Lattice enthalpy and entropy and gibbs free energy Flashcards
lattice enthalpy definition
is the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions
lattice enthalpy basic info
only in ionic compounds (not covalent as no ions)
strength of ionic bond directly linked to lattice enthalpy
always exo as bond formed
more exo = stronger attraction between ions
cant be directly measure as impossible to form 1 mole of ionic compound
How do we determine lattice enthalpy
born-haber cycle and hess’ law
sig of lattice enthalpy value
more negative = more exo
stronger attraction between opp charged ions
stronger ionic bonds in giant ionic lattice
higher melting point
symbol for lattice enthalpy
DeltaLEH
factors affecting lattice enthalpy
ionic size-
NOT ATOMIC RADIUS BUT IONIC RADIUS
smaller ions are more strongly attracted to larger ones
greater the diff stronger the attraction and greater DeltaLEH
Ionic charge-
ions of greater charge more strongly attract than ions of smaller charge
Mg 2+ attracts stonger than Na+
Mg in compound has higher DeltaLEH
Standard enthalpy change of formation
KEY
exo or endo
enthalpy change when 1 mole of a compound is formed from its elements in standard states and conditions
usually exo
e.g. Mg(s) + Cl2(g) –> MgCl2(s)
standard enthalpy change of atomisation
exo or endo
enthalpy change when 1 mole of gaseous atoms is formed from its elements in standard states and conditions
endo as bond are always broken
e.g. 1/2 Cl2(g) –> Cl(g)
First ionisation energy
KEY
exo or endo
enthalpy change when one electron is removed from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
endo as energy needed to overcome nuclear attraction
e,g Na(g) —> Na+(g) + e-
second ionisation energy
exo or endo
enthalpy change when 1 electron is removed from every ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions
Endo (more than 1st as greater proton : e- so stronger nuclear attraction)
Ca+(g) —> Ca2+(g) + e-
first electron affinity
deltaEA1H
exo or endo
enthalpy change when 1 electron is added to each atom in one mole of gaseous atoms to form 1 mole of gaseous 1- ions
exo because electron is attracted to the nucleus
Cl(g) + e- —> Cl-(g)
second electron affinity
DeltaEA2H
exo or endo
enthalpy change when 1 electron is added to each atom of 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions
endo as electrons repel each other need to overcome
Cl-(g) + e- —> Cl2-(g)
Born-Haber cycle key features
elements in standard states have zero enthalpy (shown on datum line)
all DeltaH values pointing up = exo
pointing down = endo
clockwise=anticlockwise
if change includes 2 moles then * that section by 2
DO EXAM QUESTIONS
standard enthalpy change of solution
DeltaSolH
the enthalpy change when 1 mole of an compound dissolves completely in water under standard conditions
can be exo or endo
NaCl (s) –> Na+(aq) + Cl-(aq)
2 processes that occur when ionic compound dissolves
ionic lattice dissociates into its gaseous ions
the gaseous ions are hydrated with H20 molecules
`dissociation of ionic lattice
endo or exo
endo
energy needed to overcome attraction between opp charged ions
dissociation is opp of lattice enthalpy
NaCl (s) –> Na+(g) + Cl-(g)
Hydrating gaseous ions
endo or exo
exo
energy releases when gasoues ions attract and bond to h20
+ attract slightly negative O
- attract slightly positive H
dipole dipole interaction (weak but many) form
standard enthalpy change of hydration
DeltahydH
KEY
exo or endo
enthalpy change when 1 mole of isolated gaseous ions is dissolved in H2O forming 1 mole of aqueous ions under standard conditions
Na+(g) + aq —> Na+(aq)
exo
Factors affecting Hydration value
Ionic size-
smaller ions attract + bond with H20 more strongly
more exo as stronger bond
ionic charge-
higher charged ions attract and bond with H2O more strongly
more exo as stronger bond
Enthalpy
measure of the heat content of a chemical system
entropy
measure of the dispersal of energy in a system
The entropy is greater when the system is more disordered
Basic facts of entropy
all substance have some degree of disorder (as particles in constant motion)
entropy= always +
more disorder = more entropy
entropy of pure substance increases with increasing temp
units= JK-1mol-1
how do we know a system is becoming more disordered? and why does this matter?
if entropy change (DeltaS) is positive
system becomes more energetically stable as it becomes more disordered + entropy helps explain spontaneous processes as entropy tends towards max (2nd law of thermodynamics)
Entropy of diff states
more disordered = greater entropy
entropy increases ——>
solid liquid gas
entropy increases during changes in state that cause more disorder
in exam questions answer in terms of moles of gas (if present)
standard entropy change of reaction
entropy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard states and conditions
entropy change calc
entropy of products - entropy of reactants
more disorder= entropy change is +
less disordered= entropy change is -
when is a process feasible?
a process is spontaneous/feasible is chem system becomes more stable and overall energy decreases
depends on=
temp
entropy change
enthalpy change of surrounding
Gibbs free energy change
is the balance between the enthalpy change, entropy change and temp of a process
Gibbs free energy equ
DeltaG = DeltaH - TDeltaS
units for DeltaG usually KJmol-1
units for T = K
units for DeltaS = KjK-1mol-1 (convert by / by 1000)
why do most exo reaction occur spontaneously at room temp
enthalpy content of system decreases during reaction and energy is released to surroundings
increases stability
how do some endo reactions occur at room temp
enthalpy increases during reaction
for it to be spontaneous overall energy must decrease
so must be a decrease in free energy via the entropy change
TDeltaS= greater than DeltaH
more stable
when is a reaction feasible?
when DeltaG = less than 0
if = 0 then in equilibrium (e.g. state change)
calc min temp for feasibility
calc DeltaS plus convert to KjK-1mol-1
Calc DeltaH
then set DeltaG to 0
0=DeltaH - TDeltas
T=DeltaH/DeltaS = min temp
convert to C if needed (-273)
limitations of predictions of feasibility
some have - DeltaG but still not spontaneous
not considered reaction kinetics
rate of reaction so slow (Ea too high) so reaction not observed
not thermodynamically feasible
