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charles' law

1/29/2014

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I am devoting a few entries to the laws related to the action of gases.  The next relationship is Charles' Law, the relationship of volume and temperature:

Volume / Temperature = Volume / Temperature

This is a direct relationship -- this link describes quite a bit:
http://en.wikipedia.org/wiki/Charles%27s_law
Click on the animation to the right to view a full-sized animation.

CAUTION: There cannot be any negative or zero values for temperature, so this relationship must be solved using Kelvins!

Remember, absolute zero is -273 degrees Celsius -- it's the temperature at which particle motion stops.

Kelvin temperature = Celsius temperature + 273

Here's one example of a laboratory:

My classes put ice in the cool water bath, and didn't take their finger off the stopper until a minute or two had passed.  Many times, when their finger was removed, the water shot up when rushing into the flask.  It wasn't fun for the student holding the flask, but the result was fun to see!

I had access to large, plastic buckets in one school which made it easier to submerge the flask in the cold water bath.  I didn't put glass medicine dropper valves in the stoppers, we just put our fingers of the hole in the stopper.

Here's a stopper size chart for several sizes of Erlenmeyer flask (just ignore the Product Numbers):
http://www.phytotechlab.com/pdf/FlaskStopperGuide.pdf

You can make a graph and extrapolate to absolute zero.  My students only had two points and many would miss -273 degrees Celsius, but it was exciting when we got close!

I found a simple laboratory procedure:
http://www.newwestinghouse.org/ourpages/auto/2011/5/4/41324522/Charles_s%20Law%20Lab.pdf

Here is a video of a sample mathematical solution for Charles' Law:
http://www.kentchemistry.com/links/GasLaws/charles.htm

And here is a worksheet of 5 problems with answers:
http://mmsphyschem.com/chuckL.pdf

Please let me know about your experiences, ask if you have any questions or if you have ideas for other topics for this blog.

And, check out my lab book "Chemistry on a Budget"  at:
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

I hope you're having a good week!
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Boyle's law

1/26/2014

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In my last post, I showed video of a demonstration of the combustion of methane gas using the classroom gas supply.

*Adding glycerin to the bubble solution can make the bubbles last longer.  Here's a link to a Flinn version with a "recipe" for bubble solution:
https://www.flinnsci.com/media/620299/91058.pdf

Another topic I would be teaching right now is Gas Laws.  I am aware that some of you may have taught this at the beginning of the school year (that's how a lot of textbooks are set up).

It helps to relate the study of gases to Kinetic Molecular Theory. 
Here is one overview of KM theory that has good animation of particle motion:

This video is sort of cute:
For the next few days, I am going to examine one gas relationship at a time, the first being Boyle's Law.

Boyle's Law is the relationship of pressure and volume of a sample of gas where the temperature and the number of moles of the gas are not changing,

Pressure x Volume  = Pressure x Volume

As pressure increases, volume decreases -- or as pressure decreases, volume increases -- an inverse relationship.

Gas pressure is due to the collisions of the gas particles with the walls of the container.  Going up in altitude (such as in an airplane), there is less atmosphere and the atmospheric pressure is decreased.  I have had sealed bottles reduce in volume as and airplane has gone up in the atmosphere.

Here's another cute example using a bell jar to remove the surrounding atmospheric gas and reduce the pressure of the system:

Another issue with gas pressure is during scuba diving.  As a person submerges under water, there is more pressure on the body.  This is more of an issue as a person surfaces because there are gases dissolved in the blood. 

Reducing pressure on the body (and blood), gases previously dissolved in the blood can escape, causing decompression illness (aka "the bends" ).  If you want to read more about it, go to: http://www.diversalertnetwork.org/medical/articles/Decompression_Illness_What_Is_It_and_What_Is_The_Treatment

Here's an example of the pressure changes:
The math tends to be pretty simple. One challenge for students can be the use of different pressure and volume units so that they don't cancel.   Students should be aware of different standard pressures depending on the measurement system:

1 atm = 760 mmHg =  760 Torr = 29.92 inHg  =  14.696 psi =  101.3 kPa

Also, volume units can be changed, even though milliliters is the most typical measurement students will encounter.

Including units in their work and being sure the units cancel is always important!

Here's an example of a solved problem:
http://chemistry.about.com/cs/workedproblems/a/bl011804.htm

Please let me know about your experiences, ask if you have any questions or if you have ideas for other topics for this blog.

And, check out my lab book "Chemistry on a Budget"  at:
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

Have a good week!
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Combustion of a hydrocarbon

1/22/2014

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The combustion of a hydrocarbon to produce carbon dioxide and water is a 5th type of chemical reaction.

A fun demonstration is making bubbles of methane gas and lighting them -- a taper or candle taped to a meter stick can be used to light the bubbles up high, away from observers.  Lab goggles are required!

Notice that he is using a funnel attached a rubber hose which is attached to the gas outlet.  The funnel is dipped in soap solution and when the gas is turned on, a bubble can be formed.  BTW, it is more difficult to form the bubbles during dry weather (such as the winter months).

One small challenge of the reaction is balancing -- sometime it's easy (with a hydrocarbon with an even number of carbons):

C2H6  +  O2  -->  CO2  + H2O

Because the oxygen is alone on the reactants side and split on the products side, and reactant oxygen is probably going to be the last coefficient entered to balance the reaction.

C2H6  +____O2  -->  2 CO2  +  ___ H2O

C2H6   +  ____ O2  -->  2 CO2  +  3 H2O

4 +  6 =  10

C2H6  +  5 O2  -->  2 CO2  +  3 H2O

If there is an odd number of carbons (e.g. 1, 3, 5, etc.) then you would need a fractional coefficient for the reactant oxygen and then double all of the coefficients (to get rid of the fraction).

A trick I taught my students was to automatically double the hydrocarbon coefficient -- this will lead to a whole number coefficient for the reactant oxygen:

C3H8  +  O2  -->  CO2  +  H2O

2 C3H8  +  __ O2  -->  __ CO2  + __ H2O

2 C3H8  +  __ O2  -->  __ CO2  +  8 H2O

2 C3H8  +  ___ O2 -->  6 CO2  + 8 H2O

12  +  8 = 20

2 C3H8  + 10 O2 --> 6 CO2 +  8 H2O

But wait!  The coefficients are not in the lowest ratio!  They all can be divided by two!

C3H8  +  5 O2  -->  3 CO2  + 4 H2O

Please let me know about your experiences, ask if you have any questions or if you have ideas for other topics for this blog.

And, check out my lab book "Chemistry on a Budget"  at:
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

Have a good week!

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Double Replacement Reactions

1/18/2014

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Picture
A double replacement is the changing of cations and anions in solution, sort of like changing partners at dance.
Picture
Double replacement reactions can be done with solutions very easily.  Definitely see what's available in your department before you "reinvent the wheel".
This reaction won't go completion unless a precipitate (solid) or a gas is formed, so the observation may be NR for "no reaction".

Safety: Wear goggles and aprons!  It's important to protect your eyes and skin!  And wash your hands after the lab!

A simple video is available online:
http://honorsph.startlogic.com/gmovies/movies/AgCl.mov

And this is sort of cute:
http://www.youtube.com/watch?v=SEqdapiHLNA

One example of a lab is:
https://c7chemistry.wikispaces.com/file/view/CW%20double%20replacement%20lab%20120110.pdf

This site has 2 labs, but I like (1) the chart on the 1st page and (2) the page of reactions to be completed on the 4th page.

A half sheet of  plastic overhead could be placed over the chart on Page 1 and the reactions performed right over the chart.  The chart can then by slid away for the result to be written on the paper, and then slid back.  (Thanks to Ray Hamilton at Greenwich High School for that idea!)

Another lab with fewer reactants is:
http://www.thesciencehouse.org/images/stories/learningactivities/ctc/Expt11.pdf

If you want to mention it, there's a little "Equilibrium" preview in these reactions b/c a precipitate or gas is necessary for the reaction to occur -- this way the reaction will go to completion as the product leaves the system!

These equations involve students working with (a) balancing equations and (b) solubility rules.

Balancing double replacement reactions can be easier if the polyatomic ions are counted as a unit.  For example NO3- as one unit versus one nitrogen atom and 3 oxygen atoms -- if the polyatomic ion changes, this won't work.

2 AgNO3 (aq)  +  ZnCl2 (aq) --> 2 AgCl  (s)  +   Zn(NO3)2

I did allow students to put in parentheses even though they weren't required -- they just had be used accurately.  This would make balancing with polyatomic ions a little bit easier.

For example,

Ag(NO3) (aq)  +  ZnCl2 (aq) --> 2 AgCl  (s)  +   Zn(NO3)2

where the "nitrate" unit was counted versus the individual nitrogen and oxygen.  This made balancing much easier -- sometimes students would put parentheses in the middle of the polyatomic ion -- e.g. (NO)3 vs. (NO3) -- and they would not receive credit for those examples on homework/lab/quiz/test.

I don't really have any other tricks for balancing equations -- my approach was always a very simple comparing back and forth comparison of reactants and products.  I guess I'm sort of "old school" this way.

I'd love to hear about any systems the work for your students!

Some teachers have students remember
solubility rules:

Picture
I know that the New York State Regents provide reference chart that includes a solubility chart for reference on Table F.

http://www.kentchemistry.com/newRT.pdf

This reference table is very handy, and if you're not familiar with it you might want to print yourself a copy.  I would use tables selectively and include some tables on quizzes/tests.

My schedule for this blog is changing... again.. after today, I will post new entries on two days, Wednesday and Sunday.  This way I can provide a well researched entry.

Please let me know about your experience with these labs, ask if you have any questions or if you have ideas for other topics for this blog.

And, check out my lab book "Chemistry on a Budget"  at:
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

Have a good weekend!

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Combination of Magnesium and oxygen

1/16/2014

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This lab is a classic and hopefully a roll of magnesium ribbon is available in your department.  I've worked in departments where the grams per centimeter had been measured, so a small length of magnesium ribbon could be used.  However, if the ribbon sample must be massed, a much longer sample of magnesium ribbon may need to be used (depending on the balance available) -- and you may have crumple it up to fit into the crucible.

This again uses the crucible/clay triangle/clamp/ring stand setup:
Picture
Safety: Laboratory goggles and an apron are to be worn by students.   Notice that the crucible is glowing red from the Bunsen burner flame.  This is a very hot setup, and it takes time for the crucible to cool down.  I've told this tale here before, but I've had students touch this set-up while it was still hot, flinch and knock it over.  Bring the back of your hand close to the setup to see if it's still warm.

Here's one lab that you might find useful:
http://stahlchem.wikispaces.com/file/view/MgO+aca+09.PDF

Now I'm a fan of the "less is more" approach, but it's difficult to find lab procedures that are brief for this experiment.

One concept that can be applied here is that of determining the empirical formula for the magnesium oxide formed. 
  An empirical formula is the lowest whole number ratio of the elements in a compound.  All ionic compounds (metal/nonmetal compounds) are empirical formulas (for example, CaCl2 ) but some molecular formulas can be reduced to an empirical formula -- e.g C2H4 can be reduced to CH2.

Mass data can be collected, converted to moles, the lowest ratio calculated, then reduce to a whole number ratio.  Phew, that's a lot of steps! Let's break that down:

1) collect mass data -- 80 g C and 20 g G

2) convert to moles

 80 g C  x 1 mole C / 12 g
C = 6.7 mol C

20 g H x 1 mole H /
1 g H = 20 mol H

3) calculate the lowest mole ratio-- by dividing both values by the smallest number

            6.7 mol C / 6.7  =  1 mol C

            20 mol H / 6.7 = 2.99 mol H


4)
  Get a whole number ratio by (a) rounding off the numbers or (b) multiplying the ratios by 2 or 3, then r.

Rounding off works well in this case:

1 mol C             2.99
mol H rounds to 3 mol H

So the empirical formula in this case is CH3 !

I hope this lab works out well for you.  Please ask if you have any questions or ideas for other topics

Check out my lab book "Chemistry on a Budget"
!
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

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Decomposition of sodium bicarbonate

1/14/2014

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A decomposition reaction can easily be performed by heating sodium hydrogen carbonate  (aka sodium bicarbonate or baking soda):

                                                    2 NaHCO3 -->  Na2CO3  +  H2O  +  CO2

The carbon dioxide and water leave the system as a gas.  I have performed this reaction in a crucible in a clay-triangle on a ring-stand and over a Bunsen burner, but I lost a lot of product with the rapid release of the gases.  I could actually see the stream of white powder leaving the system when performing this lab. 

Picture
I never tried to use a cooler setting on the Bunsen burner by closing off some of the air vents.  This would result in a flame that is more yellow than blue.  If you end up trying this, write and let me know!I started performing this lab by heating the crucibles in an laboratory oven. 

Remember that the crucible gets very hot!  I've had a few students touch the crucible, flinch and knock over the entire setup.  Be careful! Remind students to detect the heat with the back of their hand, and to use their crucible tongs!

The results can be very good and lead to some good values to work with stoichiometrically.

The Theoretical Yield can be calculated based on the mass of sodium hydrogen carbonate:

# grams NaHCO3 x 1 mol NaHCO3/84 g NaHCO3  = # mol NaHCO3

# mol NaHCO3 x  1 mol Na2CO3 / 2 mol NaHCO3  = # mol Na2CO3

# mol Na2CO3 x  106 g Na2CO3 / 1 mol Na2CO3  =  # gramsNa2CO3

I separated these 3 calculations to aid students in focusing on what is happening and not get lost in the calculations.  Of course, these 3 calculations can be put together:

# grams NaHCO3 x 1 mol NaHCO3/84 g NaHCO3  x  1 mol Na2CO3 / 2 mol NaHCO3  x  106 g Na2CO3 / 1 mol Na2CO3  =  # gramsNa2CO3

Then the lab measurements can be used to calculate an Actual Yield.

These values can then be used to calculate a Percent Error.

                    (experimental value) − (true value)
% error  =  ―――――――――――――  × 100
                                       true value

The absolute value is typically reported (negative symbols are not included).

Here is one lab procedure you could use:
http://phs.princetonk12.org/teachers/FOV2-0001F947/AccelChem/Labs/A19Lab-DecompBakeSoda.pdf

Please write if you have any questions or comments!








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New Schedule

1/11/2014

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Happy Saturday!  I'm starting my revised writing schedule of Saturday, Tuesday, Thursday.  I'd love to hear your comments / questions!

There are several chemical reactions that are good experiences for high school chemistry students, and I'm going to focus on a few through the month of January.

A nice looking experiment is the single replacement reaction between copper wire and silver nitrate solution. 
Picture
The materials may already be in your science department -- you might have a little trouble obtaining silver nitrate crystals or solution.

Here is link to a brief YouTube video showing the lab:
http://www.youtube.com/watch?v=PNaZqnFwIIQ

There are several variations available on the Internet -- here's one that I like:
http://www.e-bookspdf.org/view/aHR0cDovL3FzaWIuZmlsZXMud29yZHByZXNzLmNvbS8yMDEwLzEwL2NvcHBlcl9hbmRfc2lsdmVyX25pdHJhdGVsYWIucGRm/VGhlIFNpbHZlciBOaXRyYXRlL2NvcHBlciBSZWFjdGlvbg==

Let me know how about your experiences with this lab!

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2nd entry

1/10/2014

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TGIF!  Well, it seems that people are stopping by this site, and I'm fixing a few items, including when I'll blog.  It looks more like Saturday, Tuesday, Thursday for now (today is the exception).  So, we'll see how it goes.  I'd love to hear from you!

Around this time of year, I was typically teaching stoichiometry, a big challenge for the chemistry student.  My favorite approach to stoichiometry is the factor-label method because it was less memorization and less room for error.

This is very unit based, a habit I encourage in students.  For example,

5 grams Cu x 1 mole Cu/64 grams Cu =  0.08 mole Cu

Yes, it's the same as memorizing to divide when converting grams to moles, but there less chance for error when a student is in the habit of making sure the units cancel.

There are various "maps" that provide an image of the multiply/divide route, and they are useful.
Here's one:
Picture
Years ago, my students named the various "locations" on the map.  The center was "Moletown" and you could go to "Massville" or to "Particle Pass" or to "Gas Volume Junction" -- unique names do help students remember facts better.

Let me know some of your mole successes and challenges!

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Day one

1/8/2014

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Welcome to my blog!  All of this is new, so bear with me as there may be some problems/ challenges/changes along the way. 

If you are teaching, you're about  halfway through the school year -- congratulations!  If you are going to graduate and start looking for a teaching job, good luck!  It's a big challenge, but you can do it! 

I'd love to hear from you -- until then it might seem a little random while I try to put useful information up.  I might be writing just M/W/F until I think that schedule needs to change.

Hmmm, it's Wednesday -- for a lot of you it's the end of the marking period, so you're grading or getting that last test in, so I'll let you keep at it. 
Bye for now!
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    Author

    Marjorie R. Heesemann is a chemistry teacher with 15 years of experience who is now working to develop resources for the Chemistry classroom.

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