hey it's professor Dave, let's talk about solubility
we now know what a solution is so we want to know how to determine whether two
substances will be miscible or immiscible, that is whether they will or
won't mix to form a homogeneous solution. we must also understand that there is a
limit to how many solute particles can fit in a solution of a given volume
because there are a certain number of interactions a solute particle must make
with solvent molecules in order to dissolve and once all the solvent
molecules are occupied and making such interactions the solution is said to be
saturated meaning no more solute can dissolve and join the solution. if more
solid is added it will just sit at the bottom and there will be a dynamic
equilibrium where solute particles leave solution to join the solid at the same
rate that particles leave the solid to join the solution. the solubility of a
solute is the amount that dissolves in a given quantity of a particular solvent
at a given temperature. for example these substances will dissolve in 100 grams of
water to the extent shown at the indicated temperatures. a supersaturated
solution is one that contains more solute than a saturated solution. this
can happen when a solution is prepared at a very high temperature where
solubility is heightened and then allowed to slowly cool. this is a very
unstable situation so any disturbance causes immediate precipitation. let's
look at what causes a discrepancy in solubility between different ionic
solids. first we must ask the question: if an ionic solid is held together by ion
ion forces, the strongest intermolecular force. why would they want to dissolve?
one version of the answer includes a concept called entropy, or the tendency
to go towards more disorder, but we will get to that later. for now the answer is
that each
ion can participate in a number of ion-dipole interactions in solution which
could potentially add up to store more energy than there is in the solid
lattice. if this is the case the substance will likely be soluble. we have
to compare two values for any ionic solid, the hydration energy and the
lattice energy. the hydration energy is the energy that results when a network
of ions maximize interactions with solvent particles. lattice energy is the
energy that holds the solid lattice together. whichever one is greater will
determine whether the substance dissolves or not. so here is sodium
chloride as a solid and here we have dissociated ions making ion-dipole
interactions. how do we calculate these energies? it can be tricky to do
experimentally so we actually calculate lattice energy indirectly by using
something called The Born-Haber cycle. first we use heat of formation data to
show the change in enthalpy related with going from elemental sodium and chlorine
to sodium chloride. there's the delta H for the reaction then separately we find
enthalpy change associated with first vaporizing and ionizing sodium and then
splitting chlorine molecules into individual atoms and ionizing those
until we have individual sodium and chloride ions not interacting with each
other
wherever that sits on an energy diagram, the distance to NaCl solid where they are
interacting must be the energy stored in the interactions, or the lattice energy
just look at tabulated data like this and if the hydration energy of a
substance is greater than the lattice energy it should dissolve as it will be
energetically favorable for it to do so. we can also just refer to this handy
table and memorize the compounds we know to be water-insoluble. thanks for
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