NH4Cl - Teacher Notes
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In the Shakashiri demo, a glass tube 1-m in length and with an outside diameter of 22 mm is used. In building
this activity, many different clear tubes were tested, including:
o glass buret tube
o acrylic tubes of varying length and diameter from TAP Plastic stores
o clear plastic straws
o polystyrene pipettes with ends sawed off (BD bioscience serological individually packed)
Cotton balls are used in the demo materials, while pyrex/glass wool, cotton balls, and cotton swabs were tested.
o cork with q-tip seems simplest/easiest way to handle chemicals.
Various injecting mechanisms were tried:
o what is used in the demo, which is rubber stoppers with cotton balls soaked with solution.
o corks with cotton
o glass wool with rubber stoppers and syringes to inject a set volume of each substance (amount of glass
wool varied, not controlled for)
o corks with q-tip sticking out of each end
o corks with q-tip pretty flush with cork end
Varying concentrations were tested to try to keep the toxicity of chemicals to a minimum
o Lab calls for 12M HCl and 6M NH3
o 6M HCl with both 6M NH3 and 3M NH3 in varying tubes - HCl did not travel down the tube and the NH4Cl
ring formed at the HCl end. Seems that lower concentrations of HCl does not have a vapor pressure (VP)
therefore it will not form gas to diffuse down tube.
Tube lined with pH paper to visualize the gas travelling/diffusing down the tube
o Found that the presence of pH paper significantly hindered the HCl from traveling down the tube, and at
some point, stops it completely. Unsure as to what the interaction is. Hypothesize that the paper is
actually absorbing the HCl vapor.
o HCl has a very defined front as it travels down tube, while NH3 has a very diffused front with some
particles travelling faster down the tube and pH paper changing colors slightly to indicate as such, while
the end of tube pH paper changes color to darker blue slowly.
Cork with cotton swab
Rubber stopper with cotton swab
Acrylic tube, cotton ball, cork, pH paper lining inside
Pipette, cotton swab, cork, pH lined
Pipette with holes, cotton swab, cork, pH taped to outside to cover holes
Acrylic tube, cotton swab, cork at different time intervals – ring migration
PROCEDURE/DATA/ANALYSIS
Engage
Teacher
Student
1. Show demo of the reaction between
ammonia and hydrochloric acid. This can be
done by soaking the tips of two q-tips with each
chemical and holding them near each other. The
teacher will talk briefly about the chemical
reaction that is happening and about diffusion.
(5-7 min)
1. Students will observe the demonstration and should see a white precipitate
form in the air as the two chemicals come into contact and react with each other.
Students will complete the first question on their handout.
T: Ammonia (NH3) is a gas and hydrochloric
acid (HCl) is also a gas. They are both in
solution, but are both very volatile, which
means they readily become gases. So even
though the Q-tip is soaked with a liquid or
aqueous solution (chemical dissolved in water),
gas is forming from each of the solution soaked
Q-tips. When the two chemicals come in
contact, they chemically react and form a white
solid precipitate called ammonium chloride.
Talk a bit about diffusion in terms of things the
students are familiar with, like various smells perfume, flatulence, etc. You may even spray
some perfume at the front of the class and have
someone be the timer and have students raise
their hand when they smell the perfume and
collect some of that data. Do all gases travel at
the same speed? What do they know about this?
Do they know the relationship between kinetic
energy and temperature at this point? (Do they
need to? or is this something that I want to
focus on in this lesson?) What determines how
fast you will smell something? How do you
know the gas is moving if you cannot see it?
Will the people in the back of the room ever
smell the perfume before the people in the front
of the room?
Student Handout 1
Q1. On the above image of a tube, draw where you think the two gases will meet
and form the ammonium chloride ring. Explain why you drew what you drew,
where you drew it.
**diffusion: the tendency of molecules in a gas to move from areas of high
concentration to areas of low concentration.
How does the gas start at the cotton ball and end up somewhere in the middle of
the tube? Why does it do this?
2. Acid/base connection: Take a strip of pH
paper and dip one in each of the two solutions.
Show the students and have them answer the
next question. Try holding a strip of pH paper
above the solution or away from the Q-tip and
see if the color changes at all. You can talk a
bit about what is happening or have students
hypothesize as to what is happening to the
paper, why it is changing colors.
Q2. Write down your observations from the pH paper demonstration and answer
the following questions using the information provided.
chemical
observations
A/B
NH3, ammonia ______________________
HCl, hydrochloric acid_________________
Low pH (<7) indicates an acid whereas high pH (>7) indicates a base. Use the pH
paper key to identify each chemical as either an acid (A) or a base (B) above. Salts
are ionic compounds that form from a cation (+) and an anion (-) and are neutral
(pH=7). What color will the pH paper be in the presence of a neutral compound?
___________________________
3. Talk explicitly about the chemical reaction
taking place:
NH3 (g) + HCl (g) → NH4Cl (s) ← a salt.
NH4+ and ClOn the board, identify what color each will turn
Q3. With this new information, in the tube above, draw a strip of pH paper that
will run through the tube. Color in or label what color you think different parts of
the paper will be. Briefly explain what you have drawn and why.
the pH paper to provide students with a
visual/reference
Explore/Explain
Teacher
Student
4. Brief instructions for the activity. Each
group should have 4-5 students. First thing they
should do is assign roles. (Maybe this happens
before engage...and they are answering their
questions in the presence of their group
members). In doc, insert check boxes at each
action step.
P1. In your groups, assign each member a role.
What is your role? _______________
Once you are assigned a role, review the procedure and make sure you are clear of
your role and when it is pertinent. Make notes in the text and margin to emphasize
important steps and things to remember. You may choose to use a highlighter.
P2. Label one end of the clear tube “HCl” and the other “NH3”. Come to a group
consensus about where you think the ammonium chloride ring will form in the
tube, and use the overhead marker to make a mark on the tube.
P3. Cut a strip of pH paper ~1 inch shorter than the length of the tube and
slide/center it into the tube.
P4. Clamp the tube horizontally between the two stands, as shown in the image.
Make sure the pH strip is centered. You may use a small piece of tape if
necessary, to hold it down.
P5. Place a small cotton ball in the hollow of each stopper. Dip one of the
stoppers holding a cotton ball into a 50-mL beaker containing the HCl and the
other into a 50-mL beaker containing the NH3.
P6. Simultaneously insert the two stoppers into the appropriate ends of the tube
and start the timer. Make observations and record them on your worksheet (Q4).
P7. When you see the ammonium chloride ring form, draw a line with the
overhead marker to mark on the tube where it formed and make note of the time.
Make some measurements and answer Q5.
Check in with students/groups during their
procedure, especially near the end and illicit
ideas about what is happening and what they
are observing. Hopefully, students will be
marvelling at how cool it is to see the pH paper
change colors and visualize the movement of
the gases.
Q4. Draw what you observed in the experiment. Use colors and words to explain.
Does traveling further mean that it was
traveling faster? What other factors might you
take into account that you think might affect
how far the gas travels?
Q5. Draw a line on the tube where the white ring formed. Measure how far the
ring is from each end/chemical.
NH3:_____
HCl:_____
Which gas traveled further? _____
How much further did it travel? _____
Q6. Compare these results to your predictions. Discuss with your group how they
compare. Was your prediction correct? Do you think your reasoning is accurate?
Was your prediction very different from what actually occurred? Why might that
be? Come up with some explanations and/or questions. Write down a few things
from your discussion.
Option 1: Bring the class back together at this
point to discuss findings and guide toward
understanding.
Option 2: Groups work at their own pace and
check in with you at this point and get their
next handout. Students may choose to work
together in a group to answer the questions or
within their groups in smaller pairs or triplets.
Student Handout 2 (Not to be passed out before completing Student Handout 1)
Q7. You know that gas molecules are in constant, random motion in a straight
line. A gas molecule will continue moving in a straight line until it hits something,
at which point it will change directions. We will analogize a pool ball to a gas
molecule. You can imagine a pool ball continuing in a straight line until it hits
something, then it moves in another direction until it hits something else, just like
a gas molecule, except gas molecule constantly moving. Imagine two (identical)
pool balls that are pushed with the same force along the same straight line from
opposite ends, where do you think they would collide? Why?
Experimental data
Expected: 1.46, 1.28
Trial
Tube
Ends
[HCl]
[NH3]
pH paper?
NH3/HCl
=
distance
ratio
Time
1
glass
syringe
12M
6M
Yes
48.5/13.3
3.65
32.5
min
2
glass
syringe
12M
6M
No
36/26
1.38
7.5 min
3
glass
syringe
6M
3M
No
N/A
@HCl end
8 min
4
glass
cork/q-tip
12M
6M
No
27.5/26.5
1.04
7 min
4b
“
“
“
“
“
5
plastic
cork/1-tip
6M
3M
No
N/A
@HCl end
6 min
6
plastic
cork/cotton
12M
6M
No
18.2/10.8
1.69
1.5 min
7
plastic
cork/q-tip
12M
6M
Yes
20.2/8.5
2.38
4 min
8
plastic
cork/q-tip
12M
6M
No
15.8/11
1.44
1m23s
8b
“
“
“
“
“
15.3/11.5
1.33
1m30s
8c
“
“
10M
6M
“
19.5/7
2.79
3 min
8d
“
“
12M
3M
“
14.3/12.5
1.14x
1m30s
8e
“
“
10M
3M
“
9
plastic w/
holes
cork/q-tip
12M
6M
Yes, taped along outside
along holes
17.0/9.5
1.79
2m30s
10
plastic
cork/q-tip
12M
6M
Yes, soaked w DI H2O
along inside
22.9/3.7
6.19
5m42s
11
plastic
cork/q-tip
12M
6M
Yes, filter paper soaked w
universal indicator
20.4/6
3.4
~4 min
12
straw
q-tip
12M
6M
N
17/7.2
2.36
<40s
12b
“
“
“
“
“
11.2/8.3
1.35
<30s
12c
straw
q-tip
10M
6M
N
15.5/4
3.88
<1 min
12d
straw
q-tip
10M
3M
N
13
straw w
holes
q-tip
12M
6M
Yes, taped along outside
Acylic tube (TAP) 1 cm in diameter inside, cork/q-tip, 12M/6M, no pH, 33 cm long
ring formed at 1m30s, 12.5 cm from HCl end and 20.5 from NH3 end (20.5/12.5 = 1.64x)
Then it migrated farther and farther toward HCl.
time elapse
time increment
NH3
(33-HCl)
HCl
Ratio
(NH3/HCl)
Rate of ring move
(0.5/inc) cm/s
1m30s
0
20.5
12.5
1.64
0
2m30s
60s
21
12
1.75
0.0083
3m8s
38s
21.5
11.5
1.87
0.013
4m
42s
22
11
2
0.012
5m
60s
22.5
10.5
2.14
0.0083
6m
60s
23
10
2.3
0.0083
7m10s
70s
23.5
9.5
2.47
0.0071
8m20s
70s
24
9
2.67
0.0071
10m
100s
24.5
8.5
2.88
0.005
12m
120s
25
8
3.13
0.0042
14m
120s
25.5
7.5
3.4
0.0042
17m
150s
26
7.0
3.7
0.0033
Timed trial using glass buret tube, glass wool, rubber stopper, 1cc syringe with luer lock, 12M HCl, 6M NH3, one tube with
pH paper lining length of the tube, one tube without pH paper. Length of tube = 62 cm. Injected 0.5 mL of each chemical
at each end.
Time
HCl moved ___ cm
toward center
cm/min
total
distance/total
time
cm/min
(section
distance/2.5 min)
NH3 moved ___
cm toward center
no pH control
~7.5 min
26 cm
3.47
4.8
36 cm
36 cm NH3 /
26 cm HCl
= 1.38x farther
NH3 than HCl
w/o pH paper
2.5 min
(150s)
7 cm
2.8
6.8
17 cm
5 min
8.5 cm (1.5 cm)
1.7 (0.6)
4.4 (2)
22 cm (5 cm)
7.5 min
9.5 cm (1.0)
1.27 (0.4)
3.53 (1.8)
26.5 cm (4.5)
10 min
10 cm (0.5)
1 (0.2)
3 (1.4)
30 (3.5)
12.5 min
10.5 (0.5)
0.84 (0.2)
2.64 (1.2)
33 (3)
15 min
11 (0.5)
0.73 (0.2)
2.4 (1.2)
36 (3)
17.5 min
11.5 (0.5)
0.66 (0.2)
2.11 (0.4)
37 (1)
20 min
11.5 (0)
0.58 (0)
2.0 (1.2)
40 (3)
22.5 min
12.5 (1.0)
0.56 (0.4)
1.8 (0.4)
41 (1)
25 min
12.75 (0.25)
0.51 (0.1)
1.76 (1.2)
44 (3)
27.5 min
13.3 (0.55)
0.48 (0.22)
1.67 (0.8)
46 (2)
32.5 min
13.3 (0)
0.41 (0)
1.49 (1)
48.5 (2.5)
48.5 cm NH3
/
13.3 cm HCl
= 3.65x farther
NH3 than HCl
w/ pH paper
(1)
http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Rate_Laws/Gas_Phase_Kinetics/Kinetic_Molecular_Theory_of_Ga
ses