants
units = JK^-1mol^-1
what is standard entropy
1 mole
100KPa
298K
for a feasible reaction what must the entropy be
positive
how to work out total entropy
entropy system + entropy surroundings
system = products - reactants
surroundingds = -AH in JKmol^1 / T in K
what does gibbs free energy tell us
if a reaction is feasible or not
how to work out gibbs free energy
AH - T AS system
AG - Jmol^-1
AH - Jmol_1
T - K
AS system - JK^-1mol^-1
what must gibbs free energy be for reaction to be feasible
negative or 0
how to calculate the temperature of feasibility
T + AH / AS system
T - K
AH - convert from KJ mol^-1 to Jmol^-1
in terms of equilibrium constants what happens when gibbs is negative
reaction is theoretically feasible
equilibrium constants are large
greater than 1
in terms of equilibrium constants what happens when gibbs is positive
reaction is not theoretically feasible
equilibrium constants are small
less than 1
how to work out equilibrium constants using gibbs free energy
lnk = AG / - RT
k = e ^- ln
