Lab 7 Flashcards
what does this experiment study
equilibrium systems that exist between a sparingly soluble salt and its environment
Ionic solids
some of which are the products of acid-bases reactions, have varying solubility in water
what represents the equilibrium of dissolution of compounds such as ionic solids
Ksp
When sparingly soluble salts are added to water, agitated, and allowed to equilibrate
you get saturated solutions
Equilibrium in saturated solutions differs from ionic equilibrium in aqueous acids and
bases in that they …..
are heterogeneous mixtures and therefore involve heterogeneous equilibria
solid material is in contact with a liquid aq phase containing separated ions of the solid
Ions move back
and forth from the aqueous phase to the solid phase at the same rate and the system is said to be in a
state of dynamic equilibrium
For example, the sparingly soluble salt calcium iodate, Ca(IO3)2, dissolve in water to a very small
extent according to the reaction:
Ca(IO3)2 = (w/H2O) = Ca2+ + 2IO3
- eq lies to the left
- concentration of ions is small
Ksp= [Ca2+][IO3-]^2
- solubility product constant expressions are often
restated in terms of the molar solubility of the
salt.
molar solubility
The molar solubility of a substance is the maximum amount (in moles) of solute that dissolves per
litre of solution.
therefore we can rewrite
Ksp=(s)(2s)^2=(4s)^3
common ion effect
according to Le chat adding excess calcium or calcium iodate ions can cause equilibrium to shift left
- We will study this effect via a redox titration of calcium iodate solution containing an additional source of iodate ion (from KIO3).
to find Ksp
we need to know the concentration of both Ca+2 and IO3– in solution at equilibrium. However, since the concentrations of the two ions are stoichiometrically related, we only
need to find one to know the other.
The concentration of iodate, IO3-, is most easily determined through
a redox titration with standardized thiosulfate, S2O32-, in the presence of iodide, I-, using starch as an
indicator.
iodate is used because
Iodate is a fairly good oxidizing agent and will oxidize I- (supplied by KI) to give iodine, I2, in acidic solution (7.4).
use of starch indicator
Starch is used as an indicator in this titration because it reacts with I2 reversibly to form a dark blue –
black complex
- Since I2 is consumed in the titration the dark colour of the complex will fade.
when is endpoint reached
The endpoint is reached when one drop of Na2S2O3 causes the disappearance of the last trace of blue – black
colour
