QuestionDry Lab: Synthesis of Bleach by Electrolysis Goals: Determine the… Dry Lab: Synthesis of Bleach by ElectrolysisGoals: Determine the effects of electrical current and electrolyte concentration on the efficiency andyield of bleach production by electrolysis. Use the resulting data to determine optimum conditions forthe synthesis of bleach from seawater.IntroductionIn this experiment, you will synthesize the active molecule in bleach (sodium hypochlorite) throughsaltwater electrolysis and determine the concentration of your product through an iodometric titration.Bleach is a very strong oxidizer with many commercial applications. It can be used as a disinfectingagent to kill bacteria in water, making it useful for providing clean drinking water.In terms of Green Chemistry, this experiment serves as a good example for Design for EnergyEfficiency as you will be determining the conditions under which bleach is produced most efficiently.Electrolysis reactions are widely used in the production of many of today’s commercial chemicals. Inthis lab, we will focus on making sodium hypochlorite (NaClO), commonly known as bleach, throughthe electrolysis of an aqueous solution of NaCl, or table salt. Although this technique is no longer usedcommercially to produce bleach, the basic chemical reactions are the same using our simpleelectrochemical reaction.Image transcription textpower supply electrons electronscarbon steel anode cathode evolvedevolved chlorine hydrogen… Show more… Show morefigure 1 (JEPS, Mind42)Sodium hypochlorite, NaClO, the solution is clear and slightly yellowish. It is a strong oxidizing agent andis most commonly known for its ability to bleach colors. However, it can also be used as a disinfectantfor drinking water, a useful property that can be used to help provide third-world countries with cleanwater. Other industrial applications include agriculture, food, paper production, and textiles. Sodiumhypochlorite is also added to wastewater to reduce odors.The process of making bleach through the electrolysis of saltwater involves several intermediatereactions. First, if an external current is applied between 2 electrodes immersed in saltwater, chlorineions are oxidized to form chlorine gas at the anode, while water molecules are reduced to producehydrogen gas and hydroxide ions at the cathode. The electrolysis half-reaction is as follows:Image transcription text2 Cl(ag) ” Cl2(q) + 2e anode 2 H,O(qq ) + 2e -> H2(q) + 20H (gg)cathode… Show moreThe chlorine gas produced at the anode can then react with dissolved hydroxide ions produced at thecathode to form hypochlorite ion:Image transcription textCl2(9) + 20H (aq) > Cl'(aq) + CIO (aq) + H2O(1)… Show moreThe hypochlorite anion, ClO-, is the weak conjugate base of hypochlorous acid, HClO (pKa 7.2).Commercial Clorox Bleach is formulated at high pH to keep it stable in the ClO- form. For ourpurposes, we will refer to aqueous sodium hypochlorite (NaClO(aq)) as ‘bleach’ and assume a formulamass of 74.4 g?ol-1 for calculations.The theoretical yield of an electrolysis reaction can be predicted from Faraday’s Law of ElectrolysisImage transcription textQ = It 1 mole e F = 9.6485 X 10*C… Show moreWhere Q is charge in coulombs, I is current supplied in amperes and t is in time in seconds. Byrearranging the Faraday constant and using equation 4 to substitute the product currently (I) x time (t) forcharge in coulombs (Q) we can calculate the moles of electrons (n) supplied to the electrolysis reactionfrom the magnitude and duration of the applied current:Image transcription textn F… Show more(equation 6)Then, using the stoichiometry of the balanced electrochemical equation, the theoretical yield of bleachfor a given current and duration can be calculated.Titration is used to determine the actual concentration of bleach in a sample. Normally, titration uses atitrant-indicator system in which an indicator reacts with excess titrant to form a differently coloredproduct, but such a system does not exist for sodium hypochlorite. Instead, a multi-step process is usedto titrate bleach solutions. The sodium hypochlorite is first converted to hypochlorous acid (HClO)(reaction 1). The hypochlorous acid is then titrated using iodine and starch as an indicator systemaccording to the detailed steps below:Image transcription textNaCIO(go) + H, SO (qq) – HCIO (aq) +NaHSO ( ag) (reaction 1) HCIO(ag) +H2 SO 4(ag) + 3 I(aq) &qu… Show more… Show moreImage transcription textI3(aq) + starch(ag) > [13][starch aq)(reaction 3) red purple [I;][ starch ](ag)+ 2523( aq) -+ 31(ag) + S4… Show more… Show moreIf excess sulfuric acid is added during reaction 1, all of the bleach presents will be converted tohypochlorous acid. Then, if excess iodine is added in reaction 2, all of the hypochlorous acids will beconverted to purple triiodide-starch complex in reaction 3. Therefore, by titrating the triiodide-starchcomplex with thiosulfate ion (S2O3-2) we are indirectly titrating bleach using a visually strikingindicator system which changes from vibrant purple to clear at the endpoint.In part 1 you will conduct several electrolysis reactions with different concentrations of dissolved saltand different applied currents. In part 2, you will use the titration strategy described above to measurethe amount of bleach produced.Procedure Part 1: ElectrolysisYou will work in groups of 2-3 for this experiment. Each group will perform electrolysis and titrationsfor both a controlled trial and one assigned experimental trial (see table 1). All groups will complete acontrol condition first (a check to see if all groups are doing the correct procedures) with 6.0 M NaClsolution and an applied current of 0.400 A.1. For both your control trial and your assigned experimental trial: Prepare a salt solution withyour intended concentration. Weigh out the appropriate amount of salt, quantitatively transfer toa 100.00 mL volumetric flask and dilute to the mark with Distilled water. Using a graduatedcylinder, measure out 80.0 mL of the NaCl solution into a narrow 150 mL beaker.2. Obtain a constant current generator and a LabQuest. The generator should have a set of twoalligator clips attached via wires. Examine the exposed metal portions of the alligator clips. Ifthe clips are dirty or corroded, use sandpaper to remove the corrosion.3. Attach the alligator clips to the graphite rods. Be careful with the rods, as they are fragile andwill break easily. Place the rods into the beaker, but be sure the rods and the alligator clips donot touch each other, and that the alligator clips do not touch the solution. Do NOT start thereaction if they are, as you will destroy the clips (see Figure 1 for a diagram of the setup).4.Image transcription textCathode Curry Anode… Show morefigure 25. Turn on the LabQuest and go to page 11 for settings. Also, set a timer for 30 minutes (1800seconds). Plugin the constant current supply and turn the knob clockwise to achieve maximumcurrent supply. The measurement of your current (A) should be displayed on the Lab Questscreen. Adjust the knob accordingly on your constant current supply to achieve the current yourequire for your specific trial conditions. If you cannot quite achieve the desired amperage justmake a note of the actual amperage for your trials. Press the green triangle “Play” button on thebottom left of the Lab Quest screen to start plotting data. Bubbles will begin forming from bothrods. If this is not the case, please check your setup, and ask your instructor for help if needed.6. Keep an eye on the current making sure that it is as close to your intended amperage aspossible. The LabQuest will not automatically stop the reaction, so keep track of time and turnoff the current after 30 minutes. After turning off the current, carefully remove the graphiterods, and stir the solution with a glass stirring rod.7. Record the average current by using the “statistics” function in the LabQuest (First, Tap/dragover the region of interest). See Appendix A for more details on operating the LabQuest witha constant current unit. Now that you have accomplished the control electrolysis reaction, set upyour assigned electrolysis reaction. While the next reaction is running, move on to Part 2, thetitration of your solution.See the electrolysis dry lab datasheet for the resulting data.Procedure Part 2: TitrationTitrate the bleach solution to determine its concentration. You will perform 3 titrations using portions(aliquot is the correct term) of the bleach solution that you created in the electrolysis reaction in Part 1.Your group of two students will be performing titrations on both the control solution and an assignedelectrolysis trial, you will be executing a [total of 6 titrations during the lab period].1. In the 250 mL beaker, combine the following reagents. The solution will turn a dark red. Themore concentrated the bleach, the darker the color will be.a) 25.0 mL out of your 80.0 mL electrolysis solution (made in Part 1)b) 15.0 mL of Distilled waterc) 20.0 mL of 10% KI solutiond) 20.0 mL of 2M sulfuric acid2. Prepare a buret for titration with 0.250 M thiosulfate solution. Rinse with a small amount ofthiosulfate, then fill, drain to remove air, and start the titration below 0.00 mL mark as always.Record the starting volume to two decimal places.3. Begin titrating the solution. The color of the solution will begin to fade. Once the color becomesa golden yellow (which is Before the endpoint), add 5 drops of 10% starch solution. The colorshould turn a very dark purple-black. If the color is not dark enough, add 5 more drops of starchsolution.4. Finish the titration. There will be a very sharp endpoint when the purple color disappears. Notethe final volume to two decimal places, and record the change in volume in your data table.Calculate the [NaClO] for trial 1.5. Perform 2 more titrations with new 25.0 mL aliquots of electrolysis solution. Calculate [NaClO]for each trial, determine with your group/TA if any data should be tossed, and write the averageon the board.6. Rinse out the electrolysis beaker, electrodes (clean/wipe them) and then add 80.0 mL of yourassigned solutions and repeat the procedures. Report averages on the class data table.See the electrolysis dry lab datasheet for the resulting data.Analysis Part 1: Electrolysis1. Which gases were being formed at each electrode?2. Why are graphite rods used instead of a metal electrode, such as iron, copper, or stainless steel?3. Based on the chemical reactions which lead to the formation of bleach, why would it be betterto have the chlorine gas spend as much time in the solution as possible in order to produce morebleach?Analysis Part 2: Titration1. Explain the four distinct color changes (initial red color, red to yellow, yellow to purple, purpleto clear) in each step of the titration, and cite the chemical reaction # from the introduction thatare causing these changes.2. Use the volume of sodium thiosulfate solution added to compute the moles of sodiumthiosulfate added, and the sodium hypochlorite concentration in molar and percent by massconcentration units for each trial.3. To find the theoretical yield, use Faraday’s law as described in the introduction.4. Calculate the percent yield for each trial.5. Compared your class control % mass to commercial bleach (6.25% mass). How much moreconcentrated is a bottle of the bleach than your solution? (calculate a number: e.g. 6.25 / 1.25 = 6.0times more concentrated)6. What was the effect of changing the salt concentration and current on the final concentration ofbleach?7. What are the factors and tradeoffs (energy, time, etc.) to consider between using a lower currentand using a higher current? Which would you prefer and why?ScienceChemistryCHM MISC
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