Today is 17 July 2008 -- Periodic Table and Air Pressure lessons

Remember, period 1.  If you are going to claim extra credit for spelling names with the periodic table symbols, your images have to be emailed to me or transferred to my computer by Friday.  Send them to abinc@aol.com.  Remember also your homework and notebooks are due on Friday.  Do not procrastinate!

Period 2 students, remember that your barometers are due on Friday for me to inspect.  That means completely assembled with card pointer and calculations done to show what a change in pressure would read.  Tomorrow -- Thursday -- we either fly a hot air balloon or study how to find absolute zero. 

Yeah, yeah -- I know.  Buzz Lightyear would say, "To absolute zero -- and beyond!"

About your Friday quiz:  Period 1 will be heavy on the Periodic table and trends.  I strongly advise you to answer the questions for HOLT Ch. 4 (Periodic table) in your workbooks.  Period 2 will be problem solving for gas laws and molarity.  Expect something on partial pressure and Graham's law of diffusion.  YOU WILL NEED YOUR SLIDERULES, SO GET COMFORTABLE USING THEM.  Ditto scientific notation.  

Today is 22 July 2009 -- We continued exploration of Boyle's law

We used mini marshmallows and a 60 ml syringe to study pressure/volume relationships.  When we put our thumbs over the nozzle and pressed on the plunger, the marshmallow shrank.  This had to be because the marshmallow was full of air.  When we pulled a vacuum on the syringe, the marshmallow grew larger because the air in it expanded.  This really demonstrated the inverse relation on a fixed amount of gas by volume and pressure.  If the pressure goes up, the volume does down, and vice versa. 

Students were asked to write up a paragraph reflecting their understanding of this principle. 

Then we explored pressure itself.  We made a water barometer.  Students had to understand that mercury, used in barometers since the time of Torricelli, are prohibited in schools.  Mercury, being 13.543 times as dense as water can be used in a glass column 760 mm long to measure the pressure of the atmosphere.  In other words, a column of mercury 760 mm long will balance the pressure of the atmosphere.  You learned that the weight of a column of air one inch square, from sea level to the stratosphere weighs 14.7 lbs.  We call that "air pressure" and it is stated as lbs per sq. inch.  Since we can't use mercury, we used water.  Students found that the column of water required to balance the atmosphere was about 33 feet.  (760 mm x 13.534 g/cc)/(25.4 mm per inch x 12 inches per foot).  We took a length of plastic tubing 35 feet long, filled it with water and hung it from the 3rd floor landing.  The top was sealed and the bottom was in a bucket of water.  The water column in the tube fell to 33 feet, creating a vacuum in the top of the tube.  The students were told to follow a specified report format.

We then started talking about Charles' law which relates temperature and volume.  The students saw a demonstration of what happens when a balloon is fitted to a flask, and the flask is heated.  The air inside the flask expands, forcing the balloon to inflate.  When water is put in the flask, the balloon expands even more as the water is vaporized. 

Students had a homework assignment to express the temperatures 212, 32 and 800 degrees F in Kelvins.  They were shown the formulas:  K = degrees C + 273.15;  degrees C = (degrees F - 32) x (5/9).

Tomorrow we will continue with Charles' law, solving problems.  We would also like to do a lab on finding absolute zero.

Today is 22 July 2009 -- We are studying the Gas Laws in some detail

We studied the meaning of the gas laws in some detail.  Robert Boyle discovered the relationship between pressure and volume.  He found that as you increased the pressure on a fixed amount of gas, the volume decreased, and vice versa.  In other words, pressure is inversely proportional to volume. That is, P α 1/V.  As an equation, this can be written as PV=K where K is a constant.  

Then, the French scientist Jacques Charles, while working for the Montgolfier brothers to help them with their hot air balloon, discovered that volume of a fixed amount of gas is directly proportional to temperature.  That is, V α T.  As an equation, this is V/T = K where K is a constant.  

And another Frenchman, Joseph Gay-Lussac, found that pressure of a fixed amount of gas was also proportional to temperature.  That is, P α T.  As an equation, this is P/T = K.  

We demonstrated Boyle's law with a plastic syringe and mini marshmallows.  As pressure was applied, the marshmallow (inside the syringe) "shrunk."  When the plunger was pulled back, air inside the marshmallow caused it to expand.  We demonstrated Charles' law by fitting a balloon to a flask and heating it.  As the air was heated, the air inflated.  When we added a small amount of water and heated it, the steam inflated the balloon quite a bit.  After the water had all evaporated, the temperature of the steam continued to increase and the balloon inflated even more.  For considering Gay-Lussac's law, we discussed what happened when an aerosol can of hair spray was used.  It takes energy to expand and that energy comes from the molecules inside the can.  Consequently, as the container valve is operated, the spray leaves it and heat is removed from the rest of the spray in the container, cooling it.   This is how air conditioners work, too. 

The three laws were combined in the -- logically enough -- "combined gas law," P₁V₁/T₁ = P₂V₂/T₂.  This was based on the fact that PV/T = a constant, so if the same amount of gas has any condition changed, the other conditions will adjust to maintain the constant.

The combined gas law says that for a fixed amount of gas at a given pressure, temperature and volume, if you change anything, at least one other condition will change.  For example, for a gas initially at 1 atmosphere pressure (same as 14.7 pounds per square inch, or 760 mm Hg, or 760 torr), occupying 1 liter volume, and at zero degrees Celsius, if it is warmed by 10 degrees, but the container volume is kept constant, then the pressure will go up.  If the container is allowed to expand, the pressure will stay constant. 

Since PV/T = constant = K, a good question is "What is the nature of the constant? Just what IS 'K'?"  By specifying the conditions to apply to just one mole of gas, we can explore this point.  Through experiments it was discovered that one mole of gas occupies 22.414 liters at standard temperature and pressure, STP.  What is STP?  It is a pressure of one atmosphere at zero degrees Celsius.  Let K =nR where "n" is the number of moles and R is titled the "Gas Constant."  Then PV/T = nR.  This is usually written "PV = nRT."  Here, we rearrange to solve for R.  That is R = PV/nT.

By the way, T is in units of Kelvins, or K.  This unit was named in honor of William Thompson who was dubbed "Lord Kelvin" due to the importance of his discovery.  He found absolute zero, the temperature at which motion stops.  It is 273.15 below zero on the Celsius scale.

So, to solve the ideal gas law for R we have: R = (1 atm)(22.414 liters)/(1 mole) (273.15 K).

This yields R = 0.0802 liter ‧ atm/mole ‧ K.

Clearly, if you use different units for the volume or pressure, the value of R will change. 

Importantly, when you are given problems, you can generally just plug in the appropriate values to solve them. 

FOR TOMORROW:  We will explore the experiment by Lord Kelvin to find absolute zero.  We will examine how the volume of a gas changes with temperature, and then extrapolate on a graph to the intersect with the temperature axis.  That intersect represents absolute zero. 

REMINDERS:

1. Have your class notes ready to hand in so I can grade them for your three-week report.

2. You will have another on-line quiz over the weekend.  It will cover the gas laws and the barometer.

3.  If you can finish  the absolute zero experiment on Thursday, I'll have you do another one on Friday involving calculation of the amount of reactants and products from the volume of a gas produced in the reaction. 

Today is 28 July 2009 -- Today we focused on the carbon dioxide evolution lab in our study of the Gas Laws

Students calculated how much vinegar (containing 5% acetic acid) did it take to react with a weighed amount of bicarbonate of soda.  The bicarb was put in a flask and the vinegar was put in a balloon.  The balloon was fastened to the flask and the vinegar tipped into it.  The carbon dioxide that was produced caused the balloon to inflate. The students determined the volume of the balloon.  From the volume, they determined how much bicarbonate had actually reacted and calculated a yield of actual to theoretical.  Formulas used were:

1.  Gross weight - tare weight = net weight

2.  moles = mass (in grams) / atomic weight (sodium bicarbonate)

3.  Stoichiometric ratio bicarb to acetic acid determined.

4.  grams acetic acid needed = moles x atomic weight of acetic acid.

5.  weight of acetic acid in grams x 100%/5% = grams of vinegar needed

6.  grams vinegar = ml vinegar

7.  balloon circumference (C) = Pi x diameter (d)

8.  d = 2 radius (r)

9.  Volume of balloon (V) = 4/3 x Pi x r(exponent 3)

10.  moles CO2 = P x V/R x T where R = gas constant (0.082 liters.atm/mol.K) and T is temperature in Kelvins

11.  K = oC + 273.15

12.  mole ratio CO2:NaHCO3

13.  moles NaHCO3 x atomic weight NaHCO3 = grams NaHCO3 actually reacted

14.  grams Actual / grams used in equation 1 x 100% = yield

Students were told to save the solution that results.  It should be clear.  If not, it means that some of the bicarb was left unreacted.  The solution, now sodium acetate, is something that we want to use in our study of solutions.  Specifically, when sodium acetate is in a "supersaturated solution," meaning there is more sodium acetate than should actually be capable of being dissolved, and a "seed crystal" is injected, the solution will crystalize and release heat.  It is packaged and used as hand warmers.  I would like to set you up to make hand warmers by evaporating the solution you have.  I need something to activate the solution, which is what I am now looking for.  Commercial warmers use a thin, stainless steel disk. 

Many of you need practice solving problems.  I'm giving you a few (below) to practice on:

1.  A "molar solution" is made by dissolving one mole of a substance in water, then topping up to 1 liter.  The concentration is thus one mole per liter.  What is the concentration of 4 grams of table salt, sodium chloride, in one liter of solution?

2.  Hydrochloric acid, HCl, is typically used at a concentration of 1 Molar, or one mole per liter of solution.  How many grams of HCl is needed to make a 1 Molar solution?

3.  Concentrated hydrochloric acid is 37.5% by weight.  It is made by bubbling HCl gas through water until the water is saturated.  How many grams of HCl gas are in 1000 grams of concentrated HCl acid?  How many moles is this? 

What we did in class 30 July 2010 -- "Gas Laws"

We started our study of the Gas Laws.  We described Gay-Lussac, Boyle and Charles' Laws.  We worked sample problems and discussed the history of how the laws were developed.  I said that math applied to science continues to be a challenge for many students, and that when they understood the basics, their minds would be free to study other matters -- that would help them become better students.  To help in this, I offered to teach how to make and use the slide rule to multiply and divide, and other math functions used in science.  This would be (basically) on-line so it could be studied out of class. 

Homework:  Select AT LEAST three problems from EACH of the problem groups on page 446 of your text (Holt -- Chemistry) starting with problem 35 and going to problem 51.  It would be best if you do all 17 problems, but 9 is acceptable.  REMEMBER:  that is 3 from Nos. 35 - 40; 3 from 41 - 46 and 3 from 47- 51.

On Monday, 2 August, my students needed to bring their water bottles with the steel wool inside ("In Rust We Trust" experiment).  We will check the final volume and weights.  My students will then have until Wednesday to hand in the report of the experiment