13 - energetics 2 Flashcards
what is lattice energy?
for ionic compounds
energy change when one mole of the ionic solid is formed from its gaseous ions
if under standard conditions then it is the standard lattice energy
what factors affect the magnitude of lattice energy?
- charge - greater charge = greater le
- cation-to-anion interactions - more interactions = greater le
- distance between centre of cation and anion
covalent interactions between ions also affects lattice energy
what is electron affinity?
the energy change when each atom in one mole of atoms in the gaseous state gain an electron to form a -1 ion
what is experimental lattice energy?
value of the lattice energy calculated from the knowledge of other energy changes, all of which can be determined experimentally
what assumptions are made for theoretical lattice energy?
- ions are in contact with one another
- ions are perfectly spherical
- charge on each ion is evenly distributed around the centre so that each ion can be considered as point charges
type of lattice and inter-ionic distance found through x-ray crystallography
what two factors affect the extent at which anions are polarised?
- a high charge and small size of cation (high density)
- high charge and large size of anion
if electrons from an anion are pulled towards the cation, the anion is said to be…
polarised
what kind of cation has a high polarising power?
high charge and small radius
which anions are easiest to polarise?
high charge and large radius
when an ion is placed in water it immediately reacts. what kind of bond is formed?
water is polar
ion-dipole interaction
for transition metals - dative covalent bond
what factors affect the magnitude of hydration enthalpy?
- charge - greater charge means electrostatic forces of attraction between ion and water molecules is stronger
- ionic radius - larger ions = lower electrostatic force of attraction between ion and water molecules
what is a spontaneous process?
one that takes place without intervention (such as heating)
the reverse of a spontaneous reaction never happens on its own
e.g. two gases in a container will mix spontaneously however they will not separate on their own accord
total entropy increases in spontaneous process
what is entropy?
property of matter (like density or energy)
it is a quantity associated with randomness or disorder
the greater the degree of disorder, the greater the entropy
e.g. entropy of a liquid is higher than that of a solid
total entropy change ΔStotal =
ΔSsystem + ΔSsurroundings
for reaction to be spontaneous ΔStotal must be positive
entropy change of system ΔSsystem =
ΣS(products) - ΣS(reactants)
what units are entropy values in?
J K-1 mol-1
whereas enthalpy changes usually kJ mol-1
entropy change of surroundings ΔSsurroundings =
- ΔH/T
T in Kelvin
ΔH = enthalpy change of reaction
how does temperature affect entropy?
cold object with increased heat increases entropy
hot object with increased heat also increases entropy but not as much since the molecules were already vigorously moving around
what happens to entropy if you increase the number of particles?
increased number of ways the particles can be arranged = increased entropy
what changes take place when an ionic solid dissolves in water?
- lattice structure is broken down (endothermic)
- ions become hydrated (exothermic)
what factors affect the solubility of a solid?
- entropy of the system
- enthalpy change of solution
- temperature
ΔG =
ΔH - TΔSsystem
thermodynamically feasible if Gibbs is negative
ΔG =
relationship between K and Gibbs
-RTlnK
K = e^(-ΔG/RT)
R is the gas constant
what does is mean for K when ΔG is so large/so small?
ΔG is super big (positive) equilibrium lies to the reactant side
ΔG is so small (negative) equilibrium lies to the product side
+40kJ mol-1 and -40kJ mol-1 so large/small that either all reactants or all products
what two factors need to be considered when applying Gibbs?
- kinetic stability - activation energy and how many collisions occur
- non standard conditions
what does it mean if ΔsolG* is negative?
salt is soluble
if it is positive then salt is insoluble
ΔsolG* =
ΔsolH* - TΔSsystem
NOTE TO SELF
13.3 not complete as beyond scope - revisit textbook if not understanding solubility, strength of acids
