The Double Switch
Precipitates in Double Displacement Reactions
Many ionic compounds (salts) are soluble. Generally, solubility is thought of as the tendency of one
substance to dissolve in another substance. For qualitative purposes, we use such terms as “soluble,”
“insoluble” and “slightly soluble” to describe these tendencies. Soluble means the substance dissolves
completely. Insoluble means that the substance will not dissolve. Slightly soluble really means that only a
small amount will dissolve, but not enough for you to notice. Salts dissolve in water by a process known as
dissociation. In this process, the crystal lattice of the solid salt breaks down to allow free ions to move
throughout the solution. This new solution composed of an ionic salt dissolved in water is called an
aqueous (aq) solution. For example, when table salt – NaCl – is placed into water it will dissociate into its
constituent ions Na1+ and Cl1- with the total number of positive charges equal to the total number of
negative charges.
If aqueous solutions of two different ionic compounds interact, one of two things will occur. If all of the
ions remain free (soluble), then nothing will happen. That is, the mixture of solutions will remain clear, or
transparent. This result likely means that there is no reaction between the two substances that were mixed.
However, if two oppositely charged ions are attracted to one another strongly enough, they may bond
together to form an ionic compound that is insoluble in water – it does not dissolve. In such cases, a
precipitate (s) forms.
A precipitate is a solid substance that separates from solution during a chemical reaction. Precipitation in
chemical solutions occurs when two chemicals react to form a product that is insoluble in water and falls
out of solution. A precipitate can be identified by the cloudy, milky, gelatinous, or grainy appearance it
gives to the mixture.
The Process: A calcium sulfate precipitate can be produced by the reaction of calcium chloride and
lithium sulfate. A balanced chemical equation to describe the reaction is written like this:
1CaCl2 (aq) + 1Li2SO4 (aq) → 2LiCl (aq) + 1CaSO4 (s)
Notice the reaction that forms the CaSO4 precipitate is a double-displacement reaction in which the cations
and anions of the reactants trade partners to form the products – a double switch…
To write a total ionic equation, rewrite all aqueous substances as their component ions and keep all solid
substances (the precipitate) unchanged. In other words – work backwards and undo your compounds
changing everything but the precipitate back into ions..
CaCl2 (aq)
+
Li2SO4 (aq) → 2LiCl (aq) + CaSO4 (s)
Ca2+ (aq) + 2Cl1- (aq) + 2Li1+ (aq) + SO42- (aq) → 2Li1+(aq) + 2Cl1- (aq) + CaSO4 (s)
Cancel all components of the reaction that are identical on both the reactant and product side of the
reaction. These are called spectator ions.
Ca2+ (aq) + 2Cl1- (aq) + 2Li1+ (aq) + SO42- (aq) → 2Li1+ (aq)
+ 2Cl1- (aq) + CaSO4(s)
Write the net ionic equation by rewriting the reaction equation including only those reactants and products
that change in the reaction.
Ca2+ (aq) + SO42- (aq) → CaSO4 (s) = Calcium Sulfate
Make sure your net ionic equation is balanced.
Purpose:
In this lab you will observe double displacement (precipitation) reactions by mixing aqueous solutions of
various ionic compounds. You will identify reactants, write proper formulas for products, balance chemical
equations, write total and net ionic equations and, identify precipitates.
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Procedure:
1.
Collect the following: A piece of Saran Wrap and the following solutions: “M” means molarity, a unit
of concentration giving the number of moles of solute (the compound) dissolved in 1000 ml of water:
These solutions have 0.1 moles of solute (the compound) dissolved in 1.0 liters of water for a
concentration of 0.1 M
0.1 M Na3PO4
0.1 M NaNO3
0.1 M BaCl2
0.1 M Pb(NO3)2
0.1 M K2CO3
0.1 M MgSO4
0.2 M AgNO3
2.
In this lab, you will need to test each possible combination of solutions. To do this, first make a data
table in your notebook that has a place for every combination of the provided compounds that can be
mixed. When you are finished with the experiment you should have fifteen entries in your data table.
A sample table is provided.
3.
Spread the saran wrap out on the BLACK table top. This makes most of the precipitates easier too see.
4.
Using the pipettes provided, carefully place two drops of the indicated solutions on the surface of the
saran wrap. Keep track of what you have placed and where. DO NOT TOUCH THE TIP OF THE
PIPETTE TO ANY SURFACE.
5.
Mix each combination as noted in the data table. Again, two drops will do. Observe and record your
observations in your data table. If you don't see a reaction, indicate "No reaction". If a precipitate
forms, write "precipitate", and give a description of the precipitate and its color.
6.
When you are sure you have investigated all the possible combinations of solutions, clean up by
gathering up your saran wrap and placing it in the trash. If any of the chemicals were spilled onto the
desktop, wipe it clean with a wet paper towel. Wash your hands is necessary.
Data and Observations:
Your Data Table with descriptions goes here. There is a LARGE table on the web-site for your use.
From your first data table identify the combinations of solutions that formed a precipitate. Now for ONLY
those combinations that formed a precipitate: Pick 5 different reactions and complete the following:
•
•
•
•
•
•
Identify the Reactants
Write out a chemical reaction in words
Write out a balanced chemical equation using the proper chemical formulas.
Remember to use the state of matter designations.
Write out the total ionic equation. Make sure it is balanced.
Write out the net ionic equation. Make sure it is balanced.
Identify the Precipitate
*Use the General Rules for Solubility provided for you on your Useful Information sheet to determine
which product is the precipitate that forms. One EXAMPLE has been done for you.
Error Analysis: For your error analysis, refer back to your original data table, and the solubility rules
listed below.
•
•
Are there any combinations of solutions that should have formed a precipitate and did not?
Are there any combinations of solutions that formed a precipitate and shouldn't have?
Regardless, discuss 2 possible sources of error and how the errors may have affected your results.
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Post Lab - Questions: Answer the following Questions based on your lab data.
1.
2.
3.
4.
5.
6.
7.
8.
9.
Define, in your own words, the following terms which were discussed in this lab:
(a) aqueous solution
(b) precipitate
(c) dissociation
(d) ionic compound
Which cations commonly formed precipitates?
What are the sign(s) that a double displacement reaction has occurred?
How could you determine if a double replacement reaction occurs without actually doing the reaction?
Which solutions produced the fewest precipitates? Salts of these anions may be described as soluble
with only a few exceptions.Name the cations that are the exceptions to the rule for each anion identified.
Which solutions produced the most precipitates? Salts of these anions may be described as insoluble
with only a few exceptions. Name the cations that are exceptions to the rule for each anion identified.
What might account for any differences between your observations and those predictions based on the
solubility rules?
Although we did not test the bromide (Br1-) anion, would you expect most bromide salts (AlBr3, NH4Br,
BaBr2, etc.) to be soluble or insoluble? What is the likely exception to this rule? Hint: Use the periodic
table to see which of the anions you tested would behave like bromide.
Using four criteria for double displacement reactions, together with the rules of solubility, predict
whether a double displacement reaction will occur in each example below. If reaction will occur,
complete and balance the overall equation, properly indicating all conditions. If you believe no reaction
will occur, write “no reaction” on the right-hand side of the equation.
(a.) K2S (aq) + CuSO4 (aq) →
(b.) KOH (aq) + NH4Cl (aq) →
(c.) (NH4)2SO4 (aq) + NaCl (aq) →
(d.) CoCl3 (aq) + NaOH (aq) →
(e.) Na2CO3 (aq) + Sr(NO3)2 (aq) →
10. Complete the following table to summarize the general solubility rules of ionic compounds.
Ionic Compounds
Carbonates
Halogen Ions
Hydroxides
Nitrates
Phosphates
Sulfates
Alkali Metal Ions
Ammonium Salts
Illustration:
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A neat and relevant sketch
Soluble / Insoluble
Exceptions
AgNO3
Data Table 1
Na3PO4
K2CO3
NaNO3
MgSO4
Pb(NO3) 2
BaCl2
AgNO3
Na3PO4
K2CO3
NaNO3
MgSO4
EXAMPLE: Analyzing a Precipitate Reaction
Identify the
Reactants:
Word
Equation
Balanced
equation - with
conditions
Total Ionic
Equation
Al2(SO4)3 = Aluminum Sulfate
Ba(NO3)2 = Barium Nitrate
Aluminum Sulfate + Barium Nitrate →
1Al2(SO4)3 (aq)
2Al3+
Net Ionic
Equation
Identify the
precipitate
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+
3SO42-
+
+
3Ba(NO3)2(aq)
3Ba2+ +
3Ba2+
6NO31-
+
Aluminum Nitrate + Barium Sulfate
→
2Al(NO3)3( (aq)
→
2Al3+
3SO42-
→
+
3BaSO4
BaSO4 = Barium Sulfate
+
6NO31-
(s)
3BaSO4
+
(s)
3BaSO4 (s)