13.1 Flashcards
define standard lattice energy
the energy change when one mole of the ionic solid is formed from its gaseous ions under standard conditions of 100kPa and a stated temperature
what factors effect standard lattice energy
number of ions in the compound - the more ions in a molecule the more negative the lattice energy
the amount of cation to anion interactions - the more interactions the more negative the lattice energy
the sum of their atomic radii - smaller atomic radius the more negative the lattice energy
define standard enthalpy change of atomisation
enthalpy change measured under standard conditions of a stated temperature and 100kPa when one mole of gaseous atoms is formed from an element in its standard state
what is the equation for standard lattice energy of NaCl
Na+(aq) + Cl-(aq) -> NaCl(s)
what is the equation for standard enthalpy change of atomisation of chlorine and sodium
1/2Cl2(g) -> Cl(g)
Na(s) -> Na(g)
define first electron affinity
the energy change when each atom in a mole of atoms in the gaseous state gains an electron to form a 1- ion
what is the equation for the first electron affinity of chlorine
Cl(g) + e- -> Cl-(g)
in general are first electron affinities positive or negative and why are noble gases an exception
first electron affinity is usually negative but noble gases are positive because of the repulsion caused by the electrons in the outer shell mean that an additional electron would have to occupy a new valence shell
in general are the second electron affinities positive or negative
positive
what is the equation for the second electron affinity of oxygen
O-(g) + e- -> O^2-(g)
what assumptions are made in calculating lattice energy
ions are in contact with one another
ions are perfectly spherical
the charge of an ion is evenly distributed around the center so each ion can be considered as point charges
what could cause the experimental value for lattice energy to be more negative than the expected value
if the bonding in the lattice has considerable covalent character
what is covalent character
covalency in bonding is caused by polarization of the anion by the cation which results in distortion of the electron density within the anion resulting in a higher electron density near the cation
what are fajans rules for the extent of polarisation of an anion
polarisation will be increased by:
a high charge and small size cation
a high charge and large size anion
what is polarising power
the ability of a cation to attract electrons from the anion
what increases a cations polarising power and how can it be measured
high charge
small radius
by calculating charge density
define enthalpy change of solution
the enthalpy change when one mole of an ionic solid dissolves in water to form an infinitely dilute solution
what is the equation for the enthalpy change of solution of sodium chloride
NaCl(s) -(aq)-> Na+(aq) + Cl-(aq)
is dilution and endo or exothermic process and why
it is an endothermic process as ions in solution move further apart
what is #solH and how can it be found
refers to an infinitely dilute solution
found through extrapolation
what is the point of infinite dilution
the point at which further dilution has no measurable effect on #solH
define enthalpy change of hydration
the enthalpy change when one mole of an ion in its gaseous state is completely hydrated by water
is hydration an exo or endothermic process
exothermic
what is the equation for enthalpy change of hydration for chlorine
Cl-(g) -(aq)-> Cl-(aq)
how can water interact with the cations in solution
the dipole negative oxygen have an ion-dipole attraction which in some cases (often transition metals) can become a dative bond with the lone pair of electrons form the oxygen
how can water interact with the anions in solution
the dipole positive hydrogen have ion-dipole interactions which can form hydrogen bonds using the anions lone pair of electrons (e.g. chlorine)
what factors effect the value of the hydration enthalpy and why
increased charge = more negative hydration enthalpy as the ion has stronger interactions with the water molecules therefor there are stronger electrostatic forces of attraction between them
increased atomic radius = less negative hydration enthalpy because as ions become larger the electrostatic force of attraction between them decreases
what is the trend in hydration enthalpy going down a group
hydration enthalpy becomes less negative due to an increase in atomic radius
how does charge density effect hydration enthalpy
the greater the charge density the more negative the hydration enthalpy
