thermodynamics1 - phase transitions Flashcards
(38 cards)
why do things evaporate when heated?
heat energy is converted to kinetic energy, breaking IMF and causing liquid to turn to gas, works because ΔG<0
gibbs free energy definition
the maximum amount of useable energy available to do useful work in a system at constant temp and pressure
ΔG definition
change in gibbs free energy, which relates to whether or not a process is spontaneous under constant temp and pressure
what does it mean for a process to be energetically favourable?
this means its ΔG<0, the process can occur spontaneously
for a reaction A <-> B, how can gibbs free energy be used to tell which reaction is more favourable?
if Ga<Gb process is more favourable to A
if Gb<Ga process is more favourable to B
where does the position of equilibrium lie in relation to gibbs free energy?
equilibrium lies where G is as low as possible
for the phase transition reaction
liquid -> gas
how can gibbs free energy be used to determine favourability?
if Gliquid<Ggas>0, gas turns to liquid
if Ggas<Gliquid then ΔG<0, liquid turns to gas
at boiling point Ggas = Gliquid, ΔG=0</Ggas>
how can ΔG be minimised?
ΔH should be -ve
TΔS should be +ve
enthalpy definition
energy to do with formation and breaking of inter and intra molecular forces/bonds
is bond forming favourable?
bond forming is exothermic (-) meaning its favourable
is bond breaking favourable
bond breaking is endothermic (+) meaning its not favourable
entropy definition
a measure of disorder - high entropy is more favourable
how does ΔS change with temperature?
temperature favours entropy, at high temperatures ΔS is larger
how does temperature affect ΔG=ΔH-TΔS
at low temperatures, enthalpy dominates
at high temperatures, entropy dominates
linking to gibbs free energy equation, how does temperature affect the
liquid-> gas phase transition?
at low temp, enthalpy dominates, breaking IMF/bonds costs energy so liquid is more stable/favourable
at high temp, entropy dominates, gases have greater entropy so gas is more stable/favourable
is salt dissolving in water entropically or enthalpically favourable?
entropically favourable so favoured at high temps, although molecules are still ordered so not as favourble as expected
enthalpy depends on particular IMF/bonds
is gas dissolving in water entropically or enthalpically favourable
entropically unfavourable, gases have higher entropy, so favoured at low temps
enthalpically favourable as IMF/bonds are formed between gas molecules
how can the boiling point of a substance be related to gibbs free energy?
ΔG=ΔH-TΔS
if T<b.p.>b.p. gas is favourable
when T = boiling point, ΔG=0</b.p.>
what are standard conditions?
pressure = 1bar ~ 1 atm
temperature = 298.15K
states under these conditions
all solution concentrations = 1M
- importantly, when considering ΔG equation, standard temperature is not specified as T is a variable in the equation
what is troutons rule?
for many liquids, ΔSvap (standard) is ~ constant
(around 70-100 J K^-1mol^-1)
what are the limitations of troutons rule?
doesn’t work if there is hydrogen bonding in the liquid
because hydrogen bonding results in local ordering, so each molecules is not quite free to rotate so entropy is lower than expected, and there is an even greater gain of freedom when becoming a gas so ΔS vap (standard) is higher ~109
how does pressure affect gibbs free energy of a gas?
as pressure increases, volume decreases as gas is compressed, meaning entropy decreases as there is less freedom/disorder, so G increases
why is Gliquid = Gliquid(standard)?
this is because G liquid doesn’t change much so we assume it stays constant
why is ΔGvap(standard) = 0 at boiling point?
at boiling point Ggas = Ggas(standard)
- if p = 1bar
as phases are in equilibrium at bp, Ggas=Gliquid so ΔGvap(standard) = 0
