NH4Cl - Teacher Notes 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 tubeo acrylic tubes of varying length and diameter from TAP Plastic storeso clear plastic strawso 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 cottono 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 endo corks with q-tip pretty flush with cork end

Varying concentrations were tested to try to keep the toxicity of chemicals to a minimumo Lab calls for 12M HCl and 6M NH3o 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 tubeo 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/ANALYSISEngage

Teacher Student1.  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 1Q1.  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 Cl-On 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/ExplainTeacher 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 dataExpected: 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 longring 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/mintotal 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_Gases