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For many of us, chemistry is about mixing chemicals, balancing equations, and endless measuring. But there's also an artistic, aesthetic, even philosophical appeal to it.
Precipitation reactions were used in this sense, but of course understanding it is a prerequisite. |
"In the tin-vat, commonly used for calico-printing, the indigo is reduced by a solution of stannous oxide in caustic potash or soda. The bath is usually mixed with an acid solution of tin, so as to neutralize the alkali and precipitate the indigo-white; the precipitate is then used for printing." - Henry Watts, 'A Dictionary of Chemistry and the Allied Branches of Other Sciences', 1875.
Precipitation reactions occur when different aqueous solutions (usually of salts) are mixed, and a solid substance results. Remember solutions are homogeneous mixtures in which each part of the solution has the same composition. A solution can be colored (such as a blue copper sulfate solution) but it is always clear. Clear means that the solution is transparent, not cloudy, turbid, translucent, or opaque.
Most of the precipitation reactions that we will deal with involve aqueous salt solutions. Remember salts are compounds which consist of metal cations like Na+, Ca2+, Cu2+ (or the one nonmetal molecular ion that we have discussed, ammonium - NH4+) ionically bonded to nonmetal anions such as Cl-, (or molecular anions such as hydroxide - OH-, sulfate - SO42-, phosphate - PO43-, nitrate - NO3-, and carbonate - CO32-), dissolved in water. Salts can be divided into two types: those soluble in water, and those insoluble in water. You should know some simple solubility rules which will allow you to know which salts are soluble in water.
Cheat Sheet
Most of the precipitation reactions that we will deal with involve aqueous salt solutions. Remember salts are compounds which consist of metal cations like Na+, Ca2+, Cu2+ (or the one nonmetal molecular ion that we have discussed, ammonium - NH4+) ionically bonded to nonmetal anions such as Cl-, (or molecular anions such as hydroxide - OH-, sulfate - SO42-, phosphate - PO43-, nitrate - NO3-, and carbonate - CO32-), dissolved in water. Salts can be divided into two types: those soluble in water, and those insoluble in water. You should know some simple solubility rules which will allow you to know which salts are soluble in water.
Cheat Sheet
- nitrate NO3- salts are soluble
- salts containing Group 1 metals (Li, Na, K, 1+ charge) and NH4+ are soluble
- most Cl-, Br-, and I -salts are soluble, with the notable exceptions of salts that contain silver, Ag+ and lead, Pb2+
- most sulfate SO42- are soluble with the exceptions of salts containing barium, Ba2+, Pb2+ and Ca2+
- most hydroxides OH- are just slightly soluble, with the exceptions of the very soluble NaOH and KOH
- most phosphates PO43- and carbonates - CO32- are only slightly soluble
Ionic Conductivity
When salts dissolve in water, they seem to "melt away". What happened to the ions? An import clue comes from the fact that the resulting solution conducts electricity while pure water doesn't. We demonstrated this but putting a light bulb connected to two electrodes into water or a sodium chloride solution and connected it to an electrical outlet.
Before we can understand this, we need to have a quick review of electricity. As a water current is a flow of water, a current of electricity can be viewed as a flow of electrons. Consider our hypothetical copper wire. Copper is a metallic element, but does it exists as copper atoms in the wire? This can't be since atoms of any element except the Group VIII Nobel gas elements are reactive. Something must be holding the copper particles together and cause the wire to be a good conductor of electricity (i.e. it can carry a current of electrons). In fact, the copper particles don't exist as atom but rather as positive ions which have given up an outer shell electron. These copper ions surrounded by a sea of mobile electrons account for the stability of the solid copper and the fact that it can conduct electricity - which is a flow of charged particles - in this case of electrons.
Why does a solution of NaCl conduct a current? Somehow when NaCl dissolves, it must formed individually charged particles. We know the NaCl consists of an array of Na+ and Cl- ions. If when NaCl dissolves, it separates into individual molecules of neutral NaCl, then the solution would not contain particles with an overall net charge. This suggests that the individual ions which comprise a "molecule" of the NaCl separate from each other -i.e. they dissociate - in water. The O (slightly negative) on water interacts with the metal ion (positively charged cation), while the H (slightly positive) on water interacts with the nonmetal ion (negatively charged anion), separating the two ions. Charge particles are required for a flow of electric current. If the molecules of salt didn't separate into ions, the smallest particles in solution would be neutral salt molecules. NaCl and other soluble salts are strong electrolytes.
When salts dissolve in water, they seem to "melt away". What happened to the ions? An import clue comes from the fact that the resulting solution conducts electricity while pure water doesn't. We demonstrated this but putting a light bulb connected to two electrodes into water or a sodium chloride solution and connected it to an electrical outlet.
Before we can understand this, we need to have a quick review of electricity. As a water current is a flow of water, a current of electricity can be viewed as a flow of electrons. Consider our hypothetical copper wire. Copper is a metallic element, but does it exists as copper atoms in the wire? This can't be since atoms of any element except the Group VIII Nobel gas elements are reactive. Something must be holding the copper particles together and cause the wire to be a good conductor of electricity (i.e. it can carry a current of electrons). In fact, the copper particles don't exist as atom but rather as positive ions which have given up an outer shell electron. These copper ions surrounded by a sea of mobile electrons account for the stability of the solid copper and the fact that it can conduct electricity - which is a flow of charged particles - in this case of electrons.
Why does a solution of NaCl conduct a current? Somehow when NaCl dissolves, it must formed individually charged particles. We know the NaCl consists of an array of Na+ and Cl- ions. If when NaCl dissolves, it separates into individual molecules of neutral NaCl, then the solution would not contain particles with an overall net charge. This suggests that the individual ions which comprise a "molecule" of the NaCl separate from each other -i.e. they dissociate - in water. The O (slightly negative) on water interacts with the metal ion (positively charged cation), while the H (slightly positive) on water interacts with the nonmetal ion (negatively charged anion), separating the two ions. Charge particles are required for a flow of electric current. If the molecules of salt didn't separate into ions, the smallest particles in solution would be neutral salt molecules. NaCl and other soluble salts are strong electrolytes.
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Precipitation reactions occur when different salt solutions are mixed, which result in the formation of an insoluble salt, or precipitate. When precipitation occurs, the cation of one of the soluble salts interacts with the anion of the other soluble salt to form an insoluble salt. The other ions which remain soluble are called spectator ions.. Consider the following example when aqueous solutions of NaCl (sodium chloride) and AgNO3 (silver nitrate) are added to each other. A white cloudy precipitate forms. What is the likely product that formed a precipitate?
Consider all the ions in solution. If the positive ions approached each other, they would repel, so we can't form compounds like NaAg. The same is true of the negative ions. Ions of opposite charge can attract each other so we could reform NaCl and AgNO3. However, these are soluble salts and would immediately dissociate again into individual ions. The other possibilities are AgCl and NaNO3. The later would likewise dissociate since we know that Na and NO3 are soluble. The only alternative is AgCl. We can write a series of chemical equations to explain these interactions: |
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molecular equation: write down the species as molecules as below-
NaCl (aq) + AgNO3 (aq) --> AgCl (s) + NaNO3 (aq) where the (aq) for aqueous implies that the salt is in solution as is soluble.
ionic equation: write down the species as they actually occur in the solution:
Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) --> AgCl(s) + Na+(aq) + NO3-(aq)
net ionic equation: remove identical ions that appear on both sides fo the ionic equation that act as "spectators" in the reaction. They are called spectator ions.
Ag+(aq) + Cl-(aq) --> AgCl(s)
NaCl (aq) + AgNO3 (aq) --> AgCl (s) + NaNO3 (aq) where the (aq) for aqueous implies that the salt is in solution as is soluble.
ionic equation: write down the species as they actually occur in the solution:
Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) --> AgCl(s) + Na+(aq) + NO3-(aq)
net ionic equation: remove identical ions that appear on both sides fo the ionic equation that act as "spectators" in the reaction. They are called spectator ions.
Ag+(aq) + Cl-(aq) --> AgCl(s)
I know it's rather dry, but as a reward for understanding how the whole thing works, here's a special treat for the inner artiste in all of us.
Ponder this
It was mentioned that these reactions occur in aqueous solutions, but would it work with solvents other than water? Ethanol, acetone, or turpentine perhaps?
Why do different reactions behave in different ways? They are, as explained, essentially solids. And yet some form granules, others gelatinise or plasticize.
Discuss
Precipitation reactions are important in industrial chemistry. They are much more controllable, the inputs are measurable and thus results are predictable, which facilitates commercial production.
During the 19th century, industrial chemistry really took off. Research and discuss the sort of chemistry being used in that period. What are they producing? What process did they use? What were their options before, and what are the reasons for using the new methods? Are they for reasons of safety, economy, quality?
Further reading
Precipitation Reactions, at US Davis' ChemWiki
Precipitation Reactions in the Environment, for an example of the practical use of precipitation reactions
Test tube reactions, a video of several chemicals reactions with resulting precipitates
