ions + solutions - debeye-huckel theory

Question 1

how do dissolved species interact in an ideal solution?

Answer 1

they don’t - therefore the more dilute a solution is the more it tends towards ideality

Question 2

outline debeye-huckel theory

Answer 2

the idea that non-ideal behaviour in solutions arises (at least partly) because ions are still interacting with each other electrostatically

Question 3

explain the relationship between concentration and ideality

Answer 3

at infinte dilution, a dissolved ion has no interactions, as concentration/ions increase, electrostatic interactions increases, until infinite concentration where dissolved ions can interact favourably with oppositely charged ions already present in solution

Question 4

is ion distribution in solution random?

Answer 4

distribution is neither random nor uniform due to favourable electrostatic interactions between oppositely charged ions - the ionic atmosphere moves to favour a lower energy state, therefore its arrangement is not random

Question 5

give 6 important assumptions of debeye-huckel theory

Answer 5

• the only important interaction between ions is the coulombic interactions between ions (essentially the attraction between oppositely charged particles)
• the solute is completely dissociated into ions (strong electrolyte)
• spherical symmetry is assumed, therefore not polarised and the ionic atmosphere around bust also be a spherically symmetric cloud of charge density
• there is no electrostriction, so the volume/shape of the system are unaffected by the presence of the solution
• solvent has no molecular structure, it is thought of as a dielectric medium that just modulates the interaction of 1 charge with another
• ions cannot approach closer than some distance ao, as they are fully solvated

Question 6

when is debeye-huckel theory most accurate?

Answer 6

for dilute solutions - among the assumptions are some mathematical simplifications equivalent to assuming a dilute solution

Question 7

what is the debeye-huckel limiting law?

Answer 7

ln γ± = -A * z+ * z- * √I
where the parameter A is made up of fundamental constants, temperature and the dielectric constant of the solvent (no adjustable parameters)

Question 8

explain the inert ion effect using the debeye-huckel limiting law

Answer 8

the law states that ion activity is also a function of ionic strength, and depends on all ions present
other ions present increase ionic strength so activity of ions <1 which makes the salt mroe soluble

Question 9

explain the process of salting in and salting out

Answer 9

this is all based on the inert/common ion effects:
salting in is when salt is added to increase another compounds solubility
salting out is when even more salt is added causing the previously dissolved salt to precipitate out

Question 10

how is the debeye-huckel law further improved?

Answer 10

this law is a limit of a more extended law, which has a further constant B which has no adjustable parameters and is calculated from fundamental constants
ao is an empirical fudge factor as the loosely defined closest approach of ions, and so this can be used as an adjustable parameter to fine tune the law to increase accuracy for real systems

Question 11

why do more recent deviations of the debeye-huckel law fail when tried outside of the particular niche they were developed for? (bear in mind debeye-huckel law is not perfect, more concentrated systems diverge from the model)

Answer 11

these laws all make many assumptions which aren’t true:
- ions aren’t fully separated, they can associate with each other and may not have fully separated solvation shells
- spherical symmetry is often untrue, especially for large ions which have important shapes, sizes and polariseabilities
- solvents aren’t purely dielectric solvents and can have structure (especially water)
- non-coulombic interactions can be present, particularly ion-solvent interactions

Question 12

what are 2 common types of structure formed by solvated ions?

Answer 12

although the main ionic structure dissolves, ions still have a tendency for cations and anions to pair up
- inner sphere complexes are formed where ions are adjacent are both surrounded by a single solvation shell
- outer sphere complexes are formed where each ion has a separate solvation shell but some solvent molecules in the shells are shared

Question 13

what kinds of ions are most likely to form inner/outer sphere complexes?

Answer 13

multivalent cations, sulphate, phosphate, bicarbonate, or other oxoanions which have lots of structure