Minneapolis Community Ionic Precipitation Reactions in Aqueous Solutions Essay

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Minneapolis Community and Technical College
Introductory Chemistry Laboratory
Experiment: Ionic Precipitation Reactions in Aqueous Solutions
Objectives:
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Identify the ions present in various aqueous solutions.
Systematically combine solutions and identify the reactions that form precipitates.
For the reactions that involve a precipitate, use solubility rules to identify the insoluble product.
Write the correct chemical formulas for precipitates formed.
Write a balanced chemical equation.
Write a net ionic equation (for reactions that formed precipitate).
Text references:
aqueous, dissociation, cation, anion, polyatomic ion, soluble, precipitate, electrolyte, solution, solvent,
solute.
Discussion:
Aqueous Solutions:
When one substance dissolves in another substance, a solution is formed. A solution is a homogeneous
mixture in which the components are uniformly mixed. A solution consists of solute (the species that
is dissolved) and solvent (the medium in which the solute has dissolved). The solvent is usually present
in larger amount than the solute. When water is the solvent, the solution is called aqueous solution.
When an ionic compound dissolves in water, it dissociates into its constituent ions. Such a compound
is a strong electrolyte and conducts electricity well in dilute aqueous solutions. For example, when
NaCl dissolves in water, it dissociates into separate Na+ and Cl- ions.
This process occurs as polar water molecules orient themselves
around the sodium and chloride ions and pull them free from
the solid crystal. Once removed from the solid crystal, the ions
remain separated and surrounded by water molecules.
Therefore, the solution now consists of mostly water, and
sodium and chloride ions. For all practical purposes, there are
no undissociated NaCl units floating around.
(source of diagram:
https://www.khanacademy.org/science/chemistry/chemical-reactionsstoichiome/types-of-chemical-reactions/a/complete-ionic-and-net-ionic-equations)
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Precipitation Reactions: A chemical reaction that involves the formation of an insoluble product
(precipitate; solid) is called a Precipitation Reaction. The reactants are soluble, but the product formed
would be insoluble and separates out as a solid.
In today’s experiment, we will be combining different aqueous solutions of ionic compounds and
observing them for signs of a precipitation reaction. In cases where a reaction occurs, an insoluble
solid product (precipitate) will be formed. In precipitation reactions, a color change alone does not
indicate a reaction has occurred. A solid must be formed.
Let us consider the possible reaction of aqueous solution of NaCl with aqueous solution of AgNO3. We
would place a few drops of the NaCl solution in the reaction container followed by a few drops of
AgNO3 solution and observe an immediate cloudiness that indicates a solid precipitate has formed. Ah
hah! A precipitation chemical reaction has occurred!
To determine the identity of the solid product formed:
In order to determine the possible identity of the solid product that forms, we first identify the ions
present in each of the two aqueous solutions we started with: Na+ & Cl- (from NaCl) and Ag+ & NO3(from AgNO3).
Next, we examine the ions for possible new combinations that may lead to a reasonable product
formula.
Several guidelines help here. First, the combination of ions (NaCl, AgNO3) that existed in solution prior
to the experiment had been soluble and therefore should remain as such without separating out as solid
after the reaction. This allows us to eliminate combinations like NaCl and AgNO3 from the list of
possibilities.
Second, the ions that have the same charge won’t combine and in fact will repel one another. Thus,
NaAg and ClNO3 are eliminated.
This leaves us with only two other possibilities, AgCl and NaNO3. From the knowledge of Solubility
Rules (listed below), we can determine which of these two products is insoluble. Solubility Rule #1
indicates that nitrate salts are soluble. Therefore, NaNO3 cannot be the precipitate in this reaction.
Solubility Rule #3 states that most chloride salts are soluble. HOWEVER, AgCl is listed as an
exception to this rule. In this case, it is AgCl which is the precipitate.
Solubility Rules:
1. Most nitrate and acetate compounds are soluble.
2. Most compounds of Li+, Na+, K+, and NH4+ are soluble.
3. Most chloride compounds are soluble. The important exceptions are the ones containing Ag+
and Pb2+.
4. Most sulfate compounds are soluble. The important exceptions are the ones containing Ba2+,
Pb2+, Sr2+, and Ca2+.
5. Most hydroxide and sulfide compounds are insoluble. The important exceptions are the ones
containing Li+, Na+, K+, NH4+, Ca2+, Sr2+ and Ba2+.
6. Most carbonate and phosphate compounds are insoluble except for the ones containing cations
listed in rule #2.
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Chemical Equations representing the Precipitation Reactions:
Once the chemical identity of the solid product is determined, we can then determine the balanced
molecular equation, the complete ionic equation as well as the net ionic equation, describing the
chemistry that has occurred.
a) The balanced molecular equation for the reaction of aqueous AgNO3 with NaCl is written as:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
Note that in the above equation, the physical state of the AgCl product is denoted by the letter “s”, to
indicate that it is a solid, the precipitate. The number of atoms of each element is same before and after
the reaction, indicating that the equation is balanced.
b) The complete ionic equation, indicates which reactants and products exist as ions and which ones
do not:
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl- (aq) → AgCl(s) + Na+(aq) + NO3- (aq)
The ions that actually undergo change in the chemical reaction and participate in the formation of the
insoluble product are called participating ions. In the above reaction, Ag+ and Cl- are the participating
ions. Those that do not undergo change are called spectator ions. In the above reaction, Na+ and NO3are the spectator ions.
c) The net ionic equation displays only the participating ions on the reactant side, and the precipitate
on the product side. The physical states of the reactants and products are also indicated. The spectator
ions are not included.
Ag+(aq) + Cl-(aq) → AgCl(s)
Safety Warning! Several of the chemicals (salts of heavy metals like strontium) used in today’s
experiment can be harmful if spilled on the skin. Should spills occur be sure to notify the lab instructor
and immediately begin rinsing the affected area with water. Be sure to wash your hands before leaving
the lab.
Aqueous Solutions
The aqueous solutions you will be using today are provided in reagent bottles. Each bottle is labeled
with the name and concentration of the reactant (reagent) that it contains. For example,
Sodium Chloride
1.0M
The information 1.0M in the second line of the label, refers to the concentration of the solution, where
M stands for Molarity. The sodium chloride solution was prepared prior to this experiment by dissolving
an appropriate amount of solid sodium chloride (the solute) in a carefully measured amount of water
(the solvent) to get 1.0M concentration
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Always watch carefully the concentration of the solution you are using because there can be more than
one concentration of a particular aqueous solution available. Using the wrong concentration can have
surprising, unexpected and sometimes dangerous consequences.
LAB PROCEDURE:
Watch the video (https://mediaspace.minnstate.edu/edit/1_qkcwdwpq) and record the observations.
1. Obtain a pair of the required solutions and a clean test tube. Add 0.5 – 1 mL of the first solution,
then about equal amount of the second solution into the same test tube. Mix them well.
2. Observe the contents of the test tube. A precipitation reaction is said to have occurred if the
solution turns cloudy (that is, a precipitate forms). In other words, if you do NOT observe the
cloudiness or any forms of solid, there is NO precipitation reaction. Record your observations
on the data sheets (pages 5 – 8). If a reaction occurs, note the color of the solid formed as well
as any other observations. Write “Clear” if no precipitation is observed. Check the solubility
rules to make sure your results agree with your solubility rules.
3. Use a clean and dry test tube and repeat the steps 1 & 2 for each of the 7 reactions. Be very
careful not to mix up droppers as this would lead to contamination of the dropper bottles.
While adding the drops of chemicals to the test tubes, do not touch the dropper tips to the
inside of the test tubes.
4. Work on the names of the products, balanced molecular equations and net ionic equations. For
the ones that didn’t not react, still write out the names of the products, balanced molecular
equation. However, there will be no net ionic equations.
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Ionic Precipitation Reactions in Aqueous Solutions
Name_______________________________
Date___________
Solubility Rules
1. Most nitrate and acetate compounds are soluble.
2. Most compounds of Li+, Na+, K+, and NH4+ are soluble.
3. Most chloride compounds are soluble. The important exceptions are the ones
containing Ag+ and Pb2+.
4. Most sulfate compounds are soluble. The important exceptions are the ones
containing Ba2+, Pb2+, Sr2+, and Ca2+.
5. Most hydroxide and sulfide compounds are insoluble. The important exceptions are
the ones containing Li+, Na+, K+, NH4+, Ca2+, Sr2+ and Ba2+.
6. Most carbonate and phosphate compounds are insoluble except for the ones
containing cations listed in rule #2.
Watch the video and record the observations.
(https://mediaspace.minnstate.edu/media/ionic%20precipitation%20video/1_qkcwdwpq)
1. Add calcium nitrate and potassium chloride solutions to test tube #1; mix well.
1) Observation:
2) Complete the names of the products for the following equation indicating which
product is the precipitate (if there is a precipitate).
calcium nitrate(aq) + potassium chloride(aq) →
_________________ + _________________
3) Balanced Molecular Equation:
____________ + ___________ → ____________ + ____________
4) Net Ionic Equation:
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____________ + ___________ → ____________
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2. Add strontium chloride and aluminum sulfate solutions to test tube #2; mix well.
1) Observation:
2) Complete the names of the products for the following equation indicating which
product is the precipitate (if there is a precipitate).
strontium chloride(aq) + aluminum sulfate(aq) →
__________________ + __________________
3) Balanced Molecular Equation:
____________ + ___________ → ____________ + ____________
4) Net Ionic Equation:
____________ + ___________ → ____________
3. Add iron(III) chloride and potassium hydroxide solutions to test tube #3; mix well.
1) Observation:
2) Complete the names of the products for the following equation indicating which
product is the precipitate (if there is a precipitate).
iron(III) chloride(aq) + potassium hydroxide(aq) →
_________________ + _________________
3) Balanced Molecular Equation:
____________ + ___________ → ____________ + ____________
4) Net Ionic Equation:
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____________ + ___________ → ____________
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4. Add magnesium sulfate and sodium hydroxide solutions to test tube #4; mix well.
1) Observation:
2) Complete the names of the products for the following equation indicating which
product is the precipitate (if there is a precipitate).
magnesium sulfate(aq) + sodium hydroxide(aq) →
________________ + _________________
3) Balanced Molecular Equation:
____________ + ___________ → ____________ + ____________
4) Net Ionic Equation:
____________ + ___________ → ____________
5. Add sodium chloride and silver nitrate solutions to test-tube #5; mix well.
1) Observation:
2) Complete the names of the products for the following equation indicating which
product is the precipitate (if there is a precipitate).
sodium chloride(aq) + silver nitrate(aq) →
__________________ + __________________
3) Balanced Molecular Equation:
____________ + ___________ → ____________ + ____________
4) Net Ionic Equation:
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____________ + ___________ → ____________
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6. Add ammonium carbonate and calcium chloride solutions to test tube #6 and mix
well.
1) Observation:
2) Complete the names of the products for the following equation indicating which
product is the precipitate (if there is a precipitate).
ammonium carbonate(aq) + calcium chloride(aq) →
________________ + _________________
3) Balanced Molecular Equation:
____________ + ___________ → ____________ + ____________
4) Net Ionic Equation:
____________ + ___________ → ____________
7. Add copper(II) sulfate and ammonium hydroxide solutions to test tube#7, mix well.
1) Observation:
2) Complete the names of the products for the following equation indicating which product is
the precipitate (if there is a precipitate).
copper(II) sulfate(aq) + ammonium hydroxide(aq) →
_________________ + _________________
3) Balanced Molecular Equation:
____________ + ___________ → ____________ + ____________
4) Net Ionic Equation:
____________ + ___________ → ____________
Submit pages 5-8 (in one PDF file) as the lab report by due date.
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