Lesson: Double Replacement Reactions
Field Museum Extensions
a.
Related Exhibitions.
1. Rocks and Minerals. Double replacement reactions are responsible for
a large amount of the minerals and rocks in the earth’s crust. Which
minerals are created by double replacement reactions? Can you write a
double replacement reaction to produce one of these minerals? (Think of
dissolved compounds that could occur in nature—they need to be watersoluble. A solubility table from your text or from another reference might
be useful.)
(Chemical sedimentary rocks are deposited from solution. The minerals in
these rocks are precipitated in double replacement reactions. An example
of this type of mineral would be calcite, or calcium carbonate. A student
might create a plausible double replacement reaction such as
CaCl2 (aq) + Na2CO3 (aq) → CaCO3 (s) + 2 NaCl)
2. Grainger Hall of Gems and Evolving Planet. Double replacement
reactions are a means in which the remains of living things are fossilized.
Focus on the opal fossil (#7) in the Hall of Gems. What ion is in the solid
precipitate that replaced the shell material of this organism (which is also
in the petrified wood and the reptile fossil in nearby cases)? What is a
characteristic of many compounds that contain this ion? Can you propose
double replacement reactions that might have occurred in solution during
the fossilization processes? (Think of dissolved ions that could occur in
nature—the starting compounds must be water soluble. A solubility table
from your text or from another reference might be useful.) Once you are
familiar with this process, proceed to the Evolving Planet exhibit and find
the presentation entitled Fossilization: “Here’s a step-by-step guide to
becoming a fossil.” Can you add to this step-by-step guide, providing
more information on the chemistry involved? How would you write it so
that the average museum visitor would understand the double replacement
reactions?
(The fossils in the Hall of Gems contain the silicate ion. Many
compounds that contain this ion are not soluble in water. When
concentrations of metal ions such as calcium ion or magnesium ion reach a
certain level, the silicate salt will begin to precipitate. Calcium ion is very
common in bone and shells. As the calcium ion begins to dissolve into a
solution rich in silicate ion around the bone or shell, it precipitates. The
weather-resistant calcium silicate now has replaced the old calcium
mineral of the bone or shell. Eventually, the more easily weathered
minerals will be completely replaced by the less reactive and less soluble
silicate minerals. The form of the bone or shell has thus been preserved.
A student might create a plausible double replacement reaction such as
2 CaSO4 (aq) + Na4SiO4 (aq) → Ca2SiO4 (s) + 2 Na2SO4 (aq)
3. Africa—Rift Lakes. A fundamental part of double replacement reactions
is the precipitation of ionic compounds from solution. Why do these
compounds come out of solution while other ionic compounds stay
dissolved? Some of the economic activites around Lake Magadi rely on
the precipitation of ionic compounds. What compounds are these? Are
these soluble or insoluble? What happens to make these compounds
precipitate?
How is this precipitation process different than double replacement
reactions? If you look up the solubilities of these compounds at 25
Celsius, can you determine which will come out of solution first if they are
in equal concentration (g/100ml)? What is the explanation for one
compound being less soluble than the other?
(Trona is the solid crust around Lake Magadi and is a mixture of sodium
carbonate and sodium chloride. The solubility of sodium chloride in water
at 25 degrees Celsius is 36 g per every 100 g of water. The solubility of
sodium carbonate at 25 degrees Celsius is 23.5 g per every 100 g of water.
At this temperature, if the same number of grams of sodium carbonate and
sodium chloride are dissolved in a volume of water, the sodium carbonate
will precipitate first. The reason for this is that there is a stronger bond
between the sodium ions and carbonate ions compared to the bond
between sodium ions and chloride ions. When the sodium and carbonate
ions are close enough, they will bond and come out of solution. At this
point, the sodium and chloride ions would not be close enough in solution
to bond. The difference between the precipitation in Lake Magadi and the
precipitation in a double replacement reaction is that in Lake Magadi,
precipitation is occuring because solvent is evaporating, making the ions
in solution so concentrated that they bond and come out of solution. In a
double replacement reaction, two separate solutions are mixed together,
and two previously separated oppositely charge ions that strongly bond are
now in the same mixture, where they can then bond and come out of
solution. In a double replacement reaction, there need not be any
evaporation of solvent.)
b.
Harris Educational Loan Center.
1. Fossils from Rocks near Chicago Exhibit Case. These fossils are not
composed of the actual shell matter of the living organism. They have
been turned into rock via double replacement reactions.
(Discussion can revolve around the precipitation of carbonate, phosphate,
and silicate compounds, replacing the shell materials of the organism.)
2. Rocks and Minerals Experience Box. Compare sedimentary rocks with
igneous and metamorphic rocks. Describe the chemical processes (double
replacement reactions) that can create these rocks.
(Students can practice writing double replacement reactions such as
CaCl2 (aq) + Na2CO3 (aq) → CaCO3 (s) + 2 NaCl (aq) that could
plausibly create the minerals in common chemical sedimentary rocks such
as limestone.)
c.
Field Museum Science/Website Resources.
1. Exploring the Arctic Seafloor—Photographs by Chris Linder. When
chemists write double replacement reactions, they write the “molecular
equation” form for convenience. But this is not the whole story of what’s
in solution. For example, the double replacement reaction,
2 Na3PO4 (aq) + 3 CaCl2 (aq) → Ca3(PO4)2 (s) + 6 NaCl (aq),
here written in molecular equation form, is compact and shows the
dissolved chemicals and the precipitate. Yet, the solution contains free
and mobile ions. In other words, the dissolved ionic compounds are
dissociated. Chemists know that NaCl has dissociated into free and
mobile sodium ions and chloride ions. So to show the formulas of
independant ions in solutions, you would write Na+ (aq) + Cl- (aq),
instead of NaCl (aq). This is important information because solutions of
independant ions in solution have the unique property of having electrical
conductivity. And the presence and concentration of ions in solution can
be measured using devices that detect the electrical conductivity of
solutions. Start at the “Exploring the Arctic Seafloor” exhibit site
(http://www.fieldmuseum.org/exhibits/arctic_tempexhib.htm) and then
move to the companion sites: “Dive and Discover II”
(http://www.divediscover.whoi.edu/expedition11/) and “Ocean
Instruments”
(http://www.whoi.edu/instruments/viewInstrument.do?id=1003) to see
some how research on thermal vents in the arctic ocean is being conducted
and see what types of instruments are being used. What is a CTD device?
How is it used to locate life forms around thermal vents? What ions might
be detected with this device? Would a CTD be able to detect the presence
of a chemical such as methane, CH4? Can you give some examples of
dissociation reaction equations for salts found in ocean water?
(When students learn to write complete ionic reaction equations from the
molecular equations of double replacement reactions, there might be some
confusion as to the reasons and meanings behind each equation form.
Reinforcing the idea of “free and mobile” ions in solution when looking at
aqueous ionic compounds is important as this idea is the basis of
understanding many topics in chemistry, including acid/base chemistry
and equilibrium. In these websites, the CTD (conductivity, temperature,
depth) tool is mentioned as a crucial component of the investigations. The
life forms around thermal vents have adapted to utilize the chemical
energy contained in reduced elements contained in certain ions. Large
concentrations of these ions, which create high conductivities in the ocean
water, are linked to the presence of the vents and of the life that may be
found around them. Any ions would be detected with a conductivity
device, although certain types—ion selective electrodes—can detect the
presence and concentrations of specific ions. A conductivity device would
not be able to detect the presence of a molecular compound such as CH4
since it does not have a charge and does not affect the conductivity of the
solution. Students can practice writing dissociation reaction equations for
ocean salts such as Na2SO4 (s)  2 Na+ (aq) + SO4 -2 (aq), and this
practice will help them to convert their molecular equations for double
replacement reactions to complete ionic reaction equations.)