states of matter - mixtures Flashcards
(22 cards)
what is daltons law + its function?
xa = Pa / Ptotal
where xa= mole fraction of a
Pa = partial pressure of a
Ptotal = total pressure of mixture
this is true for any component a, b, etc
it can be used to work out mole fractions/partial pressures of the componentss of a gaseous mixture
partial pressure definition
the pressure that would be exerted by the gas if it was alone in the system
why is mixing liquids more complicated than mixing gases?
gases will always fully mix, as there is only 1 single gas phase, regardless of how many components are present
whereas liquids may mix, or they may not
ideal mixture (of liquids) definition
formed when the IMF between molecules in both components are identical, so the IMF of the initial liquids and final liquid mixture are identical, making ΔmixH = 0
ΔmixH definition
= enthalpy of mixing
the change in energy associated with mixing any 2 substances - existing IMF are broken which uses energy, and new IMF are formed, releasing energy
what is the driving force of mixing?
entropy
for an ideal solution ΔmixS must be +ve for the mixture to be able to form spontaneously, when ΔmixG<0
(remember ΔmixH = 0 for ideal!)
in practice, how can we form ideal mixtures?
2 components must be similar in chemical nature so they have similar IMF and must be similar in size and shape
e.g. heptane and hexane or benzene and toluene (methylbenzene) form almost ideal mixtures
there must also be no volume change on mixing (as a result of new IMF forming, changing arrangement of particles)
how do real mixtures deviate from ideal mixtures?
ΔmixH and ΔmixV do not = 0 for real mixtures
what impacts ΔmixH + how?
strength of IMF between components
if A-B > A-A/B-B then ΔmixH<0 so ΔmixG«0
- components mix
if A-B < A-A/B-B then ΔmixH>0 slightly so ΔmixG<0 slightly
- components may mix
if A-B «_space;A-A/B-B then ΔmixH»0 so ΔmixG»0
- components do not mix
(T)ΔmixS is always negative!
how does IMF of liquid mixtures link to vapour pressure?
the smaller the IMF in a mixture of 2 liquids, the more molecules will be able to escape at any particular temperature, exerting vapour pressure
- in an ideal mixture, the tendency of the 2 different types of molecules to escape is unchanged
what is Raoults law?
Pa = xa*Pa^o
where Pa = vapour pressure due to a
xa = mole fraction a
Pa^o = vapour pressure of pure a
for mixture of a + b
Ptotal = Pa + Pb
where Ptotal = total pressure
the partial vapour pressure due to a component in a mixture = vapour pressure of pure component at the same temp x mole frac
what does it mean if a mixture obeys Raoults law?
it is ideal
how is Raoults law adapted to fit non ideal mixtures?
the activity coefficient γ is added - this allows for deviation from ideality
- can be seen on pressure/temperature composition diagrams as the adapted law gives curved lines, whereas ideal mixtures would give straight lines
- why curves? there is no deviation at phase boundaries because vapour pressures of pure a and b don’t change
- positive deviation = dome curve above expected/ideal line, negative deviation = dip curve below ideal line
limitations of Raoults law
few solutions obey ideal behaviour so it is an approximation at best
we know molecules can escape to exert vapour pressure - how is this different for ideal + non-ideal solutions?
ideal: IMF A-B = A-A and Pb =xbPb^o
non ideal:
IMF A-B < A-A/B-B so positive deviation from raoults law, Pb > xbPb^o
- b molecules easily escape to vapour, so vapour pressure increases
or
IMF A-B > A-A/B-B so negative deviation from raoults law, Pb < xb*Pb^o
- b molecules don’t easily escape to vapour, so vapour pressure decreases
what does γ = for an ideal solution?
γ = 1
typically, how do mixture IMF compare to pure components?
usually IMF in mixtures are weaker than in mure liquids, so molecules escape more readily to vapour, real vapour pressure is greater than ideal so we expect positive deviation from raoults law, thus γ >1
what is global activity coefficient?
γglobal = γa + γb
Ptotal(real) = γglobal * Ptotal(ideal)
when does raoults law work best + why?
works best for the component in excess, the solvent
this is because it is surrounded by more of its own molecules, so interactions are more similar to pure liquid, so it obeys raoults law better
what is henrys law?
Pa = Kaxa
where Pa = vapour pressure a
Ka = henrys constant for a
xa = mole fraction a
experimentally is was found than Pa depends on xa in a linear fashion
when does henrys law work best?
henrys law works best for the component not in excess, the solute
what does it mean if a mixture obeys henrys law?
it is an ideal-dilute solution
