An Exercise in Freezing Point Depression

Week One: Gathering DataYour TA will divide your lab section into five groups of 3 to 5 students per group.He or she will then assign one of the following 5 deicers to each group: NaCl, KCl,KC 2H 3O 2, MgCl 2, or CaCl 2. If your group consists of 4 or 5 students, it will be most timeefficient to split up and use two computers. You’ll need to calibrate a temperature sensor(Part IA) for each computer. One group should begin by preparing the two beakers forPart IIA, then calibrating the temperature sensor and finally proceeding with Part III.The second group will also do Part I A and will then proceed with trial 1 of Part IB.All of Part IB needs to be done using the same computer, but each group member shouldbe part of at least two freezing point depression trials—first as an “assistant/learner” andthen as the “leader/trainer”. All group members will need to keep a record of all datacollected on Data Sheet I. If you calculate and plot ∆T vs. m as you go, and if you obtain agood straight line for trials 1 through 4, you may omit trial 5 and proceed to trial zero.Omitting trial 5 will significantly reduce the amount of chemical used.Week Two: Comparing Data on Deicers and Evaluating their Effectiveness1. Compare the damage to nails in each deicer after one week.2. If not completed during week one, compare concrete samples.3. Each group in your lab section will share its experimental results and observationsfor Parts I, II and III. See Post-Lab Questions 2 through 4 for further directions.EXPERIMENTAL SECTIONWaste TreatmentBecause road deicing chemicals would be applied by the ton, we have chosenenvironmentally safe substances to study. Unless announced to the contrary by your TA,all solutions and dry solids may be rinsed down the drain with running water.Part I Freezing Point DepressionA. Calibration of Temperature ProbeYou will be calibrating the temperature probe for your instrument using two knowntemperatures – that of boiling H 2O and that of freezing H 2O. You will need to obtain oneor more temperature probes for your group.10-4

B. The Corrosive Effects of Deicers on Concrete and Iron Surfaces During RepeatedFreeze - Thaw CyclesConcrete scaling refers to damage done to concrete surfaces under various conditions. Theamount of scaling is rated from 1 to 5, with 1 being no surface damage during the studyand 5 being the most significant damage.Two sets of concrete samples were prepared in plastic containers. One set consists ofconcrete in 1300 mL beakers. In the second set, the concrete samples were placed in 800mL containers and a U – shaped iron bar was imbedded in the concrete. Each concretesample was covered with one of 9 different solutions and was then subjected to 10 freezethawcycles between about -17°C and room temperature. Because of the time required toprepare the concrete samples and to put them through a number of freeze – thaw cycles,the samples were prepared and treated outside of the regular laboratory and over aperiod of several weeks. The resulting concrete samples will be on display during the twoweeks spent on Experiment 10.1. Examine the concrete samples which have no iron bars present, estimate theamount of scaling on each sample and report the results, which will be subjective,on Data Sheet IIB.Do not touch the concrete surfaces!2. Also describe the extent of corrosion on the U – shaped bars on a scale of 1 to 5 andreport the results on Data Sheet IIB. Once again the results, while reported to thebest of your ability, are qualitative. For a given deicer, did the presence of the ironbar affect the amount of scaling on the concrete surface?3. Look around campus and around town for places where rusty iron bars and flakedconcrete reveal damage to bridges, parking ramps, roadways, etc. (Hint: A photoof serious rust or crumbling concrete, with the location noted, would be a goodaddition to your lab report or to a poster on deicers.10-8

Part III: Determining the Enthalpy of Solution, ∆H solnMany factors go into making a deicer both effective and practical. An obvious one,perhaps so obvious we usually don’t mention it, is solubility. Since freezing pointdepression is proportional to the molality of the dissolved solute, a solute with lowsolubility in water would be useless as a deicer. Therefore, you wouldn’t expect to seeCaCO 3used as a deicer because of its low solubility although it has other problems aswell.Another factor to consider in selecting a deicer is its enthalpy of solution. If a soliddissolves exothermically, it will provide heat to its surroundings and tend to increase theamount of ice melted per gram. From Section 11.2 of your text (Zumdahl, 5 th Ed.), we seethat the dissolving process can be considered to take place in three steps. According toHess’ Law, the overall enthalpy of solution is the sum of the enthalpies of the threeindividual steps. The first two steps require the separating of solvent particles and soluteparticles (or “expanding” the solvent and solute) and are always endothermic. The thirdstep is the coming together of the solute and solvent particles. If this step is sufficientlyexothermic, the overall process is exothermic. (Consider the rule learned earlier that “likedissolves like”. The stronger the attraction between solvent and solute particles, the moreexothermic, or less endothermic, we would expect the third step to be.)For an ionic solid such as NH 4Cl, the overall reaction for dissolution isNH 4Cl(s) + H 2O(l) → NH 4 + (aq) + Cl - (aq)∆H soln? If a salt dissolves endothermically, will the temperature of the resulting solution increase ordecrease? Will the ∆H solnyou calculate be a positive or a negative number?In this part of the experiment you will determine the approximate enthalpy of solution foryour deicer. During week two, you will also collect ∆H solnvalues for the other deicers anduse these values to help rank the deicers from the salt which dissolves with the largestnegative enthalpy of solution to the one which dissolves with the largest positive enthalpyof solution.10-9

Part A. Determination of the Calorimeter ConstantRefer to Experiment 5 for a sample calculation of a calorimeter constant. Yourcalorimeter is not perfectly insulating; that is, a small amount of heat is either absorbed orreleased by the calorimeter each time you run an experiment. The amount of heat gainedor lost by the calorimeter per degree change in temperature is called the calorimeterconstant. It is the heat capacity of your calorimeter. You will determine this constant bymixing warm water with cool water in your calorimeter.1. Using your small graduated cylinder, measure approximately 5 mL of cool water(approximately 10°C – you may use the water from your ice bath used in thecalibration of your temperature sensor) and record its volume to the nearest 0.02mL. Pour the water into a calorimeter well. Using a clamp place the temperatureprobe into the well such that it is reading the temperature of the water but is nottouching the sides or bottom of the well.2. Go to the “set up run” menu and adjust the run to 5 minutes with a sampling rate of60 samples per minute. Begin the run and then quickly, using the same smallgraduated cylinder as before, measure approximately 5 mL of hot water (about 30 -35°C) from the hot-water tap and record its volume to the nearest 0.02 mL.Immediately before mixing the two solutions read and record the initial temperatureof the hot water using the digital thermometer.3. Pour the hot water into the well and stir until the temperature stops rising. Click onthe stop run button. To obtain the initial temperature of the cold water, use themouse to highlight the portion of the graph immediately prior to the addition of thewarm water and then click on the “stat” button (a box will appear that will give youthe average temperature of the water for the portion of the graph you highlighted).Repeat this procedure to obtain the maximum temperature reached. This is the finaltemperature of the solution. Record these initial and final temperatures in yournotebook. For all temperature measurements, do not rest the probe on the bottomof the flask!10-10

Part B. Determination of ∆H solnfor your Deicer1. Refer to Table 10.1 to confirm that the suggested mass of sample will dissolvecompletely in 10 mL of water.2. Use a small graduated cylinder to measure 10 mL of water into a calorimeter well.Record the exact volume (to the nearest 0.02 ml) on Data Sheet III. Suspend acalibrated temperature sensor in the water.Suggested Sample Sizes:CaCl 2 0.6 to 1.0 g KCl 1.5 to 2.0 g MgCl 2 0.25 to 0.3 gNaCl 1.25 to 1.75 g KC 2 H 3 O 2 1.5 to 2.0 g3. Weigh the indicated amount of salt in a small, clean, dry beaker and record its mass.Begin the run and when the temperature sensor reading has reached a constanttemperature add the salt to the well. Stir with the temperature sensor until thetemperature reaches a maximum or a minimum. Record the initial temperature ofthe water as well as the maximum or minimum temperature reached. You can nowpredict the sign of ∆H soln4. Rinse the well thoroughly with DI water and repeat the procedure, adjusting themass of sample used, sampling rate, etc. if necessary.5. Referring to the sample calculation below, calculate the enthalpy (heat) of solution foryour deicer in J/g and kJ/mol. If the specific heat of your deicer is not given in Table 10-1,use the specific heat of water, 4.184 J/g°C, for your solution of dissolved deicer.Table 10.1Solubility (g/100mL H 2O) and Specific Heat (J/g°C) of an Aqueous Solution ofSelected Ionic CompoundsCompound Solubility(°C)Solubility(100°C)Specific Heat (J/g°C)of Salt SolutionNH 4 Cl 29.7 (0°C) 75.8 3.31CaCl 2 74.5 (20°C) 159 3.75KCl 34.7 (20°C) 56.7 3.45MgCl 2 54.2 (20°C) 72.7 3.94NaC 2 H 3 O 2 119 (0°C) 170 3.32NaC 2 H 3 O 2 •3H 2 O 76.2 (0°C) 138 (50°C) 3.32KC 2 H 3 O 2 253 (20°C) 492 (62°C) -----------NaCl 35.7 (0°C) 39.1 3.6010-11

Sample Calculation: Determine the Enthalpy of Solution of Ammonium Chloride.A student is asked to add ~2g of NH 4Cl to 10mL of water and to use the change intemperature to calculate ∆H soln. The reaction below corresponds to the enthalpy ofsolution of ammonium chloride.NH 4 Cl(s) + H 2 O(l) → NH 4 + (aq) + Cl - (aq)∆H solnA student first checked the solubility of NH 4Cl to make sure that a 2 g sample woulddissolve in 10 mL of water. She reasoned that because 2.97 g will dissolve in 10 mL ofwater at 0°C and since the solubility of NH 4Cl increases with temperature, a 2 g sampleshould readily dissolve at room temperature. She then stirred 2.105 g of NH 4Cl into 10.00mL of DI water and observed that the temperature changed from 24.2°C to 11.1°C. Thecalorimetry constant for the well used was 5.8 J/°C. If q solnis the heat transfer associatedwith the dissolving of the salt sample and q is the heat lost or gained by the solution ofwater and salt, thenq soln + q + q cal = 0q soln + (mass of salt + mass of water)(C sp solution)(∆T) + (5.8 J/°C)(∆T) = 0q soln + (2.105g NH 4 Cl + 10.0 mLx 0.997g/mL)(3.31 J/g°C)(11.1°C – 24.2°C) +(5.8 J/°C)(11.1°C – 24.2°C) = 0q soln − 524 J − 76 J = 0 and q soln ≅ 6.00 x 10 2 J∆H(J/g) = (6.00 X10 2 J)÷(2.105 g) = 285 J/g∆H (kJ/mol) = (285 J/g) (1 kJ/ 1000 J) (53.49g/mol) = 15.2 kJ/mol10-12

Name: _________________________________________ Date:___________________Pre-Lab Questions for Experiment 10Four students have completed Part I of Experiment 10. Their deicer was NH 4Cl (molarmass = 53.49 g/mol). The data for trials 1,2, and 3 is given in the following table.For trial 0, the freezing point of water, the temperature was 0°C and m = 0. Refer to thisdata to answer the questions below.1. Complete the Table below. Show your work for parts c, d, e, and f.Trial 1 Trial 2 Trial 3Mass of deicer used 1.920 g 4.119g 5.861gMass of water added 50.525g 49.872g 48.955gMass of ice melted 10.584g 23.916g 32.074gMinimum temperature reached -2.11° -3.88°C -4.70°Ca) Mass of water in final solutionb) kg of water in final solutionc) Moles of deicer usedd) ∆Te) Molality of final solution*f) Grams of ice melted/ g deicer* The value required here is "m" not "i x m"c)d)e)f)10-13

2. Prepare a plot of ∆T vs. m. Include trials 0 through 3. Use Excel or the graph paperat the back of the Manual. Each part (a, b, and c below) must be entirely correct toreceive credit for that part.a) Put your name on the graph, provide a title, and label the axes.b) Plot the 4 data points and draw the “best” straight line.c) Calculate the slope of your line, ∆T = imK fd) Calculate “i” from this slope. (Slope = iK f)e) What is the theoretical value of "i" for NH 4Cl? How does your value compareto the theoretical value of “i” for NH 4Cl?f) Use the experimental value of “i” to determine the “colligative molality, i x m,where m is the calculated molality of the compound (not of the ions.)10-14

Name: _________________________________________ Date:___________________Partner(s): _________________________________________ TA: ____________________Data Sheet I: Data and Calculations for Part ICalibration Data: Slope: ______________ Intercept: ______________Deicer: ______________ Molar Mass of Deicer: ___________For each trial, record the experimental data.Trial 0 1 2 3 4 51. Mass of cup ~2 g2. Mass of deicer 03. Mass of cold H 2 O ~ 50 g4. Minimum temperature5. Total mass after adding ice ~65 g6. Mass of cup + remaining ice xxxFor each trial you carried out, complete the table below:Trial 0 1 2 3 4 57. Mass of deicer (2) xxx8. Mass of cup, cold water, xxxand deicer (1+2+3)9. Mass of ice added (5-8) xxx10. Mass of ice remaining xxx(6-1)11. Mass of ice melted xxx(9-10)12. Mass of final solution xxx(5-6)13. Mass of water in final xxxsolution (12-2)Complete the table below (see Post-Lab Question 1.) Show complete calculations for 2 trialson the next page.Trial 0 1 2 3 4 5Moles of deicer usedKilogram of water in the finalsolutionMolality of final solution∆T = |0°C - T f |Grams of ice melted per gramof deicerxxxxxxxxxxxxxxx10-15

Show the calculations required for any two of Trials 1 through 4.Trial Number: _______Trial Number: _______10-16

Name: _________________________________________ Date:___________________Partner(s): _________________________________________ TA: ____________________Data Sheet IIA:Observation of the Corrosive Effects of Deicers onIron NailsWrite a brief description of the nail surfacesTime Exposed Plain H 2 O H 2 O + DeicerAfter 30 minutesAfter 1 hourEnd of Lab (total time nailsexposed = _______)After 1 weekCorrosion Results After One Week for the Remaining DeicersSee Data Sheet IIB for directions for "rating" the samples.Deicer Sample Observation Rating10-17

Data Sheet IIB:The Corrosive Effects of Deicers on Concrete and theSurfaces of Iron Embedded in ConcreteCAUTION: Do not physically disturb the surface of the iron rod or the surface of theconcrete during your examination of the samples.ConcreteRatingConcrete Sample Rating1 2 3 4 5Some Scalingsurface penetrates“powder” the surfaceno scaling,surface intactFairly deepcorrosion,large cracksand flakesSoftsurface,deepscalingIron BarRatingIron Bar Sample Rating1 2 3 4 5Orange Mostlysurface orange, sizesplotches intactEntirelyintact,no colorchangeEntire barorange,someflakingBar visiblysmaller aspieces flakeoffUse your best judgement in assigning the ratings.Deicer Solution Concrete Sample Rating Iron Bar Sample RatingWater10-18

Name ___________________________________________ Date ____________________Partner __________________________________________ TA _____________________Experiment 5 DATA SHEETPart A. Calibration of the Temperature Probe1. a) Record the trial Calibration Curve Equation here: ______________________________b) Record the Calibration Curve Equation here: _________________________________2. Does the probe measure temperature directly? Explain.Part B. Calorimeter Constant DeterminationTrial 1 Trial 21. Volume of cold water ________ mL ________ mL2. Mass of cold water ________ g ________ g3. Temperature of cold water ________ °C ________ °C4. Volume of hot water ________ mL ________ mL5. Mass of hot water ________ g ________ g6. Temperature of hot water ________ °C ________ °C7. Final temperature after mixing (T f) ________ °C ________ °C8. Calorimeter constant ________ J C° −1 ________J C° −19. Mean calorimeter constant ____________ J C° −1Show complete calculations of the calorimeter constant here.10-19

Data Sheet III: Determination of the Enthalpy of Solution of a SaltDeicer: Formula _______________________ Molar Mass ________________________Estimated grams of compound that will dissolve in 10 mL of water at 25°C: _____________gTrial 1 Trial 21. Volume of H 2O used ________ mL ________ mL2. Mass of water used ________ g ________ g3. Mass of sample used ________ g ________ g4. Initial temperature of water ________ °C ________ °C5. Final temperature of solution ________ °C ________ °C6. q soln________ J ________ J7. Enthalpy of solution per gram ________ J/g ________ J/g8. Enthalpy of solution in kJ/mol ________ kJ/mol ________ kJ/mol9. Average value of the enthalpy of solution ________ kJ/molShow a complete calculation of the enthalpy of solution below:10-20

Name ___________________________________________ Date ____________________Partner __________________________________________ TA _____________________Post-Laboratory Questions for Experiment 101. Data Treatment for Part 1a) Carry out the necessary calculations and complete the last table on Data Sheet I.b) Prepare a graph for plotting ∆T vs. molality, m for trials 0 through 4 or 5. Selecta scale such that the graph fills over half of a page. Give the graph a title andlabel the axes. (Whether the graph is done by hand or using the computer, anymissing information will result in a loss of all points for b).c) Graph your 5 or 6 data points, including DI water, and draw the best straightline through the data points. The formula for your straight line is ∆T = imK f.(An error in graphing your data points or in drawing the best straight line willresult in the loss of all points for c).d) Determine the slope of ( i x K f) and calculate “i”. Compare this to thetheoretical value of “i”. For example, we expect i ≤ 3 for MgCl 2. Show yourwork.2. Summary of ResultsUse the computer to prepare a table that summarizes the experimental results and anyother information available to your lab section on the 5 deicers. For each deicer,include as much of the following information as possible. “Average values” may referto the average of several trials, as is the case for ∆H soln, or, as in the case of ratingsurface damage, take the average of the ratings assigned by each of the groups. Youneed to submit only one table per group.a) Slope of the plot of ∆T vs molality.b) Value of “i”c) Average mass of ice melted per gram of deicerd) Solubility at 0 or 20°C (g/100 g solvent)e) Average ∆H solnin kJ/molf) Average rating of the surface damage (corrosion or scaling) of the concretesurfaces, iron bars, and iron nails10-21

3. Evaluations and ConclusionsWhat do you think is the “best” deicer for Twin Cities streets and highways? Why?Base your answer on the information obtained in this Experiment and summarized in thetable in Question 2. Explain or justify your choice. If there is a consensus within yourgroup, the group may submit a single table and paragraph here. However, if your opiniondiffers from the group’s, you may decide to write a “dissenting” opinion. Write the nameof each contributor on all joint work.4. Other Important considerationsYour Table covers only part of the information that needs to be considered when selectinga deicer. List at least 3 other factors that should be a part of the decision.10-22

Name: _________________________________________ Date:___________________Partner(s): _________________________________________ TA: ____________________Part V: Group Poster Project on “Selecting the Best Deicer forMinnesota Roads”(100 points: All contributing group members receive the same grade.)Prepare a poster using poster board of standard size and thickness. (There will be a 10point deduction from your assigned grade for failure to use the designated poster board.)Poster board is available from the lab stockroom for about $1.00 per sheet. The postershould be understandable to someone who has studied Chapter 11 in Zumdahl.Use your answers to the post-lab questions, the questions below, and any otherinformation you have obtained to prepare your group’s poster. Your TA may request anoral report from each group in addition to or instead of the preparation of a poster.1. Look up the cost of each deicer in a chemical catalogue. Use the cheapest possibleprice, which will probably be for the largest quantity and the lowest purity. Theprices paid for very large quantities (thousands of tons), as would be needed by amunicipal or state highway department, would be considerably lower, but catalogprices will hopefully at least give a relative ranking. You might also look for priceson the Internet or in a chemical trade magazine. Cargill, a privately held Minnesotabased company, is a major supplier of deicers and does extensive research on them.(Companies often do not quote prices to the public but negotiate them directly withcustomers such as MNDOT who use very large amounts of a product.)For the poster, search the library and Internet for information about theenvironmental effects of deicer chemicals. You might visit the Web pages of varioushighway departments and also nature magazines. Reference any web materials byreporting both the complete address and the date you accessed the material.a) Which deicers are the least expensive in terms of cost per ton and cost perpound?b) Compare the deicers as mass of ice melted per dollar2. Economists sometimes talk about “hidden costs” - costs that do not show upimmediately, but that do ultimately add to the “price” of a deicer. For snowremoval, some of the obvious expenses (besides buying the deicer) are the payrollfor snowplow and truck drivers, fuel and maintenance for the trucks, etc.However, most of these costs will be similar for each deicer compound.Environmental damage during mining or production, pollution and any of theresulting environmental degradation are examples of hidden costs.3. Select the “best” deicer from those studied in this experiment and write a briefparagraph to support and explain your choice.4. Work in groups of 4 to 6 to prepare a poster which will name the “best” deicer forMinnesota roads and which uses all available information to support your decision.10-23