2C: Aqueous Ionic Equilibria

buffers

solutions that resist pH changes when an acid/ base is added

• neutralize whatever has been added

How to make a buffer

mixing solution of weak acid with a soluble salt containing its conjugate base anion

OR

mixing weak base with soluble salt containing conjugate acid

common ion effect

process through which buffers work:

• equilibrium shift via addition of salt that has an ion in common with one of the dissolved species

buffer capacity

measure of the strength of the buffer = ability to properly maintain pH

buffer range

pH range over which the buffer is effective:

• buffer range within +- 1 pH of pKa

example of an acidic buffer (weak acid/ conj base)

HCOOH + H2O → HCOO- + H3O+

example of a basic buffer (weak base/ conj acid)

NH3 + H2O → NH4+ + OH-

henderson- hasselbach equation

• assumes amount of acid/base that dissociates is small relative to initial conc

• simplifies pH calculation

when does henderson-hasselbach eqn give acceptable error

if [HA] and [A-] > 0.01 M

if Ka and Kb between 10^-3 and 10³ = small K

if buffer ration [A-]/[HA] between 0.1 and 10

impact of adding a small amount of St. A to buffer

converts weak base to conjugate acid

impact of adding a small amount of St. B to buffer

converts weak acid to conj base

solubility

amount of solute that will dissolve in a litre of solution

molar solubility is related to ___

Ksp

to compare Ksp’s compounds must have the same ___

dissociation stoich

impact of adding a soluble salt that contains an ion of the insoluble salt

decreases solubility of the insoluble salt

for insoluble ionic hydroxides, low pH = high ___

pH

for insoluble ionic hydroxides, high pH = __ [OH-]

high

for insoluble ionic compounds with anions of WA’s, low pH = ___ solubility

low

precipitation

when the concentrations of the ions exceed solubility of the ionic compound

IF Q = Ksp

solution is saturated = NO PRECIP

IF Q<Ksp

solution is unsaturated = NO PRECIP

IF Q > Ksp

Above saturation = PRECIPITATE FORMS

supersaturated solution

wont precipitate unless disturbed

all compunds of group 1 ions and NH4+ are (sol/insol)

soluble

nitrates, ethanoates, and perchlorates are (sol/insol)

soluble

chorides/ bromides/ iodides are (sol/insol)

soluble

which chlorides, bromides, and iodides ARE NOT soluble

Ag+, Pb2+, Cu+, Hg2 2+

fluorides are (sol/insol)

soluble

which fluorides are insol

Pb2+, Group 2 ions

sulfates are (sol/insol)

soluble

which sulfates are insoluble

Ca2+, Sr2+, Ba2+, Ag+, Pb2+

metal hydroxides are (sol/insol)

insoluble

which metal hydroxides are soluble

group 1 ions, group 2 ions after Ca2+

carbonates and phosphates are (sol/insol)

insoluble

which carbonates/ phosphates are soluble

group 1 ions, NH4+

all common sulfides (sol/insol)

insoluble

which common sulfides are soluble

group 1 and 2 ions, NH4+

selective precipitation

addition of a reagent to precipitate certain cations within a certain Ksp bound

complex ion

central metal ion with several anions or neutral molecules

central ion of a complex ion is usually a

transition metal

attached ions in complex ion are called

ligands

higher formation constant (Ksp) favours formation of ____

complex ion

in presence of aq ligands, an ionic compound that contains a metal cation forming a complex ion will (increase/ decrease) in solubility

increase