history of Onondaga Lake and local chemical industry
the Solvay process - preparation of sodium carbonate (soda ash)
solubility in aqueous solution, saturated solutions, fractional crystallization
general solubility rules for different ions in aqueous solution
the chlor-alkali process (electrolysis)
mercury and other heavy-metal poisoning
what's in Onondaga Lake now and what can be done about it?
Onondaga Lake (Syracuse, NY) - salt and soda ash production
nearby brine springs were commercially valuable (major supplier of salt in U.S. in 1800)
evaporation of brine gives NaCl - using wood fires, coal, or solar
major uses of salt - for food processing
salt processing in the area closed down in 1926
starting in 1884 production of sodium carbonate (soda ash) from salt + limestone
2 NaCl + CaCO3 ---> Na2CO3 + CaCl2
major uses of sodium carbonate - in production of glass, paper, soap
Allied Chemical made 2000 tons per day of sodium carbonate
problem - about 500 tons per day of unsold CaCl2
CaCl2 is used in concrete, and for salting icy roads, but wasn't needed in such large quantities
excess CaCl2 was stockpiled, buried, overflowed, much of it leached into the lake
Allied Solvay plant closed in 1986
most sodium carbonate now supplied from a natural mineral (trona) found in Wyoming
The Solvay Process
overall: 2 NaCl + CaCO3 ---> Na2CO3 + CaCl2
Solvay process had to get around the insolubility of CaCO3
step (1) - CaCO3 -- (heat) ---> CaO + CO2
step (2) - NH3 + CO2 + NaCl + H2O ---> NaHCO3 + NH4Cl
(all components are water-soluble)
sodium bicarbonate converts to sodium carbonate on heating:
step (3) - 2 NaHCO3 -- (heat) --> Na2CO3 + H2O + CO2
the CO2 released can be recycled to the previous step
the ammonia can also be recycled:
step (4) - 2 NH4Cl + CaO ---> 2 NH3 + H2O + CaCl2
after all the recycling, the overall conversion is the simple exchange reaction:
overall: 2 NaCl + CaCO3 ---> Na2CO3 + CaCl2
Aqueous Solubility
how is sodium bicarbonate isolated from the aqueous solution ?
step (2) - NH3 + CO2 + NaCl + H2O ---> NaHCO3 + NH4Cl
in solution are positive ions (cations) Na+ and NH4+ and negative ions (anions) Cl- and HCO3-
solubility - the maximum amount of a compound that can dissolve in a solvent (like water)
when the maximum amount is dissolved, the solution is said to be saturated
solubility depends on temperature - solids usually dissolve more at higher temperature
at 0 C, NaHCO3 is much less soluble in water than NH4Cl
as the solution is cooled, NaHCO3 exceeds its solubility and comes out of solution
the solid precipitate can be harvested by filtration (fractional crystallization)
refer to Figure 7.4 to see how the solubility of each component changes with temperature
General Aqueous Solubility Rules
cations: most salts of ammonium (NH4+), Na+, and other group IA metals are soluble
most salts of group IIA metals are soluble, except the carbonates (e.g., CaCO3 )
anions: most nitrates are soluble
most chlorides are soluble, except silver (AgCl, used in photography)
most sulfates are soluble, except some group II metals, (e.g., BaSO4 and PbSO4 )
most carbonates, hydroxides, and sulfides are insoluble
(except the ammonium and group IA metals - see first rule)
be able to use these rules to predict solubility of a given salt (see Your Turn 7.4, 7.5)
be able to use these rules to predict when certain combinations will precipitate (Your Turn 7.8)
Mercury Contamination - the Chlor-Alkali Process
another commercial use of salt - electrolysis to give sodium hydroxide (lye) and chlorine
2 NaCl + 2 H2O -- (electrolysis) --> 2 NaOH + Cl2 + H2
mercury (Hg) is a liquid metal (quicksilver) and is used as the electrode for the electrolysis
many tons are used and recycled in the process, but some escapes and is released to the lake
originally up to 20 pounds per day, eventually lowered to less than 0.5 ounce per day
estimated that the lake contains about 47 tons of mercury
mercury is insoluble in water, as are most of its salts
however, some bacteria can convert Hg to CH3Hg+, which is water-soluble
mercury is also poisonous if its vapor is breathed
heavy-metal poisoning (e.g., mercury - Hg, lead - Pb, cadmium - Cd)
typically these interfere with enzyme reactions (which control all bodily processes)
probably by replacing the ions usually used, such as Ca or Mg
the chlor-alkali plant closed in 1988
Chemistry in Onondaga Lake
large amounts of soluble CaCl2 entered the lake as calcium ions and chloride ions
calcium reacts with carbon dioxide from the air to precipitate limestone (see Figure 7.6)
CaCl2 + CO2 + H2O ---> CaCO3 + 2 HCl
the large reserve of CaCO3 means that the lake is well-buffered from acid rain
the lake also contains many more ions (salts) than normal freshwater
the presence of high mercury concentrations means fishing and swimming is forbidden
sewage treatment plants have also contributed directly and by overflows into the lake
solutions for Onondaga Lake - generally expensive, politically difficult