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Evaluating a road deicer
Transcript of Evaluating a road deicer
-ice bath preparation
-6 test tubes of varying molalities
-test tubes in ice bath, record steady temperature
Equation used: ∆Tf=iKf
Kf = molal freezing point depression constant, units degrees Celsius per molal unit
i= van't Hoff factor
∆Tf= change in freezing point
CaCl2 molar mass: 110.98 g/mol
-if i="3" : expected freezing depression points
The members of the Under the Sea team work for Minnesota Department of Transportation (MnDOT) and are up for promotion. Their eccentric boss (who loves puzzles) says their salary and promotion depend on their research on a salt to work as a road deicer.
This winter has been especially treacherous for Minnesota drivers, so finding a deicer salt that is effective, inexpensive and has little harm to the environment is vital.
Their boss leaves them corny clues
to get through the project.
Table: Freezing Point Depression
white pellets, not powder
experimental temp. results near expected
CaCl2 can work in about -28 °C/-20 °F ground temp.
Sources of error
some CaCl2 may not have dissolved
last test tube was as cold as ice bath
Freezing Point of Depression Discussion
Fill beaker with ice, rock salt, water (-15°C). Add 20mL DI water to 6 test tubes, along with 2g CaCl2 in tube 2, 3g in tube 3, 4g in tube 4, 5g in tube 5, 6g in tube 6. Record final temp for all 6 test tubes inside ice bath
∆Hdiss = (-mwater Cwater ∆Twater – Ccal ∆Twater)/nsolute
Ccal=[-(m hot water)(C water)(Tfinal-Tinitial hot water) - (m cold water)(C water) (Tfinal-Tinitial cold water)] / Tfinal -Tinitial hot water*
*Ccal initial temp (1st calculation try)
Heat of Dissolution
Heat of Dissolution
Literature value for CaCl2: -82.8 kJ/mol
Our average value: -88.6 kJ/mol (old: -80.8 kJ/mol) "switch-up"
Discussion of results
negative value=exothermic; confirms our hot observation (touch)
heat released helps ice melt
Sources of error:
used 1g/mL for density calculation to find mass of water instead of using scale
calculation to determine Ccal off (8.53)
Heat of Dissolution Discussion
Freezing point of depression:
showed CaCl2 depressed freezing point greatly
Heat of dissolution:
showed CaCl2 exothermic (released heat in water)
Both show CaCl2 as an effective deicer
Trend in "i" value
more dilute --> less ion pairing
our values did not show trend
Connection Between Freezing Point of Depression and
Heat of Dissolution
- Low toxicity
- Not hazardous to animal
- Heat generated
- Browning of leaves
- Compare to NaCl ($0.05/Kg)
Environmental Effects of Calcium Chloride and Cost
Evaluating a Road Deicer
By Under the Sea members: Lammi Aga, Victor Kittleson, Zoe Ryan, Mariel Skogman
Is CaCl2 an effective deicer? Why?
can work in low temperatures
little environmental harm
easy to transport or put in brine
exothermic: heat released also aids in melting ice
loses effectiveness as temp. drops
To put this info partly in context,
Enthalpy of dissolution must be explained next.
Calorimeters -- remember those from chem class?
Don't let it leak heat or your lab may not pass.
Check off freezing point of depression and enthalpy of dissolution
And then take the data of your salt for final resolution.
How well will it work on an icy road?
Economically? Environmentally? How well does it bode?
You choose to research calcium chloride. Your boss sends you a new clue:
At about 0 degrees Celsius water does freeze,
But you can change that if you follow these keys.
Add salt to water in varying amounts.
Molality, dontcha know, is what counts.
All Minnesotans, take heart:
Soon the snow shall depart,
But until spring is here,
Icy roads must we clear.
Take note of a salt to break the ice.
Collisions and broken hips are far from nice.
Conclusion to clues
You've come to the end of your hunt for promotion.
Your efforts and conclusions show your devotion.
Even if CaCl2 isn't always the best in our vault,
You still will be worth your weight in salt.
1) Minneapolis Community and Technical College. http://www.mctcteach.org/chemistry/C1152/C1152_handouts/lab_handouts/Freezing_Point_Depression.pdf
2)Minnesota Pollution Control Agency, 2014. Minnesota Pollution Control Agency website. http://www.pca.state.mn.us/index.php/water/water-types-and-programs/minnesotas-impaired-waters-and-tmdls/tmdl-projects/special-projects/metro-area-chloride-project/road-salt-and-water-quality.html (accessed Feb. 24, 2014).
3) Silberberg, Martin.
Chemistry: University of Minnesota;
Mcraw-Hill Learning Solutions: United States, 2012; p. 489)
4) Tata Chemicals Europe. http://www.tatachemicals.com/europe/products/pdf/Calcium_Chloride/information_deicing.pdf
4.3 °C=(i)(1.86°C/molal)(0.9176 molal)
Ccal=[-(5g)(4.186 J/g·°C)(15.1°C-22.5°C)-(5g)(4.186 J/g·°C)(15.1°C-5.0°C)]/[15.1°C-22.5°C]
∆Hdiss=[-(5g)(4.186 J/g·°C)(22.9°C)-(11.7J/°C)(22.9°C)]/[0.0090 mol]
• For molality: molality=mol solute(CaCl
) / kg solvent(H2O)
• For "i": ∆T=iKf
*The cryoscopic constant: Kf of H2O = 1.86 °C/molal
The Meaning of "i":
• "i" represents the number of particles in a substance, this number has an effect on colligative properties.
• CaCl2 has three, Ca+, Cl-, Cl-, which makes it harder for water to freeze, as now it has 3 particles getting in the way of crystal formation.
• This is a graph of our data in y=mx+b form;
∆T = i × kf ×m
where “i” is the Van’t Hoff factor, kf = 1.86 °C/m and m = molal concentration of the solution
• Results in the slope being the "i" value.
Trend in van't Hoff factor"i"
*Note; ∆T is equal to the difference of Tf and Ti
•30% CaCl2 solution is the limit for its freezing point deprivation.
•lThis limit we explored, 2.741molal is equivalent to a 30% CaCl2 solution. (6g in 20ml water, quick conversion; 6/20 = .3 = 30% solution)
•Effectiveness of CaCl2 solutions drop drastically after concentrations > 30% solution. It is only linear up to the point we tested.
•(Common road deicers are 30-32% CaCl2, effective up to -25celsius)
Ccal=-5.923 J/°C (8.53)
∆Hdissolution= -88.6 kJ/mol (-80.8 kJ/mol)
Freezing point of depression lowest point: -15.1 ºC at 2.70
"i" average= 2.9
Ccal average= 11.7 J/ºC (-5.9J/ºC)
∆Hdiss= -80.8 kJ/mol (-88.62 kJ/mol)
Lit value ∆Hdiss= -82.8 kJ/mol
Slight environmental harm
*Relates molality/freezing point deprivation
•Trend: higher "i" value compounds = more deprivation of freezing point.