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Double-replacement Reactions
ABSTRACT: In this lab double-replacement reactions were utilized to observe forming
precipitates and to balance equations of newly formed solutions. Precipitates were
found by combining a solution containing cations and anions to another solution of
cations and anions. The double-replacement reactions were calculated using basic
mathematic knowledge about balancing equations. Seven of the twelve reactions
yielded a precipitate; all reactions were successfully balanced.
INTRODUCTION: There are five main reactions in chemistry that describe the way
atoms bond with one another: direct combination (synthesis), combustion,
decomposition, single-replacement, and double-replacement. The atoms are found in a
gas, liquid, solid, or aqueous state (dissolves in water). Everything in our world is result
of chemical reactions.
This lab will explore double-replacement reactions, the combination of
atoms/ions reactants that form completely different products. (ex: AC +BD  AD + BC).
A double replacement takes places between a minimum of two cations and two anions
on the reactant side. These ions produce a minimum of two cations and two anions on
the product side. Different sodium based solutions, anions, will combine with cations to
produce or not produce precipitates.
PURPOSE: The purpose of this lab is to observe the double-replacement reaction of
aqueous solutions and ionic compounds and determine their balanced equations by
examining which solutions result in a precipitate.
PROCEDURE: The well plate was labeled according to Figure 27-1. Using a micropipet,
five drops of silver nitrate were dropped onto wells A1-A4. Five drops of iron (III) nitrate
were dropped onto wells B1-B4. Five drops of copper (II) nitrate were dropped onto
wells C1-C4. Five drops of sodium phosphate were added to each of the solutions in
wells A1, B1, and C1. All observations of occurring precipitates were recorded in Data
Table 1, along with the color of the precipitate. If a precipitate did not form then NR was
written, for not reaction. Five drops of sodium sulfate were added to A2, B2, and C2.
Observations were recorded in the same manner as column 1. Five drops of sodium
hydroxide were added to A3, B3, and C3. Observations were recorded. Five drops of
sodium chloride were added to A4, B4, and C4. Observations were recorded. After all
results were concluded, the solutions in row A were pulled up into a dropper and
properly disposed of per instructions set by the teacher. The sodium hydroxide was
washed down with large quantities of water. All material were cleaned and put away
properly to avoid future cross-contamination.
RESULTS: There were both qualitative and quantitative results that occurred in this lab.
The results directly gathered from experimentation were qualitative. These results
included the formation of a precipitate and any color changes that may have occurred.
The quantitative results were determined from the qualitative results that were gathered.
These results were the balanced equations that resulted from the combination of
compounds.
1
Data Table: Qualitative Results
2
3
4
Na+
PO43Yellow, liquidy,
YES precipitate
Na+
SO42NO reaction
A
Ionic
Solutions
Ag+
NO3-
Na+
OHBrown, liquidy,
YES precipitate
Na+
ClWhite liquidy
YES precipitate
White, liquidy,
YES precipitate
C
Cu2+
NO3-
White, solid,
YES precipitate
Orange ring
on outside,
NO precipitate
Lighter blue
w/a ring on
outside,
NO precipitate
Dark orange,
solid,
YES precipitate
Light teal,
liquidy,
YES precipitate
NO reaction
B
Fe3+
NO3-
NO reaction
Balanced Equations: Quantitative Results
A1:
A2:
A3:
A4:
3AgNO3 (aq) + Na3PO4 (aq)  Ag3PO4 (s) + 3NaNO3 (aq)
2AgNo3 (aq) + Na2SO4 (aq)  2NaNo3 (aq) + Ag2SO4 (aq)
AgNO3 (aq) + NaOH (aq)  AgOH (s) + NaNO3 (aq)
AgNo3 (aq) + NaCl (aq)  NaNo3 (aq) + AgCl (s)
B1:
B2:
B3:
B4:
Fe(NO3)3 (aq) + Na3PO4 (aq)  FePO4 (s) + 3NaNO3 (aq)
2Fe(NO3)3 (aq) + 3Na2SO4 (aq)  6NaNO3 (aq) + Fe2(SO4)3 (aq)
Fe(NO3)3 (aq) + 3NaOH (aq)  3NaNO3 (aq) + FeOH3 (s)
Fe(NO3)3 (aq) + 3NaCl (aq)  3NaNO3 (aq) + FeCl3 (aq)
C1:
C2:
C3:
C4:
3Cu(NO3)2 (aq) + 2Na3PO4 (aq) +  6NaNO3 (aq) + Cu3(PO4)2 (s)
Cu(NO3)2 (aq) + Na2SO4 (aq) +  2NaNO3 (aq) + CuSO4 (aq)
Cu(NO3)2 (aq) + 2NaOH (aq) +  2NaNO3 (aq) + CuOH2 (s)
Cu(NO3)2 (aq) + 2NaCl (aq) +  2NaNO3 (aq) + CuCl2 (aq)
CONCLUSION: The purpose of this lab was to observe a plethora of combined
solutions to see which would result in a precipitate, as well as to write balanced
equations for said reactions. This lab was successful, because the purpose was
accomplished. However, a few results varied from the predetermined results but only in
color, not in precipitates. This affected the results in precipitates but did not affect the
balanced equations. The results were as I expected because I have no substantial
knowledge about what solutions form precipitates when combined. The balanced
equations results were what I expected because I’ve had a lot of practice with balancing
equations.
The compounds that are soluble in water include AgNO3, Na2SO4, Fe(NO3)3,
Cu(NO3)2, and NaCl. These solutions are soluble because when mixed with one or more
solution they did not form a precipitate. Because the precipitate is a solid, it will not
dissolve in water. Based on my observations, Ag+ forms the greatest number of
precipitates making it the least soluble. I believe that the results were both valid and
precise, as well as accurate.
DISCUSSION: One error that did occur in the lab was cross contamination. Solutions
C1 was supposed to form a blue precipitate, but my partner and I observed a white
precipitate. The most likely reason for error is unclean eye droppers. A person may
have cross contaminated the eyedropper for the sodium phosphate solutions by
touched the dropper to the iron nitrate solution and continue to put the contaminated
solution into one of the wells. This would cause the white colored precipitate instead of
the blue colored precipitate, as it did in our observations versus the intended results. My
confidence in the overall lab has not swayed. A precipitate still formed, which was the
purpose of the lab, even though the color slightly differed.
This lab helped to enhance knowledge about double-replacement reactions. A
double-replacement reaction can only take place when there are an equal number of
cations and anions on the reactant and product side. All of the solutions studied in this
experiment were double-replacement reactions. An example of a reaction that would not
work would be sodium hydrogen carbonate reacts with sodium chloride to form a new
substance. Because one of the reactants has three elements and the other only has
two, it is an exception to the rule and cannot be solved using double-replacement.
There were a small number of errors that occurred in this lab, which were all due
to cross contamination and/or human error when observing. The purpose of this lab was
fully completed. The relationship that this experiment has with the current topic being
studied in Chemistry is strong because balancing equations are being taught as well as
the different types of chemical reactions and how to classify them. Practicing these skills
in an informative, yet fun way, helps to build confidence in the learner.
A
1
Yellow precipitate
B
White precipitate
C
Blue precipitate
Intended Results
2
3
Possible white
Brown precipitate
precipitate OR
no reaction
No reaction
Red/orange
precipitate
No reaction
Blue precipitate
4
White
precipitate
No reaction
No reaction