T Experiment 43

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Stoichiometry and CalorimetryProblemHow is it possible to use calorimetry to determine the stoichiometric ratio for a reaction?

IntroductionIn this experiment, you will determine the enthalpy change for a reaction, and at the same time studythe stoichiometry of the reaction. There are two goals in this experiment. The first is to establish thestoichiometry for the reaction between sodium sulfite and household bleach, an aqueous solution ofsodium hypocilorite. The second is to determine the heat ofthe reaction. To accomplish both goals,you will mix the reactants in a series of volume ratios and measure the temperature change in eachcase. For each trial you will use a total of 24.0 mL of combined solutions. Since both reactantsolutions are 0.50 M, the ratio of volumes will be the same as the mole ratio for that trial. Thus, atrial, in which 6.0 mL of sodium sulfite is added to 18.0 mL of dilute bleach would have a 3:1 molarratio of hypochlorite ion to sulfite ion.

The trial for which you observe the largest temperature change must be the one with the correct molratio of the reactants. From the AT for this "best" combination and knowledge of the number ofmoles of each reactant present you will be able to calculate heat for the reaction.

Prelaboratory Assignment/ Read the Introduction and Procedure before you begin.r' Answer the Prelaboratory Questions.1. Hypochlorite is an oxidizing reagent. In this reaction the sulfite ion is oxidized to sulfate

and the chlorine atom in the OCI- ion will either become molecular chlorine or chlorideion.a, In addition to the method used in this experiment, how else could you decide whether

the chlorine atom in hypochlorite becomes chloride ion or molecular chlorine?b. Write balanced molecular equations for the formation of each of the two possible

chlorine products. Remember these are alkaline solutions, so hydroxide ions (fromsodium hydroxide) may also be involved.

c. How will you decide which of the two equations correctly represents the reactionbetween sulfite and hypochlorite?

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MaterialsApparatus50-mL beaker100-mL beaker250-mL beakerPaper towelsWorld of Chemistry

ReagentsSolutions of:

sodium hypochlorite, NaOCIsodium sulfite, NafO:

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Foam polystyrene cups, 6-az (2)25-mL graduated cylinders (2)Thermometer or CBl-interfaced probeSafety gogglesLab apron

as shown in Figure l. nli--" Probo or lhermornster

li

''-ii---trrr)i -"'--*.'-- ii 1 ---.6 oz. foam cuPt\lilhr" iiI \-.hl*:_.*.- -"2___-.t oo mt beakerll --'*:Fi"'"- ll

Probe or -X1Alo"_Jl--. s0 mt beakerti'rermometer lt tf=*l# illl ti I :ili ;l---{i--* PaPer lowel

ll ,4. '-'i '_J lj (wranned around\._::Y':..,, rheb€aker)

Figure L

Apparatus for measuring temperature ch4nge during the reaction

sarery *1P 4*1. Wear safety goggles and a lab apron whenever you are working in the laboratory.2, Sodium hypochlorite is a skin irritant. Avoid contact with skin.

Procedurel. Prepare the calorimeter set-up as illustrated in X'igure 1. The system consists of a 50-mL beaker,

wrapped in paper towel for insulation, then inserted into a 100-mL beaker. The lid is an inverted6-oz expanded polystyrene cup; the probe or thermometer is inserted through the base of the cup,

:Notes,/ If a computer- or CBl-interfaced prope is used, it may !e necessary to use a larger cup for the lid,trimmed to fit more closely to the top of the beakers. Most interfaced thermistor probes have bushingsthat can be used to make a good seal with the cup.{ If a standard glass tlprmometer is used, drill a small hole in the cup using a #2 cork borer. Do this verycarefully, since a snug fit is essential for good results.{ A digital thermometer generally has a pointed tip which can be used to make its own opening.

{ Regardless of the qrye of temperature senFor used, onca it is in place it should not be removed so that theopening is not widened, thus reducing heat-retention.

2. Label separate 25-mL graduated cylinders for the two reactant solutions, NaaSOg and NaOCl,Keep these two separate to avoid contamination.

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World af Chemistry 138

3. Eacll irial"follows the same basic sequence:a. Always place the reactant solution for which the volume is larger in the calorimeter first. It is- lgcefary do this so that the thermometer or temperature probe is completely submerged.b. The lid containing the thermometer or probe is placed on ih. calorimeter and the temperature

of the solution is recordecl.c. The lid is removed and the second reagent is added all at once. Thelid, containing the

thermometer or probe, is immediately ieplaced and the calorimeter system is gently swirled toensure complete mixing.

r' The lid is removed and the contents of the system are rinsed down the drain.r' The inner beaker of the calorimeter is rinsed with distilled water and dried, in preparationfor the next trial.

Trial1

234567

Mixing Volumessodium sodium

hypochlorite sulfite18.0 mL 6.0 mL16.0 8.0l4-4 9.6 l\ote: 14,5: 9.5 is close enough12.0 t2.09.6 r4,48 o 16.06 0 19 0

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Cleaning UpL. All solutions remaining after the reaction may safely be poured down the drain.2. Clean all glassware and return it to its proper location.3. Your teacher will instruct you about what to do with the foam cups and CBL apparatus.4, Wash your hands thoroughly before leaving the laboratory.

Analysis and ConclusionsComplete the Analysis and Conclusions section for this experiment either on your Report Sheet orin your lab report as directed by your teacher.

If Vgy are not using a Report Sheet for this experiment design a Summary table with the followingheadings:

Trial Mol OCI/mol SOr'- q (J) I

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1.

2,

For each trial calculate q, the quantity of heat released by the reaction and enter your result in the

Summary Table. Assume that each of the solution combinations has the same specific heat as

water, 4-.1g4 J/g"C, Note: The total mass of solution being heated is 24.0 g in each case.

To determine the stoichiometry of this reaction we need to establish the mole ratio that produces

the maximum amount of heat.a. Use the volume and concentration of sodium

sulfite ion present in each reaction system.sulfite to calculate the number of moles of

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b. IJse the numbers of moles just calculated and the value of qcalculate values of AH for each reaction in kJ/mol NazSOr.

for each reaction (from 1) toRecord these values in the

Summary Table.3. Based on your answer to 2b, which combination produced the greatest amount of heat

per mole of sodium sulfite consumed? This must be the reaction combination that is closest toihe actual stoichiometry of the reaction. Write the equation for the reaction using the mole ratioof sodium hypochlorite to sodium sulfite that matches your experimental result' To complete

the balancing you may need to add hydroxide ions and/or water to one side or the other.

Something ExtraL. you were told to base your decision on the number of moles of sodium sulfite consumed. That

was an arbitrary choice. Do the calculations necessary to show that the same results would be

obtained and the same decision would have been made had you based your calculations in2b on the moles of sodium hypochlorite consumed, instead of on moles of sodium sulfite.

2. Repeat the experiment, using ihe same concentrations and volume combinations, but replacethe sodium sulfite with eithera. sodium thiosulfate, Na2S2O3, orb. sodium iodide. Follow the instructions for the original experiment'

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