==Chemical Detective== TRG, PDF pg. 136-... [Notes from 17 NOV 2006] "Chemical Detective - Finding Evidence of Chemical Change" lab activity GSLG pp. 59-70 PRE-LAB: ------- > Background: Physical- vs. Chemical Change [FCPS PB5LR packet] PROVISIONING FOR LAB: -------------------- > "Activity A - In Disguise" [Answer to the question, "Where do airplanes fly?"] > ***NOTE: Be sure to use at least a walnut-sized piece of steel wool.*** The exothermic reaction is much less evident when using smaller pieces of steel wool. CuSO4 + Fe (steel wool) --> FeSO4 + Cu (single replacement reaction) Way-Cool Observation: Using a large piece of steel wool (~1/6 of steel wool pad) and 20 mL of CuSO4 solution, all of the Cu (ions?) are removed from solution (plating the steel wool) leaving only a clear- to light-greenish solution of FeSO3 -- cool! "In the chemical reaction between the steel wool and copper sulfate, the liquid left in the baggie should be clear and colorless as long as the students don't squeeze the steel wool "sponge" too much. The liquid (formerly copper sulfate) is clear and colorless because all of the copper ions have been reduced to copper metal. The iron in the steel wool has been oxidized to iron +2 in the solution. If there are enough iron ions in solution, there could be a slight green-ish tint to the solution. In most cases I saw, the copper particles on the wool were floating the liquid causing it to be turbid and dark." DrV > Flask/balloon: Alka Seltzer + H20 --> CO2(gas) Alternate procedure: Baking soda + vinegar = CO2 http://www.myschoolonline.com/folder/0,1872,2507-108666-2-18926,00.html Use wooden splints to test gas formed in flasks: [Day 1] H2O + Alka Seltzer --> CO2(gas) + (water?) ... and ... Hydrogen peroxide + yeast --> water + O2 (oxygen gas) 2H2O2 (in the presence of yeast) --> 2H2O + O2 (decomposition reaction) R | P 4 H | H 4 4 O | O 4 (2 + 2) > Post-Lab Discussion: Summarize clues to a chemical change POST-LAB DEMOS: -------------- > Post-lab demos of physical- & chemical changes: 1. water-ice melting 2. burn Mg ribbon 3. H2O + CaCl2 --> exothermic reaction [or H2O + citric acid --> endothermic reaction] 4. PbNO3 + KI --> precipitate Solution 1 - Dissolve ~33g (33.1 g) PbNO3 in 100g H2O Solution 2 - Dissolve ~17g (16.6 g) KI with 100g H2O [Note: Solutions should be 1 molar. See notes (below) re: preparing 1 molar solutions.] Pb(NO3)2 + 2KI --> PbI2 + 2K(NO3) lead nitrate (in test tube) + potassium iodide (in dropper) --> lead iodide (yellow) + potassium nitrate (colorless) Related Resources: Lead Yellow: http://www.creative-chemistry.org.uk/activities/documents/paints.pdf http://www.elmhurst.edu/~chm/demos/colorchangpowder.htm Lead Nitrate and Potassium Iodide: http://www.youtube.com/watch?v=tQ_sJTQAnRM [Optional, depending upon method used to generate CO2 gas; do demo only when Alka Seltzer + water is used in lab.] 5. vinegar + baking soda --> gas production > Activity Extensions: Flame-Testing Gases/Chemical Reactions Using Catalysts 1. Chemical reactions that produce hydrogen gas (using a catalyst): catalyst_comic: http://www.wsanford.com/~wsanford/gr8ps/01_green/07_chemical_detective/ extras/catalyst_comic/ A) Primary Procedure: Zn + H2SO4 + catalyst [CuSO4(aq)] --> ZnSO4 + H2(up arrow, or gas) Zinc + Sulfuric acid + Copper Sulfate solution --> Zinc sulfate + Hydrogen gas [As a catalyst, add a few drops of CuSO4 solution.] Capture hydrogen gas (H2) in a test tube; flame test. *** [Note: When flame-testing the gas inside the test tube, be sure to hold the test tube vertically with the flame near the bottom of the tube.] *** "Squeaky the Chihauhau" or "Pop goes the Chihuahua." 2H2 + O2 --> 2H2O + energy (Pop!) [balanced equation] hydrogen gas + oxygen gas --> water + energy <-- Note: Some mass may be converted to energy. Is this an example of Einstein's famous equation, E = mc2? Mass and energy are equivalent, not two separate domains! B) Alternate Procedure: [reactants --> products] Zn + 2HCl(aq) --> ZnCl2 + H2(up arrow, or gas) [Is this equation balanced properly? WS See DrV's answer below.*] Zinc + Hydrochloric acid --> Zinc chloride + Hydrogen gas "Why do we use a stronger solution of sulfuric acid, that is, why don't we use hydrochloric acid? Concentrated sulfuric acid is not volatile. You have to work in a hood while pouring concentrated hydrochloric acid." DV Teacher's Note: 1 molar (1 M) HCl (in plastic bottles) works just fine. If you don't have 1 molar (1 M) HCl, then prepare solution by adding 85.5 g of concentrated HCl to one liter (1 L) of water. Work in an exhaust hood! WS *"Your equations are correct. Too bad you don't take the opportunity to introduce IONS. The real reaction is: Zn + 2H (+1 ions) --> H2 + Zn (+2 ions) The 2 Cl (+1 ions) stay in solution and are not in the reaction! Zn + 2H (+1) --> H2 + Zn (+2 ions) Doesn't matter if you have Cl (-1) or SO4 (-2) ions that do not react!" DV 2. Great example of an extreme exothermic chemical reaction: "Old Foamey" - ChemFax!, Flinn Scientific Inc. http://www.wsanford.com/~wsanford/gr8ps/CBL-Graphs/01_GSLG/08_endo-_exo-/ Flinn_Old-Foamey.pdf See also, archival copy of Vernier graph of temperature versus time: http://www.wsanford.com/~wsanford/gr8ps/CBL-Graphs/01_GSLG/08_endo-_exo-/ old-foamey_ver2.cmbl Analyze/Replay... Speed: 5.0 x Original [x] Repeat == FORMATIVE & SUMMATIVE ASSESSMENT: -------------------------------- 1. Bill Nye video(s): "Phases of Matter" (26 min.); and ***"Chemical Reactions"*** (26 min.) <-- show 2nd video on videocassette 2. Chemical Equations--Balanced or Unbalanced? (by DrV & WS) - The Atomic Accounting Firm of Dewey, Cheatham, and Howe. GenEd Adaptation: At least fill in accounting ledgers by counting atoms; extra credit for balancing equations correctly. chemical-imbalance_comic: http://www.wsanford.com/~wsanford/gr8ps/01_green/07_chemical_detective/ extras/chemical-imbalance_comic/ 3. Antoine Lavoisier, the " 'father of modern chemistry,' ... stated the first version of the Law of Conservation of Mass." (See also, the companion Web site for the PBS NOVA program Einstein's Big Idea, in particular, the section entitled m is for mass.) [GT/Honors only] [Provisioning: DVD player plus large-screen display.] Subject: Einstein's Big Idea 00:55:00 - 01:08:11 Chap. 12 ==Subject: How to Prepare 1 Molar Solutions== Google Search: 1 molar solutions + formula weight Good result: Q. How would you prepare a 1 mole solution of sucrose C12H22O11? http://wiki.answers.com/Q/How_would_you_prepare_a_1_mole_solution_of_ sucrose_C12H22O11 To make a 1 molar solution of a solute: look at the formula weight printed on the label of the bottle; add the mass listed (see "formula weight") to a small volume of solvent; then add more solvent to increase the volume of the solvent to one liter (1 L). Example: The formula weight (grams) of lead nitrate (PbNO3) [Note: Cannot use subscripts because of plain text format.] is 331.23 g (1 mole) per liter of solution. If you want 100 ml of solution, then divide the formula weight by 10. In this case, use 33.1 grams to make 100 mL of solution by adding sufficient solvent (water) to the 100mL mark on a graduated vessel. Half-molar solution: Add one-half the formula weight to one (1) liter of solvent. Example: 100 mL of 0.5 molar solution: 16.6 g to make 100 mL solution by adding solvent (water) to the 100mL mark on a graduated vessel. -- Now, what about making 1 molar solutions of HCl and H2SO4? I don't think the formula weight trick works in those cases. [WS] Yes it does, but it is more complex because you use the density of the concentrated acid. I'll example another time. [DrV] == [more unsorted/unedited notes below (if you're not Mr. Sanford, then you can safely ignore the following info)...] Unrelated question from the "Chemical Detective" lab: If I have a solution of copper sulfate and water, then why doesn't water appear anywhere in the formulae for the chemical reactions involving CUSO4? For example, when we put Mg ribbon in CuSO4, the liquid bubbles; you told me the bubbles are H2 gas, which makes sense to me. Problem is, I can't reconcile our observation of gas production with the following formula (from the TRG) that shows NO GAS(es) in the products: CuS)4 + Mg --> MgSO4 + Cu When you dissolve CuSO4 in water, you form solvated ions: Cu+2 and SO4-2 and you produce in solution you form Mg+2 ions from the reaction of Mg metal with Cu+2 ions to give Cu metal. Another reaction is Mg reacting with H+ ions (CuSO4 solution is acidic) to form H2 gas and solvated Mg+2 ions in solution. Water doesn't enter into the reaction. Neither does the SO4 ion enter into the overall reaction. Hope this helps. Cu+2 + Mg -> Cu + Mg+2 all solvated. 2H+ + Mg -> H2 + Mg+2 Help! -- The iodide reaction is better than my first suggestion of hydroxide which would be best tried with some heat. The equation using KI is simple (subscripts included): 2KI + Pb(NO3)2 -> PbI2 + 2KNO3 -- Date: Tue, 14 Nov 2006 18:14:28 -0500 From: David Venezky To: 'Walter Sanford' Subject: RE: URGENT - Addendum Yes, lead iodide is also yellow. I believe we saw some potassium iodide when we were looking for iodine. Make a 1 molar solution of the KI. The reaction with 2KI + Pb(NO3)2 -> PbI2 + KNO3 (Sorry I can not get subscripts) With a hydroxide, PbO precipitate. (Call for dinner. Have to go! -----Original Message----- From: Walter Sanford [mailto:wsanford@wsanford.com] Sent: Tuesday, November 14, 2006 7:43 AM To: David Venezky Subject: URGENT - Addendum Dave, Looking at my planning notes, this is the precipitate reaction: PbNO3 + KI --> precipitate -- Date: Tue, 14 Nov 2006 18:08:17 -0500 From: David Venezky To: 'Walter Sanford' Subject: RE: ***URGENT*** Walt: Regret that I didn't get to my email until now (supper time)to answer your urgent message. I am not sure what Clemons used, but I believe you have things reversed. Most nitrate salts are soluble in water including Lead nitrate. I suspect he used lead nitrate to make a clear solution of a lead salt. A 1 molar solution (33g in 100mL of water) of lead nitrate should work. If you treat the solution with a hydroxide (sodium or potassium hydroxide) you will obtain a precipitate of yellow lead oxide (PbO). If you can wait until tomorrow afternoon, I can make the solutions for you and demonstrate the reaction. Or use the 1 molar solution of sodium hydroxide that Wargo used for her class. It should be in the middle room on the bottom shelf. Dave -----Original Message----- From: Walter Sanford [mailto:wsanford@wsanford.com] Sent: Tuesday, November 14, 2006 7:34 AM To: David Venezky Subject: ***URGENT*** Dr. Dave, Garett Clemons used to make a solution of lead nitrate that we used to demo precipitate formation. We think we have the raw materials but G-Man didn't leave the recipe with us. Here's what we recall (although our collective memory is VERY foggy). We think Garett made up a 1 molar batch of potassium nitrate solution. We have about 200 g of crystalline lead nitrate; we have no idea what the solvent was, although Kyle thinks it may have been water. We would use an eyedropper to add a solution of KNO3 (potassium nitrate), which formed the yellowish precipitate. Two questions: 1. Is there any way you can spare us a few minutes sometime today to see if you can whip up a batch for us? You KNOW you'd rather play with chemicals than rake leaves!!! 2. If not, then can you tell us how to make the solutions? As always, thanks for your support, Dave-o!