An Exercise in Freezing Point Depression

More documents

Recommendations

Info

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
Page 4: Week One: Gathering DataYour TA wil
Page 11 and 12: Part B. Determination of ∆H solnf
Page 13 and 14: Name: _____________________________
Page 15 and 16: Name: _____________________________
Page 17 and 18: Name: _____________________________
Page 19 and 20: Name ______________________________
Page 21 and 22: Name ______________________________
Page 23: Name: _____________________________

An Exercise in Freezing Point Depression

Create successful ePaper yourself

Delete template?

Save as template?