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reaction rates

3/30/2014

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Moving right along, we are entering the topics of Kinetics and Equilibrium.

Objectives:

1.  Explain how the rate of a chemical reaction is influenced by the temperature, concentration, particle size of the reactants, and catalysts using collision theory.

2. Identify and interpret information provided on a potential energy diagram; explain the effect of a catalyst on reaction mechanisms.

Here is a brief presentation about Collision Theory:
What this video refers to as a "fruitful collision" could also be referred to as an "effective collision" .

This video offers a summary of Chemical Kinetics:
Later in this blog I have a video showing a potential energy diagram -- a catalyst being used will lower the activation energy and can be shown there.

Here is a demonstration of the effect of surface area on the rate of reaction:
I've performed the lycopodium powder demonstration with just a candle  in my classroom.  I've used a micropipette to blow the powder into the candle and get a very large flame.  I just didn't do the demonstration with a can (as shown here).

A lab determining reaction rate at various concentrations or temperatures is typical for an introductory chemistry course.  If your department typically performs the Iodine clock reaction, find out what is done -- that's great if it's already being performed! 

If you are starting from scratch, something a little easier would be various effects on the rate of  reaction of Alka Seltzer tablets, which are available at the local grocery store.

One lab example is:
http://www.claireonline.ca/wp-content/uploads/2012/04/RateofReactionLab.pdf

My only edit is "thing to crush the Alka Seltzer" -- if you have it available, a mortar & pestle would work quite well.

Here is another series of labs using Alka Seltzer,  but also baking soda and vinegar:

http://classic.berksiu.org/fleetwood/tp_files/docs/251/Alkaseltzerlab13-14.pdf

Here is a basic worksheet on introducing kinetics:

http://www.gpb.org/files/pdfs/gpbclassroom/chemistry/reactionRatesWkst.pdf

The energy changes in a reaction are shown in a Potential Energy Diagram.  Here is a 12 minute video about the parts of the Potential Energy Diagram:

Here are a couple of pages with worksheets about Potential Energy Diagrams:

http://teacherweb.com/FL/StonemanDouglasHS/ASampson/Ch-22-Organic-Chemistry-review-for-test.pdf

http://fileserver.net-texts.com/asset.aspx?dl=no&id=8192

This link has 4 pages to work with:

http://www.colgurchemistry.com/Chem12/pdfs/Microsoft%20Word%20-%20Ch%2012%20Worksheet%201-2_doc.pdf

*I'd love to hear from you!  Your feedback would really help me to focus on your needs. You might be having a school vacation soon (spring breaks are around this time) -- take advantage of the time and write me about your classroom experiences!  Simply click on the "Contact" tab on the top right of this page.  

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! 

Have a good week!

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vapor pressure, bp/fp, and molality

3/26/2014

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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:

Picture
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!  

*I'd love to hear from you! Your feedback would really help me to focus on your needs, so write to me!   Simply click on the "Contact" tab. 

Have a good end of the week!

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Molarity

3/23/2014

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Continuing with the Solution theme, today's post is devoted to the Solution topic of Molarity.

Objectives:

1.  Describe how to prepare a solution of a certain molarity.

2.  Calculate molarity (M), moles of solute or liters of solution given the other two variables.

3.  Calculate the amount of concentrated solution to mix a given amount of dilute solution.

4.  Solve stoichiometry problems involving solutions of known molar concentrations.

5. Identify the limiting reactant (or reagent) in a chemical reaction and predict the amount of precipitate formed.

The solution concentration molarity is used a great deal by the chemist.  That is because by measuring the volume of the solution, you can use the molarity to calculate exactly how  much solute is contained in the sample. 

Here are two videos about mixing solutions using a volumetric flask.  This first video addresses the calculation quickly.
This second video talks a little more about technique:
Notice how in both videos they are wearing aprons and goggles.  One chemist is wearing gloves.  I've never used them b/c of the chance of the glove slipping so the glassware is dropped, and the possibility of a chemical getting inside the glove and directly on your skin.

Both videos show the use of plastic weighing dishes.  They're a cost of about $20 to $30 for 500 -- they're not typically available in a high school laboratory.  I just used pieces of scrap paper --- 8" x 11", cut in half -- I would fold the paper in half to aid in keeping the chemical on the paper, and for pouring the chemical into a volumetric flask.  Once used, the paper was simply thrown away.

Here is a 12 minute video solving mathematical problems about Molarity.  There are 4 problems, so you might not need to view all of them
Here's a practice problem worksheet, and it's got the answers at the bottom!  Bonus!

http://ptec107.wikispaces.com/file/view/S03_molarity+ws.pdf

It's very typical to order a stronger solution for your stockroom and dilute the solution according to what is needed.  Here is a brief video about dilutions:
Not all high school labs have too many pipets to use -- just do the best you can.  Because the high school lab is not typically performing research, the accuracy may not be as good as a research lab.

This is a 7 minute video (by my favorite, Mr. Post) about solving dilution problems:

Here is worksheet of Dilutions problems:
http://www.nclark.net/DilutionsWorksheet.pdf

An important application of molarity is to the math of chemical reactions (stoichiometry).  Here are a few worksheets:

http://vanmaarseveen.weebly.com/uploads/6/2/6/5/6265786/15-6solnstoichiometry.pdf

http://misterguch.brinkster.net/PRA048.pdf

This worksheet is several pages long, but it has several pages of examples with full solutions, and examples with  answers.   Perhaps you want to give the online link to your students as a supplement:

http://www.eiu.edu/eiuchem/GenChem/downloads/tutorial4.pdf

One lab that I put together in one school was to have the students calculate and mix solutions for a simple  Na2CO3 + CaCl2 reaction.  This was only because I had enough 100 mL volumetric flasks available for the students to mix their own solutions.

Perhaps you could have the class calculate and aid in mixing solutions during a class demonstration, then the students use the solutions in a 2nd laboratory.

The CaCO3 precipitate was very fine and some passed through the filter paper.  I would try larger concentrations to see if the results were improved. 

I found several examples of labs using this reaction, and here's similar lab using strontium nitrate instead of  calcium chloride:
http://www.pleasanton.k12.ca.us/fhsweb/morris/chemlab/Stoich/Stoich%20lab.pdf

This is 10 minute video discussing the concept of limiting reactant (or reagent):
Here's a page of practice problems about limiting reactants with answers:

http://teachers.sduhsd.net/jnewman/Gen%20Chem%20Worksheets/Practice%20Limiting%20Reagent%20and%20Percent%20Yield%20Problems.pdf

This is a longer page on limiting reactants which also has answers:

http://www2.palomar.edu/stem/WorkshopMaterials/Chem115/HO%205%20Limreact%20practice.pdf

*I'd love to hear from you!  Your feedback would really help me to focus on your needs, so write to me!   Simply click on the "Contact" tab.  

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! 

Have a good week!
 

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Properties of solutions

3/19/2014

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I'm moving on to the topic of Solutions, an essential concept because  many chemical reactions occur in solution.   The chemical reactions in our cells occur in solution! 

Objectives:

1.  Define the terms solution, aqueous solution, solute and solvent and give and example of each.

2.  State examples of 5 types of solutions -- solid/solid, solid/liquid, liquid/liquid, gas/liquid, gas/gas.

3. Describe the role of solvation in the dissolving process and use the rule "like dissolves like" to predict the solubility of one substance in another.

4.  Explain the difference among saturated, unsaturated, and supersaturated solutions.

5. Obtain information from a solubility curve (graph of grams of solute vs. temperature in a given amount of solvent).

A solution is a mixture of two substances.  There are two parts, the solute (typically the lesser amount) dissolved in the solvent (typically the greater amount).  Different types of solutions and examples include:

1.  Solid dissolved in solid -- one example of a homogenous (even) mixture is an alloy, where different metals are mixed -- yes, they have to be melted, but once cooled down you have a mixture of two metals.

An example of a heterogenous (uneven) mixture would be the carbon center of a wooden pencil.

2. Solid dissolved in a liquid -- this is the major focus in solution chemistry, and I will talk about concentration work in later posts.

A quick example is sugar (solute) dissolved in water (solvent).  To increase the solubility (ability to dissolve) in the solvent, increasing the amount of solvent and increasing temperature are typical methods. 

To speed up dissolving, agitating (stirring) the mixture is very useful, and increasing the surface area of the solute aids as well.  An easy comparison is the surface area of a sugar cube versus the surface area of the smaller "cubes"  in granulated sugar.

3.  Liquid dissolved in liquid -- this is also a focus in the study of chemistry. 

A method of separating a liquid mixture is by fractional distillation which depends on the boiling points of the components of the mixture.

Several textbooks have a diagram of a fractional distillation column used for separating the components of crude oil.

4. Gas dissolved in a liquid -- this is a mixture students are very familiar with b/c many drink carbonated beverage (soda) so it helps to remind students of how they keep their soda from going flat -- keep on the cap (keeping on the pressure) and keep the soda cold (lower temperature increases the solubility of a gas in liquid). 

5.  Gas dissolved in a gas -- air is a very useful example.  Here is one graph of the composition:

Picture
Here is a page devoted to solvation -- I have to admit that I don't think about it too much, but this is quite comprehensive:
http://www.wisegeek.com/what-is-solvation.htm

 One method of separating a liquid mixture is using a separatory funnel  Two solvents are used, a polar solvent and a nonpolar solvent.  Since polar and nonpolar solvents don't mix (think oil and water), it's a very simple way to divide solutes based on polarity.  One phrase used to remember this difference is "like dissolves like" -- polar dissolves polar and nonpolar dissolves nonpolar.

 I didn't use a separatory funnel until college level study (organic chemistry), but here is a brief video (less than 4 minutes) about how to use a separatory funnel:
A solution that has dissolved the maximum amount of solute in a given amount of solvent and at a constant temperature is considered saturated.  If more solute can be dissolved (where amount of solvent and temperature are constant), the solution is considered unsaturated.

A saturated solution typically has some extra undissolved solute at the bottom.  By the way, this is an example of a solution equilibrium and the solute particles are constantly precipitating and dissolving.  I'll talk about that later.

A solution that has more solute dissolved at the temperature is called supersaturated.  It is an unstable solution and it doesn't take much to disturb the system.

Here is a 10 minute video about preparing a supersaturated solution for demonstration purposes; it also includes a few other examples:
I like that he's wearing his safety goggles throughout!

The information of several solutions can be contained on a solubility curve, which is a graph of grams of solute (y-axis) dissolved in 100 g of solvent versus temperature (x-axis).  Most of the lines are increasing (solid solutes) but sometimes the line is decreasing (gas solute).

Here is an image that shows how a solubility curve can be read, that the line plots the saturated solution, any point below is an unsaturated solution, and any point above the line would be for a supersaturated solution.
Picture
An example of a solubility curve is on the NYS Chemistry Regents Reference Table, Page 3:

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

There is a file of worksheets associated with NYS Regents Chemistry in entry #9 of the Teaching Resources on this website.

Here is a class worksheet based on a given solubility curve:

http://www.docstoc.com/docs/21291723/Solubility-Curve-Practice-Problems-Worksheet-1---DOC

A lab possibility is for the student to develop a solubility curve based on laboratory data. Here's one example:

http://www.kentchemistry.com/Labs/Solubility_of_a_Salt.pdf

Remember to wear eye safety goggles and rubberized aprons!  Be careful with laboratory thermometers!  They can roll off the table and break!

*I'd love to hear from you! Your feedback would really help me to focus on your needs, so write to me!   Simply click on the "Contact" tab. 

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! 

Have a good end of the week!

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dipole-dipole forces, etc.

3/16/2014

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In my last entry, I talked about polar/nonpolar bonds and molecules, changing the focus from inside the molecule and expanding my focus to examine the overall molecule and its shape.  Now,  I'm going to talk about the attractive forces between entire molecules -- the intermolecular forces.

Objectives:

1. Describe dipole-dipole forces.

2.  Describe London dispersion forces and relate their strength to other types of intermolecular attraction.

3.  Explain how a hydrogen bond is different from other dipole-dipole forces and how it is responsible for many of water's properties.

This is brief video (2 minutes) explaining dipole-dipole intermolecular attraction:
From here, I like to examine very weak intermolecular forces (London dispersion forces or van der Waals forces) and very strong intermolecular forces (Hydrogen bonding).

London dispersion forces are the weakest intermolecular forces, typically between nonpolar molecules.  Electrons, being negatively charged, repel each other -- with this separation, a temporary separation of charge occurs, or an induced dipole.
Picture
Dipole-dipole attractions are stronger because the molecules are polar and the separation of charge is not temporary.

An extra-strong dipole-dipole attraction is hydrogen bonding, where the very small hydrogen atom bonded to a highly electronegative atom results in a greater separation of charge and results is a stronger dipole.  Examples of hydrogen bonding include the sugars in molasses, the bonds holding together the strands in DNA (strong enough to hold the two strands together but weak enough to "unzip" for replication), and water.

Here is a brief explanation -- I like how it compares the boiling point differences.  Many textbooks have a similar graph to what is shown here:
Water is a very important substance -- it is one aspect of Earth that is important for life on this planet!

Its high boiling point temperature ensures that it remains water through a large temperature range.  It has a relatively high specific heat -- the temperatures at locations by the ocean are more moderate and fluctuations are not so great as other locations -- that's one reason beach living is so desired.

The hydrogen bonding of water gives it surface tension, which allows for some insects to actually "skate" on the surface.

Here is a very brief video (4 minutes) that provides a simple summary of water's unique properties:

A laboratory experience I performed with my classes involved providing each pair of students a small plastic cup (which the student filled with water), a micropipet, a penny, and a small paper clip.  With these objects, I would guide the students through the following:

(1)  The student would examine the surface tension -- we used the paper clips to poke at the "skin" on top of the water.  

(2) The class would have a contest to see who could put the most drops of water on the head of a penny -- as you add drops, they would accumulate and a large water drop would collect on top of the penny.

(3) The students would try to float a paper clip on the surface of the water -- it is possible, just lay the paper clip flat on the water -- one way is to place the paper clip on your forefinger and immerse  your finger in the water so that the paper clip remains flat. 

Later, you can use a drop of detergent to break the surface tension on the clip will sink immediately, or the drop of water on the penny would collapse.

I didn't have a handout for this experience, I just had students record these 3 phenomena in their notebooks.

Water expands as it freezes, which is good for the life in a body of water (you wouldn't want the fish and plants to freeze!), but this expansion leads to burst water pipes and potholes in our roads.

Here is brief but very informative demonstration on how water pipes can burst:

The following video (short, under 3 minutes) with and explanation of why water expands when it freezes:
Here is a brief worksheet (w/ answers!) on intermolecular forces:

http://www.bluevalleyk12.org/education//page/download.php?fileinfo=QWRkaXRpb25hbF9JTUZfV1MucGRmOjo6L3d3dzYvc2Nob29scy9rcy9ibHVldmFsbGV5L2ltYWdlcy9kb2NtZ3IvMTY4NzJmaWxlMTgxNjM5LnBkZg==&sectiondetailid=1

Here is another worksheet that contains a little molecular geometry and intermolecular forces -- it could be a nice review!

http://www.gpb.org/files/pdfs/gpbclassroom/chemistry/molecGeoForcesWkst.pdf

*I'd love to hear from you! Your feedback would really help me to focus on your needs, so write to me!   Simply click on the "Contact" tab. 

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! 

Have a good week!
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polarity and intermolecular forces

3/12/2014

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To continue with molecular shapes, topics of Bond and Molecular Polarity  are today's focus:

Objectives:
  1. Explain the role and location of electrons in a covalent bond; distinguish between nonpolar and polar covalent bonds based on electronegativity differences. 

  2. Determine the polarity of molecules based on bond type and molecular shapes; relate the polarity and shape of molecules to the properties of a substance.

A covalent bond occurs when the electronegativity difference is 0.0 - 2.0.  An electronegativity difference with a range of 0.0 to 2.0 is considered nonpolar (neutral).  An electronegativity difference of 0.4 to 2.0 is considered to be polar, having a positively and a negatively charged end.  An electronegativity difference above 2.0 is a high enough separation of charge that the bond is considered ionic, with positive and negative ions attacted to eachother.

Below is an diagram with this information.  In this case, the nonpolar bond is called "pure" .
Picture
Here is 10 minute video providing an overview of the types of bonds and electronegativity difference:
The narrator in this video mentions the sharing of electrons in the covalent bond as a "tug of war" -- I've also used this idea, and I've actually used jump ropes to demonstrative covalent bonds in class.  

Jump ropes are inexpensive to purchase at your local store in the toy department.  I put a folded piece of paper with electrons drawn on them, and had students hold the end while I talked and moved the folded paper while talking about polar and nonpolar covalent bonds.


Some older sources classify the limit of the polar molecular bond electronegativity difference as 1.7 rather than 2.0.  There are some examples of bonds that don't fit the 1.7 guideline -- recognizing if there is a metal/nonmetal combination or a nonmetal/nonmetal combination is also important to establishing the type of bond.

Here's a worksheet on electronegativity differences and type of bond:

http://drkschemistry.wikispaces.com/file/view/Electronegativity%20WS1.pdf/413406098/Electronegativity%20WS1.pdf

Here's another worksheet which defines the ionic bond with an EN difference of 2.0 or higher:

http://www.chsdarkmatter.com/electronegativity.pdf

When more than one bond is combined in a molecule, the types of bonds and the symmetry of the molecular shape determines whether the entire molecule is polar or not.

Here's Mr. Post again, in a 14 minute video, to talk about the polarity of entire molecules.  Right at the beginning he points out that he's looking at the polarity of the entire molecule, not just the individual bonds.



In differentiating polar and nonpolar molecules, the physical properties of

 the substances are different as well.   Polar molecules have electric attraction to each other, so those molecules tend to stay together -- the boiling points are not necessarily high, but they tend to be liquids at room temperature.  Some examples are H2O (water), or C2H5OH (ethanol).

Nonpolar molecules have very little attraction to each other and tend to be gases at room temperature.    Many of the diatomic elements are good examples -- hydrogen, oxygen, nitrogen.  Iodine is a solid that turns directly into a gas, also known as sublimation.

*I'd love to hear from you! Your feedback would really help me to focus on your needs, so write to me!   Simply click on the "Contact" tab. 

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! 

Have a good end of the week!


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lewis structures and vsepr theory

3/9/2014

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During a previous post, I talked about ionic, covalent and metallic bonding -- today, I'm going to  revisit some other topics related to Bonding.

Objectives:

1.  Draw Lewis structures to show the arrangement of valence electrons among atoms in molecules and polyatomic ions; explain the difference between single, double, and triple covalent bonds.

2. Predict the shape of a molecule using VSEPR theory.

Lewis dot and line structures are part of the language of chemistry and are important for students to be able to be fluent in the language of chemistry.

Here is one teaching video to introduce the subject.  It's 20 minutes, but it's a good overview -- and, dude, this narrator is enthusiastic!
I teach my students to count how many electrons it would be if full octets are entered  (and duets if  hydrogen is involved) -- then to check if it matches the total number of valence electrons calculated.  This helps students to determine if they are dealing with exceptions to the octet rule.

Here is a very simple worksheet.  It could be used to teach Lewis structures -- you could project the image on an overhead projector to teach the structures.  You may want to switch back and forth from notes to this page during your lesson.
Picture
Here is another link to blank worksheets and a corresponding answer key:

http://teacherweb.com/WA/CloverParkHighSchool/Meldrum/Electron-Lewis-Dot-IampII-and-answers.pdf

One pair of electrons being shared is called a single bond and is drawn with a single line -- for example, H-H.  Two pairs (or four) electrons being shared is called a double bond and is drawn with two parallel lines.  An example is O2.  Three pairs (or six) electrons being shared is called a triple bond and is drawn with three parallel lines.  An example is N2.

Here is another video talking about Lewis structures and VSEPR theory.  The acronym VSEPR stands for Valence Shell Electron Pair Repulsion and helps to determine the 3-dimensional structure of a molecule whereas the Lewis structure is 2-dimensional.

The theory is that the molecule's shape will decrease the interactions of the valence electrons.  Because they are negatively charged they repel, and the molecule shape is to maintain the bonds but keep those electrons as far away from each other as possible.

Whatever shape the molecule takes, it is considered to be stable in this configuration because it minimizes the interactions of the valence electrons.  I'll talk about that more when I cover energy and potential energy diagrams in another post.

Here is a video talking about VSEPR theory -- it's longer (about 20 minutes):


The lab possibilities are mainly building models of various molecules.  Check to see if your department has molecular model kits.  To ensure that the kits are returned without pieces missing, you might want to have students sign them out and back in so they take responsibility for the completeness of the kit.

I purchased a set of Styrofoam spheres at a local craft shop (sewing stores might have them as well) for demonstration with toothpicks. I was able to purchase the Styrofoam balls in both larger and smaller sizes, so I was able to use the larger sphere as a central atom and the smaller Styrofoam balls as the attachments.

Here is a quick (short)  lab handout:

 http://www2.volstate.edu/chem/1110/Molecular_Modeling.htm

This lab is a little longer, but the chart is handy:

http://www.celinaschools.org/Downloads/MOLECMOD_Lab.pdf

*I'd love to hear from you! Your feedback would really help me to focus on your needs, so write to me!   Simply click on the "Contact" tab.

I have started a Twitter account and hope this post (or a link to it) will show up!

For other lab ideas, 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! 

Have a good week!



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heat and energy

3/5/2014

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I'm getting into heat and energy measurements, mainly b/c I talked about the heating curve on my last post.
Objectives for this post are:

  1. State the definition of the calorie; state the definition of the joule (1 kg∙m2/s2). 

  2. Solve calorimetry problems including use of the formulas Q = mc∆T, Q = mHf, Q = mHv.  (formulas listed on NYS Reference Table, Page 12).  
     
 For your reference, the NYS Reference Table can be found at:
http://www.kentchemistry.com/newRT.pdf

The calorie is a unit of energy based on heat -- it is the amount of heat required to raise the temperature of 1 gram of water 1 degree Celsius.  BTW, food calories are kilocalories -- you'll notice that food labels report Calories, the capital C being used in place of "kilo" .

A joule is an energy unit based on motion and is defined as 1 kilogram x ∙meter2/second2 

This is the unit of energy in the SI system (System International) and is based on force and motion.

FYI, 1 calorie = 4.18 joules

Many worksheets have both calorie and joule problems.

Here's a calorimetry  worksheet:

http://classic.berksiu.org/fleetwood/tp_files/docs/251/EnergyandSpecificheatcalculationworksheet.pdf

Here is a similar calorimetry worksheet with answers -- I can't find it w/o the answers:

http://winterschemistry.com/wp-content/uploads/2012/03/Specific-Heat-Answers-2013.pdf

Here is a brief video solving some calorimetry problems:
In the first problem, all of the measurements have 3 significant figures -- the specific heat constant is not considered to be limiting (and has 4 sig figs anyway) -- since you  are multiplying, the answer has 3 significant figures. 

In the second problem, again the measurements all 3 significant figures -- following the rule for multiplying & dividing, the answer also is reported with 3 significant figures.

Here is lab measuring the specific heat of a metal sample:
You can pause the video at the end to gather data to follow along with my calculations.   I'm not worrying about significant figures for this example. 


1) Using Q = mc∆T, you can calculate the amount of heat released by the hot cadmium object to increase the temperature of the water.  Use the mass of the water and the temperature change of the water.

Q = (100.0 g) (1 cal / g C) (3.0 C) --> the water changed in temperature by 25.0 C - 22.0 C = 3.0 C

The grams cancel, and the C cancel, and the answer if 300 calories.

In joules it would be:

Q =(100.0 g) (4.18 J / g C) (3 C) = 1254 J


2) Rearrange Q = mc∆T to calculate the specific heat (c) for the cadmium. 

The Law of Conservation of Energy is the basis of calorimetry.  The heat gained by the water to increase its temperature is the same amount of heat lost by the hot cadmium object.

We calculated the heat gained by the water as 300 calories.  It is now the heat lost by the hot cadmium and is used along with the mass of the cadmium and its temperature change.

c = Q / (m ∆T)

c = 300 calories / (58.953 g x 75 C) -->  the temp change of the metal is 100 C - 25C = 75 C

c = 0.67 cal / g C

This lab handout is a little long, but the illustrations are a useful reference:
http://www.sanjuan.edu/webpages/pmontbriand/files/Calorimetry%20and%20Specific%20Heat%20Lab.PDF

This lab handout is shorter:
http://www.deftstudios.com/webchem/pdf/lab19sm.pdf

Be careful transferring the hot object from the boiling water!  The video I posted above shows the object attached to a string for easier transfer from the boiling water to the cooler sample.

This reminds me of my first lab accident in the chemistry lab.  A student accidentally knocked over his boiling water bath.  I rushed over to make sure he was OK or needed medical assistance -- he said, "No, I'm ok.  My lab apron protected me." 

It is very important for your students to wear proper protective gear-- laboratory safety goggles and rubberized laboratory aprons. 

Eye protection is very important for labs involving chemicals.  They're called splash-proof goggles for a reason!

If your department doesn't have lab aprons, start finding ways the department can purchase them! Talk to the head of your department! Could it be a donation to the school?  It's very important -- don't stop until your department has the proper safety equipment!

*I'd love to hear from you -- I know that some people are reading this blog regularly, but I don't know who!  I'm writing for the chemistry teacher so I hope this information is useful.  Your feedback would really help me to focus on your needs, so please write to me!   Simply click on the "Contact" tab.

For other lab ideas, 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! 

Have a good rest of the week!


















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heating / cooling curves

3/2/2014

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 I'm going to list a few classroom objectives for each blog entry -- we'll see how it works.  It should help me to stay on task and not wander off into other topics.  Let me know what you think of it!

Objectives:

1.  Be familiar with the Celsius and Kelvin temperature scales, including:

      a.  The two fixed points on the Celsius scale; b. the basis of the Kelvin temperature scale (including the term absolute zero);  c. convert Celsius degrees to Kelvins; and,  convert Kelvins to Celsius degrees.

2.  Read a heating or cooling curve (temperature vs. time) to determine phase change temperatures. 

I probably talked about this before, but here is a diagram comparing the Fahrenheit, Celsius and Kelvin temperature scales:

Picture
This is a fairly simple concept and students master it pretty quickly.  It's just very important for Gas Law calculations, so be prepared to remind students when gas laws show up!

Here's a worksheet for practice:

http://www.teacherweb.com/LA/CEByrdHighSchool/Fox,Natalie/TEMPWS1.pdf

Another basic skill is the reading of a heating / cooling curve.  Here's one for example: 

Picture
Eventually it is used for heat calculations, but I'm not going to start that today.

A cooling curve is a downward curve -- sort of a mirror image of the curve presented. 

Here is a worksheet that you might want to use in the classroom:

http://www.cposcience.com/home/Portals/2/Media/post_sale_content/PFC2/ancillaries/skill_practice/9B_reading_a_heating_cooling_curve_PFC2.pdf

This video is a very simple explanation of heating and cooling curves:

I will be talking about heat calculations soon!


Here is a lab collecting time and temperature data that can be plotted for cooling and heating curves.  There is a brief introductory talk about Bunsen burner usage as well.

The substance being used is paradichlorobenzene, the active substance in mothballs.  It is used because its melting/freezing point is 52°C~56°C , so it's easy to use for this lab.

You can't see the narrator, but he's wearing goggles!  Protect your eyes!  *Protect your students' eyes!*

Also, be careful with this cooling bath set-up -- I know I've had a student knock it over before!

Here is a full lab manual including this experiment -- it's the 2nd experiment in the book:

http://mbss.sd5.bc.ca/sites/default/files/Chemistry%2011%20Lab%20Manual.pdf

I have chosen not to include a lab using the TI-84 temperature sensor because not everyone has access to them.  One sensor is $29 (without shipping and sales tax), so a class set of 12 is $348 before shipping and tax.  Maybe your department could buy one every year to get a class set.

Here's the page of Vernier sensors available for your examination:

http://www.vernier.com/products/sensors/

*I'd love to hear from you -- tell me about your lab experiences, ask your questions, or share your  ideas for other topics for this blog!

For other lab ideas, 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! 

Have a good week!
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    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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