Chapter 9 Flashcards
complex systems
Complexity arises due to the presence of several species that
interact with one another and water to yield two or more
simultaneous equilibria.
For example,
BaSO4(s)–> Ba 2+ + SO4 2-
SO4 2- +H3O+–>HSO4 - +H2O
2H2O–>H3O+ + OH-
If hydronium ions are added to the system, the second equilibrium is shifted to the right by the common-ion effect. The resulting decrease in sulfate concentration causes the first equilibrium to shift to the right, thus increasing the solubility of the barium sulfate.
Alternatively, if acetate ions are added to the solution, the solubility of barium sulfate increases since it forms soluble barium acetate in solution
Ba2+ +OAc- –> BaOAc+
Result- more complex equilibrium expressions
Mass Balance Equations
elate the equilibrium concentrations of various species in a solution to one another and to the analytical concentration of the various solutes.
Ex. Mass balance expression for 0.010 M solution of HCl that is in equilibrium with an excess of solid BaSO4
BaSO4(s)–> Ba 2+ + SO4 2-
SO4 2- +H3O+–>HSO4 - +H2O
2H2O–>H3O+ + OH-
Mass balance
[Ba2+]= [SO4 2-] +[HSO4]
[H3O+]+[HSO4-]=cHCl+[OH-]=0.01+[OH-]
For slightly soluble salts with stoichiometries other than 1:1, the mass-balance
expression is obtained
by multiplying the concentration of one of the ions by the stoichiometric ratio.
For example, the mass-balance expression for a saturated solution of Ag2 CrO 4 is
2[CrO 42-] = [Ag+]
Charge balance equation
Uses the concept of neutrality of solutions because the molar
concentration of positive charge in an electrolyte solution always
equals the molar concentration of negative charge.
No. mol/L positive charge = no. mol/L negative charge
Ex. Charge of 1 mol of Na+ in solution = [Na+]
charge of 1 mol of Mg2+ in solution = 2[Mg2+]
charge of 1 mol of PO43- in solution = 3[PO 43-]
charge balance equation for 0.1 M NaCl in water
mol/L positive charge = [Na +] + [H3 O +] = 0.1 + 10-7
mol/L negative charge = [Cl -] + [OH-] = 0.1 + 10-7
[Na+] + [H3 O +] = [Cl-] + [OH-] = 0.1 + 10-7
Writing the charge balance equation
AgBr (s) –>Ag+ + Br-
Ag+ + NH3–> AgNH3+
Ag(NH 3)+ + NH3 –>Ag(NH3)2+
NH3 + H2O—> NH4+ + OH-
2H2O —>H3O + + OH
[Ag+ ] + [Ag(NH3)+ ] + [Ag(NH3)2+] + [H3O +] + [NH4+] = [OH- ] + [Br- ]
Steps for solving problems involving several equilibria
- Write a set of balanced chemical equations for all pertinent equilibria.
2
. State in terms of equilibrium concentrations what quantity is being sought. - Write equilibrium-constant expressions for all equilibria developed in step 1,
and find numerical values for the constants in tables of equilibrium constants - Write mass-balance expressions for the system.
- If possible, write a charge-balance expression for the system.
- Count the number of unknown concentrations in the equations developed in steps 3, 4 and 5, and compare this number with the number of independent
equations. If the number of equations is equal to the number of unknowns, proceed to step 7. If the number is not, seek additional equations. If enough
equations cannot be developed, try to eliminate unknowns by suitable
approximations regarding the concentration of one or more of the unknowns.
If such approximations cannot be found, the problem cannot be solved. - Make suitable approximations to simplify the algebra.
- Solve the algebraic equations for the equilibrium concentrations needed to
give a provisional answer as defined in step 2.
9
. Check the validity of the approximations made in step 7 using provisional
concentrations computed in step 8.
The calculation of solubility by the systematic method.
Solubility calculations when pH is variable
Solubility of precipitates in the presence of complexing agents
dealt with examples
Gravimetric methods of analysis
based on the measurement of mass
precipitation method and volatilization method
Precipitation method
the analyte is converted to a
sparingly soluble precipitate. The precipitate is then filtered,
washed free of impurities, and converted to a product of known
composition by suitable heat treatment.
Ca 2+(aq)+C2O4 2—-> CaC2O4(s)
CaC2O4(s)—> CaO(s)+CO(g)+CO2(g)
Volatilization method
Here, the analyte or its
decomposition products are volatilized at a suitable temperature.
The volatile product is then collected and weighed, or
alternatively, the mass of the product is determined indirectly from
the loss in the mass of the sample.
NaHCO3(aq) + H 2 SO4(aq) CO 2(g) + H2 O(l) + NaHSO4(aq)
Mass to mass conversions in a chemical reaction depend on
the stoichiometric relations from the balanced chemical equation
Calculation of results from gravimetric data
percent X=(mass of X)/(mass of sample)*100 %
Factors that determine the particle size and precipitates
The particle size is related to relative supersaturation
= Q-S/S
Where Q is the concentration of species at any instant and S is its
equilibrium solubility.
Mechanism of precipitate formation
Precipitates form by two different pathways: nucleation and particle growth
In nucleation, a few ions, atoms, or molecules come together to form a
stable solid. Further precipitation then involves a competition between additional nucleation and growth on existing nuclei (particle growth). If nucleation predominates, a precipitate containing a large number of small particles results; if growth predominates, a smaller number of larger particles is produced.
Experimental control of particle size
Elevated temperature to increase the solubility of the precipitate (S)
Dilute solutions (to minimize Q)
Slow addition of the precipitating agent with good stirring
pH also controls particle size
Properties of Precipitates and Precipitating Agents
Ideally, a gravimetric precipitating agent should react specifically, or at
least selectively with the analyte. The product must:
1. Readily filtered and washed free of contaminants.
2. Of sufficiently low solubility so that no significant loss of the analyte
occurs during filtration and washing.
3. Unreactive with constituents of the atmosphere.
4. Of known composition after it is dried, or if necessary, ignited
Colloid
a solid made up of particles having diameters that are less than 10^-4 cm
Colloidal suspensions
are stable because the particle are either all positively charged or all negatively charges and thus repel each other
coagulation of colloids
i) Coagulation of a
colloidal suspension can often be brought about by a
short period of heating, accompanied by stirring.
Heating decreases the number of adsorbed ions and
thus the thickness di of the double layer.
(ii) Coagulation of colloidal suspension is effectively
done by increasing the electrolyte concentration.
The counter ions increases the vicinity of each
particle.
.
Surface adsorption
-common problem in precipitates with large
specific areas, i.e., coagulated colloids.
Can be reduced by (i) digestion (water is
expelled from the solid to give a denser
mass that has a smaller specific surface
area for adsorption).
(ii) By washing the colloid with a solution
containing volatile electrolyte.
(iii) Reprecipitation. Here the filtered solid is redissolved and reprecipitated.
Mixed-crystal formation
– from interfering ions of same size and
charge. Separation of interfering ions prior to precipitation is needed.
Occlusion and Mechanical entrapment
– occurs during rapid
precipitation formation. Foreign ions in the counter-ion layer become
trapped or occluded within the growing crystal. Mechanical entrapment
occurs when crystals lie close together during growth. Both effects can
be minimized by reducing the rate of precipitation formation, i.e., under
conditions of low supersaturation
Crystalline precipitates
are more easily filtered and purified than the
coagulated colloids
methods of improving particle size and filterability
by minimizing Q (dilute solution, slow addition or
precipitating agent, good stirring) and/or maximizing S (hot solution
and adjusting pH)
Coprecipitation
-is a process in which normally soluble compounds are carried out of solution by a precipitate. Four main types
Surface adsorption
Mixed-crystal formation
Occlusion
Mechanical entrapment
Homogeneous precipitation
is a process in which a precipitate is formed
by slow generation of a precipitating reagent homogeneously
throughout a solution
Volatilization methods
Often used for water and carbon dioxide determination
Water is quantitatively eliminated by ignition. In the direct method, it
is collected over a desiccant and its mass is determined from the
mass gain of the desiccant. In the indirect method, the amount of
water is determined by the loss of mass of the sample during heating
(less satisfactory).
Carbonates are decomposed by acids to give carbon dioxide. The
mass of CO2 is established from the increase in the mass of a solid
absorbent
