! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
1!
Precipitation Reactions and the Conservation of Mass
A CORE learning cycle lab experiment
!
!
During Oct. 5-8 (Tuesday-Thursday), the gen chem lab program will offer opportunities to make up experiments.
Guidelines were sent to you and are also available at www.interchemnet.com, at the bottom of the ICN News page
(Lab Make-up Guide Fall 2021). Questions? Ask your TA. The lab reports are due 72 hours after performing the lab.
Introduction
Goals:!
1. To observe a precipitation reaction.
2. To use an analogical model to think about chemical interactions and the conservation of mass in a
precipitation reaction.
3. To design an experiment that evaluates the conservation of mass concept.
!
The Law of Conservation of Mass
We know from data from many experiments, that atoms are conserved in physical or chemical
transformations. This experiment will help us explore this concept. The law of conservation of matter states that
the number of each type of atom is the same before a chemical reaction as after a chemical reaction. Each atom
has a particular mass associated with it and thus, mass is conserved in a chemical reaction. In other words, the
total mass of the reactants is equal to the total mass of the products.
The conservation of mass seems straight forward, but before atomic theory, the idea that mass is conserved in
chemical reactions was not obvious to experimentalists. Different types of chemical reactions, when observed, can
appear as if they have created or destroyed matter. Consider a combustion reaction, where a block of wood (made
up of a variety of compounds containing hydrogen, oxygen, and carbon atoms) reacts with oxygen. After the wood
burns, the resulting pile of ash is lighter than the block of wood. However, in a combustion reaction, atoms in the
wood are rearranged to form water (H
2
O) and carbon dioxide (CO
2
) which are released as gases into the
atmosphere. All of the atoms in the original block of wood still exist.
Precipitation reactions can also be misleading. In a precipitation reaction, two solutions containing soluble ions
(which are invisible to the eye in the solution) are mixed and react to form a new compound that is no longer soluble.
The insoluble compound, called the precipitate, is a solid that settles out of the solution. It may appear as if matter
has been created from nowhere. Familiarity with precipitation reactions will help you understand the next
experiment in this two-lab sequence about limiting reactants.
In this experiment, you will study two precipitation reactions and explore an analogical model for the reaction
to help think about what is occurring on the atomic scale.
The CORE Learning Cycle
Phase 1: Following a procedure, you and a partner in lab will make chemical observations.
Phase 2: You will review materials that develop an explanation about what you observed in lab using an
analogical model of the chemical reaction. Modeling is an important skill to help develop insight.
Phase 3: Before going back into the lab, you will be asked to apply your understanding by designing an
experiment. You and your partner will then go back into the lab to conduct your experiments and gather
experimental results.
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
2!
An analogical model
We will use nuts and bolts and assign them to different types of ions and molecules to model the
precipitation reaction. This analogy is useful for helping us to “visualize” what happens to all of the
chemical species (atoms, ions, compounds, etc.) present in a solution. In a reaction, when bonds break
and new ones form, it is important to have a way to keep track of all of the chemical species. Nuts, bolts,
and nails come in different sizes. In a box of nuts, bolts, and nails, some combinations “fit” together
securely while others do not. In a similar way, in a collection of chemical species in solution, some
combinations may readily interact, while other combinations do not.
In the analogy portion of this lab you will explore how various combinations of nuts, bolts and nails
connect, and relate what you learn from the analogy to the behavior of different chemical species in the
precipitation reaction.
!
Pre-lab Assignment
In your lab notebook, please prepare the following information and answer the questions. You must
complete the pre-lab before coming to the lab meeting or you will not be permitted to go into the
laboratory.
1. Write a 2-3 sentence introduction to the lab.
2. Create a safety information table including the chemicals used in the lab, the hazards
associated with them, and any safety handling precautions. (See example safety table on the
Student Resource Page on ICN.)
3. Consider a chemical reaction between 5 Zn
2+
(zinc
cations in water) and 5 S
2-
(sulfide anions in water) to
form 5 ZnS (solid zinc sulfide). Write a balanced chemical
equation for this reaction.
4. Now imagine that you replace Zn
2+
with a nut and S
2-
with
a bolt and draw a representation for the reaction described
above. Is this a type of analogy? Explain by annotating your
drawing.!
!
Laboratory Guide
On the following pages, you will find instructions for doing an experiment. In this experiment, pair
up with another student when doing lab work. If there is an odd number of participants in lab, one
group may have three people.
Your lab work will involve individually making observations and recording these in your own lab
notebook, and working in partnership on certain activities such as: answering questions, discussing
observations, analyzing results, and designing procedures in response to scientific questions.
As you go through the experimental guide, you will notice there are questions that are set off in the
guide (i.e. “Q:”). For example:
Q: What are the physical/observable characteristics of each of these solutions?
To develop best practices, we ask that you respond to every question in your lab notebook.
Our expectation is that you write enough to give an indication of your thinking. You do not have to write
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
3!
the question AND answer, but you must address the answer, for example: “we found that both solutions
were colorless and of similar viscosity….”
Part of the purpose of making an entry in your notebook is to allow you to remember later what you
were thinking at this point in the lab, which can be very useful when writing your lab report. It also
offers evidence of your thinking process.
Safety Suggestions
Goggles are required at all times in the lab. If you are outside lab and do not have goggles, obtain
goggles before entering he lab. There are no exceptions. Gloves and aprons are available. If you have
questions about safety, please do not hesitate to ask your laboratory instructor.
Note: This lab has portions of the procedure that must be completed in a chemical fume hood (e.g. see
phase 3).
!
Equipment*and*supplies:* *
12.5!cm!filter!paper!
Long-stem!funnel!
Ring!stand!and!ring!clamp!!
Acetone!wash!bottle!
!
!
!
!
!
!
Phase 1: Making observations
The following activities should be completed in the lab.
1. Obtain 0.1 M NaCl and 0.1 M Ag(NO
3
) solutions.
Q: What are the physical/observable characteristics of each of these solutions?
2. In an Erlenmeyer flask, mix together approximately 10 mL of 0.1 M NaCl solution and 10 mL of
0.1 M Ag(NO
3
) solution. Record the exact volumes in your lab notebook. Carefully swirl the flask to mix
the solutions.
Q: What happens when the solutions are mixed? Is there evidence of a precipitate forming?
This is the end of Phase 1.
Chemicals:*
silver!nitrate!solution!:!Ag(NO
3
)!
sodium!chloride!solution:!NaCl!
calcium!chloride!solution:!CaCl
2#
acetone!
!"#$%&'()%*+##(,
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
4!
Phase 2: Exploring the Analogy
The following activities should be completed in the breakout room.
Thinking about balanced chemical equations
A balanced chemical equation has a lot of information encoded into it. Consider the example below:
!
!
!
! !
! Ultimately, the balanced chemical equation gives you a recipe for
a reaction using ratios of moles of reactants. In the example above, it
takes two moles of sodium atoms and one mole of chlorine (Cl
2
)
molecules to create two moles of sodium chloride. The molar ratio
of sodium metal to chlorine molecules is 2:1. The molar ratio of
sodium to sodium chloride in the reaction is 1:1.
The ratio in the recipe will still hold for a much smaller number
of atoms and molecules. For example, in the reaction above, we can
also say that 2 Na atoms will react with 1 Cl
2
molecule to create 2
NaCl. The picture below represents this approach:
!
!
In summary, chemical equations provide a lot of information, such as the general ratio of the
amount of each reactant needed to create a certain amount of a particular product. Molar ratios
become useful in predicting the mass of product that can be expected in a chemical reaction.
2Na
(s)
+ Cl
2(g)
2NaCl
(s)
+
Careful(thought(is(also(needed(to(
interpret(formula s.(A s (a n (io nic(
compound,(NaCl(does(not(exist(as(
single(molecules(of("NaCl"(but(in((
a(3-dimensional(lattice(
containing(many("NaCl"(units.(
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
5!
The precipitation reaction
Now let’s take a look at the chemical equation for the precipitation reaction you performed in Phase 1.
!
!
!
Consider all of the information that you can extract from the balanced chemical equation above.
Q: What is the precipitate that is formed?
Q: What are the molar ratios of the reactants to each other? What is the molar ratio of each
reactant to the product silver chloride?
Because we are dealing with ionic compounds in the reaction shown above, we also can write the
chemical equation a different way:
!
!
!
Analogical model for the precipitation reaction
Now you are going to construct an analogical model for the reaction of sodium chloride and silver
nitrate that results in the formation of the precipitate silver chloride and aqueous sodium nitrate. In
front of you should be a variety of hardware. There are nails, bolts, and two different sizes of nuts:
(
!
!
!
!
NaCl
(aq)
+ Ag(NO
3
)
(aq)
Na(NO
3
)
(aq)
+ AgCl
(s)
!"#$%"#&'%()$#*+(,'-.$-/$,"#$0#(%,'-.$-/$1-2'+&$%")-0'2#$(.2$1')3#0$.',0(,#$,-$
40-2+%#$1-2'+&$.',0(,#$(.2$1')3#0$%")-0'2#5
!"#$%&''()(*+$
,&-(,$#'$*.+,$
/#0+,$
12&0,
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
6!
1. Assign the types of hardware to the different chemical species involved in this reaction.
In this analogical model, we will use nuts to represent the cationic species, and bolts or nails to
represent the anionic species. If a nut fits tightly with a bolt, that will represent formation of a
precipitate. If a nut slides loosely over a bolt or nail, that will represent ionic species that stay in solution.
We will give you one assignment: the chloride anion is assigned as the bolt (see picture below).
As you assign the other hardware, consider that some combinations will fit together more tightly than
other combinations. The chemical interaction between sodium cations and nitrate anions in an aqueous
solution is weak compared to the chemical interaction between silver ions and chloride ions. Make sure
your hardware assignments reflect this.
!
!
Record the assignments of your hardware in your notebook. Show them to your TA. There is only one
combination of the hardware representing Ag
+
+ Cl
-
which results in a tight connection to represent
AgCl(s). If you have any questions about your assignment, please discuss your assignments with your
TA before proceeding with the analogy.
2. Now use the hardware to create a representation of 3 sodium cations and 3 chloride anions in a
solution. Then, create a representation of 3 silver cations and 3 nitrate anions in a separate solution.
These are your reactants.
3. Next, mix the hardware representations of aqueous sodium chloride and aqueous silver nitrate to
simulate the reaction by rearranging the reactants to form the hardware representation of the
precipitate silver chloride and aqueous sodium and nitrate ions. Carefully keep track of all the hardware.
4. You should fill out the Analog and Target worksheet individually on pages 7 and 8 (remember, you
will each need to scan a legible copy of the worksheet and submit it with your lab report electronically).
As you fill out the worksheet, you may discuss what you're thinking with your partner, if this helps you
think about the chemistry at the atomic scale.
Extra Credit - Extra Challenge
If this analogy seems simplistic to you, consider extending the analogy for extra credit. This might
involve thinking about additional aspects at the atomic scale and relating them back to the analog. For
example, you might consider how you would modify the analogy to model the precipitation of silver
sulfide, Ag
2
S (2 Ag
+
cations combining with 1 S
2-
anion) or an analogy that includes solvent (water).
To receive extra credit, describe your modified analogy in your lab report in a separate section called
Extra Credit, Extra Challenge. In this section, you should identify how the modified analogy strengthened
your understanding or insight of the chemical phenomena under examination. You should also identify
explicitly where in the Analog and Target worksheet these modifications revealed themselves. Extra
credit will be worth up to 10% on your entire grade for the experiment.!!
When you have completed the worksheet, you have finished Phase 2.
Na
+
(aq)
+ Cl
-
(aq)
+ Ag
+
(aq)
+ (NO
3
)
-
(aq)
Na
+
(aq)
+ (NO
3
)
-
(aq)
+ AgCl
(s)
?
?
?
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
7!
Analog and Target Worksheet (pages 7-8) Name: ___________________
Fill out this worksheet individually as you perform the analogical reasoning activity. Work together to discuss similarities and differences. Each student must
include a scanned copy of this sheet with their lab report. Make sure your scan is entirely legible. Please label the components in your drawings.
Assignments: Bolt = _____________________ Nail = ___________________ Large nut = ___________________ Small nut = ___________________
Analog and Target
Comparison
Sodium and chloride ions in
an aqueous solution
compared to your hardware
assignments for these species
Silver and nitrate ions in an
aqueous solution compared
to your hardware
assignments for each of these
species
The action of rearranging
the hardware compared to
the chemical reaction
Silver chloride and sodium
nitrate in an aqueous solution
compared to the hardware
products of the reaction, taking
into account the conservation
of mass
Similarities: What
characteristics does the
analog (nuts and bolts
model) share with the
target (ions in solution,
precipitate)?
Differences: What
features of the nuts and
bolts model (analog) are
not representative of the
ions in solution and the
precipitate (target)?
Differences: Now think
about the target (ions in
solution and precipitate)
and describe the features
that are missing in the
analog (nuts and bolts
model).
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
8!
!
!
Analog and Target Worksheet Continued (pages 7-8) Name:
________________________!
Draw
representations of
the following
situations at the
molecular level to
model the
precipitation
reaction and the
conservation of
mass.
Remember that
there are many
water molecules
surrounding each
ion in solution.
Sodium chloride in an aqueous
solution (don’t forget the
water molecules):
Silver nitrate in an aqueous
solution (don’t forget the
water molecules):
The chemical reaction (don’t
forget the water molecules):
The products of the reaction in
an aqueous solution (don’t
forget the water molecules):
!
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
9!
Phase 3: Designing experiments
Plan your experiments in the breakout room before proceeding to lab to complete them.
Using the analogical model to make predictions
Q: Why is mass conserved in chemical reactions? Explain the law of conservation of mass at the
molecular level. Use the analogical model to justify your answer.
The experiment
Design an experiment to explore the conservation of mass in a different precipitation reaction
between calcium chloride and silver nitrate and answer the following scientific question:
Scientific Question: Is the number of moles of silver chloride produced in the chemical reaction equal to
the number of moles of silver cations and the number of moles of chloride anions in the reactants?
The balanced chemical equation for the reaction of calcium chloride and silver nitrate is shown below.
Please note the difference in charges for some of the ions compared to the sodium chloride and silver
nitrate reaction you performed in phase 1.
!
!
! !
Q: Why is it necessary to start your reaction with twice as many moles of silver nitrate (Ag(NO
3
))
as moles of calcium chloride (CaCl
2
)?
In the lab, you will have the following solutions to work with:
0.1M Ag(NO
3
) 0.1M CaCl
2
0.2M Ag(NO
3
) 0.2M CaCl
2
Use the Designing Experiments worksheet (page 11) to summarize this process. You will need to make
a copy of this to submit with your lab report. Make sure to also take careful notes of the process in
your laboratory notebook. Make sure that you check in with your laboratory instructor before starting
your experiment.
Below are some useful techniques and sample calculations to help you plan your experiment.
Separating the precipitate
A technique that will be useful in Phase 3 is called gravity filtration. Gravity filtration involves using
filter paper to separate the insoluble solids (the precipitate) from the liquid to obtain a clear solution
(the filtrate) in a new beaker.
Obtain a piece of 12.5 cm filter paper. Fold the circle in half to form a semicircle. Then fold the
semicircle in half. Open it into a cone and tear off a small piece of the upper, outside corner. This will
improve the seal between the filter paper and the
funnel. Weigh the filter paper cone (to the nearest
0.001 grams) and record the weight in your lab
notebook. The figure to the right demonstrates the
folding of filter paper.
Place the folded filter paper in a long-stem funnel that is attached to a ring stand. Place a beaker
underneath to collect the filtrate. Pour liquid from the reaction into the funnel to wet the filter paper.
Then, slowly pour the rest of the reaction through the funnel. The solids (i.e. precipitate) will be
trapped in the filter paper and the liquid (i.e. filtrate) will be collected in a beaker.
!
CaCl
2(aq)
+ 2Ag(NO
3
)
(aq)
Ca(NO
3
)
2(aq)
+ 2AgCl
(s)
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
10!
!
Drying the precipitate
When you have obtained a precipitate in the filter paper, it will still be wet.
The water molecules will add mass, so you need to remove them. In a chemical
fume hood, place a clean, dry beaker under the funnel stem (supported in a
ring clamp on a ring stand) and rinse the precipitate on the filter paper with
acetone using an acetone wash bottle. Transfer the filter paper with the
precipitate to a watch glass and then follow your TA’s instructions for drying the
precipitate. Acetone is used because it dissolves the water but it does not dissolve the precipitate.
When the precipitate is dry, weigh the filter paper and precipitate and record the weight in your lab
notebook. Note: When you are finished with this process, transfer the acetone washings to the correctly
labeled waste container.
!
!
Calculating moles from molarity
The molarity of a solution tells you how many moles of solute, e.g. CaCl
2,
are
in a liter of that solution. Molarity is a measure of concentration. For example,
if the bottle says 0.1M on it, that means there would be 0.1 moles of CaCl
2
in a
liter of that solution.
To figure out how many moles of CaCl
2
are in 10 mL of a 0.1M solution,
here’s what you can do: remember that 10 mL = 0.010 L and therefore:
!
"#"$"%&'()*+
,
-
"#$
!"#$%
&'($)
.
/ "#""$%012)+%3432
*
!
What volume of solution should I use if I want a specific number of moles?
Say you know you want to use 0.002 moles of Ag(NO
3
) in a reaction. How much solution (what
volume) should you use? If it’s a 0.1M solution, you can calculate the volume that would contain 0.002
moles this way:
!
! !
!
"#""5%012)+
,
6
-
"#$
!"#$%
&'($)
.
/ "#"5%2'()*+%789:
+
!!or 20 mL!
!
Note: if your precipitate is not completely dry by the end of the lab period, ask your TA’s permission
to record an initial mass, write your lab report and then reweigh your precipitate next week. If your
TA agrees to this, store your precipitate in your drawer during the week to dry out. Next week you can
reweigh your precipitate and use this new value to revise your lab report. (If your TA gives you
permission to do this, he/she will give one extra day to submit your final lab report.)
Reflections & post-lab discussion (group discussion)
To be completed at the end of the laboratory session.
1. In a group, discuss how the moles of AgCl produced compare to the initial moles of Ag
+
cations and Cl
-
anions in the CaCl
2
and AgNO
3
reactants. What are some possible reasons (e.g., sources of error) for
obtaining less or more product than expected.
2. In a group, discuss how the analogical model can be used to explain how your experiment supports
the conservation of mass to someone who thinks that precipitation reactions create matter.
!"#$%&'()*))+"#,-
#&',%
!"#$%&'("))
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
11!
Designing Experiments Worksheet
Names:
Experiment #:
Signatures:
Section:
Scientific Question: Is the number of moles of silver chloride produced in the chemical reaction equal to
the number of moles of silver cations and the number of moles of chloride anions in the reactants?
Please use this sheet to summarize your lab group’s experiment and findings. Before going into lab,
have your lab instructor check and initial it. This worksheet is to be scanned and submitted as part of
your lab report. Make sure that it is entirely legible!
Please describe your
proposed
experiment.
(Check in with your
lab instructor before
performing
experiments)
Instructor’s
initials:________
(attach extra pages if needed)
After you perform the experiment, please summarize the success or failure of your experiment and
any claims you think you will be able to make in your lab report.
! Fall!2021!–!Copyright!Mitchell!Bruce!and!the!ICN!Team! !
12!
Rubric'for'lab'report'(due'next'lab'meeting)'(Use'the'Gen'Chem'lab'report'template)!
Introduction:++
(10+pts)+
This section provides a short introduction or summary to the report, and should include:
•
Title of the report
•
Author name (your name)
•
Lab partner name (if you had partner(s))
•
Date of experiment
•
Overview: a few sentences describing what the lab experiment was about
We#also#recommend#including#your#section#and#TA#name#(optional)
#!
Data,+Results,+
Evidence:+
+
#
Submission#of#the#
Analog#and#Target#
and#Designing#
Experiments#
Worksheets#are#
required.#
+
(25+pts+total)+
This section organizes data, results, and evidence. The goal of the section is to describe
what you did and what data were collected. Observations can be important data to use in
analysis. See additional instructions in the Template for Submission of Lab Reports (in ICN
Resources).
This section should include:
•
Procedure: Reference the laboratory procedure that was downloaded and the date it
was accessed (Precipitation Reactions and the Conservation of Mass procedure,
accessed: 10/25/2019). Any changes in procedure should be noted.
•
Analog and Target (A&T) Worksheet:
This worksheet should be included and discussed in
the lab report. (Your scan must be legible)
•
Designing Experiments (DE) Worksheet: this w
orksheet should be included and discussed
in the lab report. (Your scan must be legible)
•
Observations:
include observations and note if they help explain any chemistry.
•
Notebook data and Calculations. You should present data such as the mass of silver
chloride and a calculation of how it was converted to moles.
Note: s
ince patterns are often critical to understanding data, you may want to present data
in Tables as well as Figures. This will increase the quality of this section.
Note:#data#from#outside#sources#can#be#used#in#this#section#to#support#data#obtained.!
Analysis+of+
Evidence+
(Reasoning):##
+
Scientific#
explanations#that#
use#evidence#and#
appropriate#
chemistry#concepts#
to#construct#claims .##
(30+pts+total)+
In#this#section,#your#d a ta ,#re s ults ,#an d#evidence,#includin g #o b s e r v ations,#are#analyzed#to #h e lp #
explain#the#chemistry#that#occurs#in#the#experiment.#Your#analysis#will#be#a#major#support#
for#any#claims#you#make .#
See additional instructions in the Template for Submission of Lab
Reports (in ICN Resources)
.#
This section should carefully analyze evidence (using reasoning) and include:
•
Data to Analysis Connection: a goal of this section is to connect data, results, and
evidence to your analysis. This is in preparation for making a claim.
•
Discuss Data and Observations:
include in your discussion data and observations that
can help you explain the chemistry involved in this lab.
•
Include an answer to the scientific question: Is the number of moles of silver
chloride produced in the chemical reaction equal to the number of moles of silver
cations and the number of moles of chloride anions in the reactants?
•
Discuss A&T Worksheet:
discuss how the analogy relates to your experimental data. Try
connecting it to the analogical model to develop your explanations of results and
underlying chemical concepts in your discussion
•
Discussion of phenomena: you can discuss
phenomena at both the submicroscopic
(molecular) level & macroscopic (visible to your eyes) level in order to explain the chemistry
of this experiment.
Note:#Data#from#outside#sources#can#be#discussed#in#this#section#to#support#analysis.!
Claim(s):#
Statement(s),#
derived#from#
evidence,#using#
scientific#
reasoning.#+++++++++++++++++++
(15+pts+total)+
This section is to present claims. It is advantageous to directly connect evidence (from your
lab investigation and outside sources) that you utilize in your discussion to develop your
claims. See additional instructions in the Template for Submission of Lab Reports (in ICN
Resources).
#
Addressing#the#scientific#question#as#part#of#your#claim#may#be#useful.
!