vapor pressure, bp/fp, and molality

Today's post adds more to the topic of Solutions and examines Vapor Pressure, Boiling Point and Freezing Point changes, and Molality (m).

Objectives:

1.  Describe what happens on a particle level at the boiling point of a liquid.

2. Determine boiling point temperatures using a vapor pressure curve; compare and contrast intermolecular forces of a substance using the data on a vapor pressure curve.

3. Explain on a particle basis why a solution has a lower vapor pressure, an elevated boiling point, and a depressed freezing point, compared to that of the pure solvent.

4.  Calculate the molality (m) of a solution, moles of solute or kilograms of solvent given the other 2 variables; calculate the change in boiling point or freezing point temperature based on the molality (m) of a solution.

Here's a video describing vapor pressure and boiling:

The vapor pressure curve is also used as a boiling point curve b/c it shows the various pressure/temperature combinations for a liquid.  If the external (or atmospheric) pressure equals the liquid's vapor pressure, boiling will occur.

A series of curves on one set of axes can be shown.  Here is one graph used in NYS Regents Chemistry:

Some questions with a graph on the page are located here:

http://drgchemistry.weebly.com/uploads/2/4/8/9/24894932/topic_4_-_vapor_pressure.pdf

Here's a lab to determine the vapor pressure of water at various temperatures so that students can graph it.  I like that it's pretty reasonable to gather the materials and to perform.

http://phs.prs.k12.nj.us/rcorell/VaporPressureWaterLab.pdf

Remember eye safety goggles and aprons

Here is a lecture about intermolecular forces and the effect on boiling point temperature.  The discussion is very fast but very comprehensive -- you may have to view it more than once to better follow this discussion:

You might want to turn up the volume on this clip as well.  BTW, he mentions Van Der Waals forces, you may know them as London dispersion forces.

You may wonder about why all this talking about vapor pressure and boiling?  Because a solute has an effect on boiling point and freezing point.  This is a video demonstrating solute affecting boiling point and freezing point.  This seems to be a student video and has a few misspellings -- oops!

The boiling point elevation of water (K b) is 0.512 degrees Celsius / molal and the freezing point depression (K f) is -1.86 degrees Celsius / molal.  Different solvents have different change values.  Here's a list:

http://www.cliffsnotes.com/sciences/chemistry/chemistry/solutions/freezing-and-boiling-points


But wait! What is molality?  You may remember my discussion Molarity a few days ago.  Here is a brief discussion (5 minutes) of molarity (M) vs. molality (m):

Here's one worksheet focusing on molality:
http://www2.hoover.k12.al.us/schools/hhs/faculty/jwalding/Worksheets/Molality.pdf

Here is a worksheet with answers and strategy suggestions:
http://www.horton.ednet.ns.ca/staff/richards/apchemistry/APAssignments/SolutionsAss4AnswerKey.pdf

Salt lowering the freezing point temperature of ice has a few practical applications.  It is commonly used on icy roads and sidewalks to lower the freezing point temperature and prevent ice formation. 

States are trying a variety of solvents other than salt to save money.  Here's Massachusetts' webpage that contains a table of the salt fighting substances:

https://www.massdot.state.ma.us/highway/Departments/SnowIce/WinterRoadTreatmentSnowRemoval.aspx 

For making ice cream, salt is mixed with the ice surrounding the system to lower its temperature and aid in freezing the cream/sugar mixture.

Here is an easy way to make ice cream using salt and ice:

I have a lab in my book "Chemistry on a Budget" that uses the Play and Freeze ice cream maker!

Check out my lab book "Chemistry on a Budget" at amazon.com:
http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget

Each lab is presented with two possible report formats -- both with the same procedure -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required.  This was to give the teacher the option of what type of report is desired!  

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Have a good end of the week!