ions + solutions - solvation

Question 1

what kind of solvent is water?

Answer 1

polar protic

Question 2

what makes water a unique solvent?

Answer 2

its ability to form hydrogen bonds and its high dielectric constant - these are important to the interactions of water

Question 3

how do solvated ions interact with water?

Answer 3

solvated ions separate from each other and are surrounded by water molecules - this is known as a solvation shell

Question 4

how does the ion affect the solvation shell?

Answer 4

solvation shells will have different structures based on the charge/polarity of the ions

Question 5

outline the structure of a solvation shell

Answer 5

solvation shells are highly structured
if the ion is +ve, the water molecules will arrange around it so that the -ve dipole/side is closest to the ion (+ vice versa)
multiple layered solvation shells are possible, each consequent solvation shell is looser and less structured than the last

Question 6

hydrodynamic radius definition

Answer 6

the size of an ion + its solvation shell

Question 7

how is an ions hydrodynamic radius relevant to its conductivity?

Answer 7

when ions move in solutions its solvation shell moves with it, and it is necessary for ions to move freely in order to conduct/carry a charge
solvated ions are much larger than individual ions and so are more easily stopped by semi-permeable membranes, meaning that the charge is more easily stopped

Question 8

electrolyte definition

Answer 8

initially referred to a substance that conducts electricity by movement of ions rather than flow of electrons, however over time it has come to be sued to refer to the dissolved ions themselves

Question 9

what is the difference between a strong and weak electrolyte?

Answer 9

a strong electrolyte fully dissociates into ions when dissolved, whereas weak electrolytes only partially dissociate

Question 10

explain why molarity calculations for dissolving ionic solids are more complicated

Answer 10

dissolving 1 mole of an ionic solid e.g. NaCl in water will produce 1 mole Na+ ions and 1 mole Cl- ions, or a 1 mole solution of NaCl but 2 mole solution of ionic species

Question 11

what is the function of the van’t hoff factor, i?

Answer 11

i = the number of moles of ionic species in a solution/salt compound

it is relevant for calculating mole fractions e.g. for raoults law, partial/osmotic pressure:
1 mol ethanol in 10 mol water = 1/10+1 = 1/11
1 mol NaCl in 10 mol water = 1/10+2 = 1/12

Question 12

what is the concentration of water in water + how is this used to classify a dilute solution?

Answer 12

Mr H2O = 18 gmol^-1, 1L = 1 dm^3 = 1kg
therefore moles = 1000/18 = 55.5… moles

so a dilute solution should be &laquo_space;1M, as we want [solute]«[water]

Question 13

molarity definition

Answer 13

a measure of moles per litre of solvent (mol dm^-3)

Question 14

give 1 limitation of molarity as a measurement

Answer 14

thermal expansion means that the volume of the solution may not be the same as after the substance has been dissolved

Question 15

molality definition

Answer 15

a measure of moles per mass (mol kg^-1)

Question 16

why is molality preferrable to molarity?

Answer 16

with molality, volume changes as a result of temperature has no effect
- although often molar/molal measurement is not distinguished as the difference in solutions under 10M is often negligable

Question 17

how does activity improve modelling of solutions?

Answer 17

real solutions often behave as if the effective concentration were less than the actual concentration - this non ideal behaviour comes from the interactions between dissolved ions
the activity coefficient accounts for this, and is typically <1 for ionic solutions

Question 18

give the activity coefficient equation

Answer 18

a = γc
where a = activity (dimensionless)
γ = activity coefficient
c = concentration normalised to 1M

conc = [X] vs activity = {X}

Question 19

what is the activity for solid substances?

Answer 19

activity of the solid phase = 1, as its normalised to a standard state

Question 20

what is the activity for gaseous substances?

Answer 20

activity of a gas is usually partial pressure, normalised to standard pressure

Question 21

why do not all ionic solids readily dissolve in water?

Answer 21

there are limits to solubility
such as solution saturation, which varies for different compounds, and the effects of temperature

Question 22

saturated solution definition

Answer 22

a solution that is in equilibrium with the undissolved solid

Question 23

sparingly soluble definition

Answer 23

when solids only dissolve a little bit before saturation is reached

Question 24

how does temperature affect solubility?

Answer 24

often solubility increases with temperature - consider ΔG=ΔH-TΔS

Question 25

what makes ions insoluble?

Answer 25

thermodynamics dissolution will happen spontaneously if ΔG<0 for the dissolution process ΔS normally favours solution over a well ordered crystal solid structure, although solvated ions still have structured solvation shells so process is not as favourable as you'd expect ΔH is a balance - ion loses binding enthalpy of being in the solid but gains the enthalpy of solvation from interacting with water therefore it is often a fine balance if ΔG0

Question 26

give fajans rules for ionicity

Answer 26

a solid is made more ionic, and therefore soluble, if: - the cation is large - the anion is small - the cation +ve charge is small the opposite case gives a more covalent compound which is less soluble

Question 27

how do highly charged anions affect ionic character + why?

Answer 27

they decrease ionic character, this is because of polarisation small highly charged cations are more polarising and large anions tend to have extensive, easily polarised electron clouds, electron density is dragged from anion to cation causing the bond to become more covalent

Question 28

give one exception to fajans rules

Answer 28

transition metal aqua complexes are much more soluble than expected from fajans rules

Question 29

what is the common ion effect?

Answer 29

the effect that a salt is less soluble in water/solvent that already contains that salt/one of its ions dissolved in it - if multiple ions are present, then with a common ion, they will suppress eachothers solubility in a le chatelier sort of way

Question 30

what is the inert/odd/other ion effect

Answer 30

the effect that a salt is more soluble in water/solvent that doesn't already contain that salt/its ions dissolved in it, this is because of non-ideality, salts will all different ion components increase the solubility of each other

Question 31

for what kind of compound are the common/inert ion effects most observable?

Answer 31

sparingly soluble salts

Question 32

solubility product definition

Answer 32

the equilibrium constant for the equilibrium between the solid and dissolved species in a saturated solution - this is usually only important for sparingly soluble substances

Question 33

what is the ideal solution approximation?

Answer 33

to assume ideality, activity coefficient can be ignored so that [X] = {X}

Question 34

ionic strength, I definition

Answer 34

reflects concentration and charges of dissolved ions I = 1/2 * ∑ [cᵢ * zᵢ²] where ion species i has charge zᵢ and concentration cᵢ

Question 35

molal ionic strength definition

Answer 35

same as ionic strength but with molality not molarity