MAIN THING: Go over the Handwarmer experiment pages, preparing for the actual experiment on Monday. If you'd like a new copy: Handwarmer HandoutIt would be really helpful to make a data table-) Anyway, read and re-read, really carefully. This is an inquiry lab, so the procedure you follow at this point is up to your team, but you will have to write down that procedure in a lab report. WE WILL DO AN ACTUAL LAB REPORT ON THIS EXPERIMENT! lab report guidelines
To practice figuring out the molar heat of solution, work the odd numbered problems on the Enthalpy Calorimetry worksheet 16-4 AGAIN! (which you did last homework period) but add in the final step of calculating the molar heat of solution. ANSWERS My math steps here are much clearer than they were in class.worksheet 16-4 again
Lastly, there are some splendid videos from Mr. Anderson on this page. http://www.bozemanscience.com/ap-chemistry/
Big Idea # 5 - Thermodynamics, videos 46-53 Each video is about 8 minutes, so start watching as you have time-- not all at once --finish up next week. Take notes.
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In Summary
:
During a chemical reaction, energy is needed to break the bonds between atoms. As new
bonds form, energy is produced. The difference between the energy required and the energy produced is called the heat of reaction. heat change heat change heat change That energy difference is usually manifested in a temperature change.
If, as is the usual case, more energy is produced than is required, the excess energy is
given off as heat. This heat causes the temperature of the chemicals, container, and
surroundings to increase. A reaction of this type is called exothermic.
There are some reactions that will proceed even though more energy is required than is
produced. This type of reaction is called endothermic and is characterized by a decrease
in the temperature of the chemicals, container, and surroundings.
Chemists measure the heat of reactions with calorimeters. The energy, as heat, is
absorbed by water in the calorimeter. The difference in the temperature of the water before and after the reaction is used to calculate the heat of reaction. The calorimeter is
insulated to prevent heat exchange with the surroundings. The heat of the reaction is then
equal to the heat absorbed by water and the calorimeter.
Yesterday we attempted to determine how much heat went into our calorimeters. Since we got such divergent results, perhaps we should check our math. I did the lab in my kitchen and got a specific heat of the calorimeter of 63 J/degree Celcius. Here is my data table:
Tinitial/cold = 13 degrees C
Thot = 66 degrees C
Tave = 39.5 degrees C
Tmix = 37.5 degrees C
qwater = (200g)(4.184)(37.5-39.5 C) = - 1673 joules
q water = - q cal ("The heat gained by the calorimeter is equal to that lost by the water, but opposite in sign.")
q cal = 1673
C cal = 1673 / 39.5 -13 = 63 J/ Celsius)
q is "energy transferred as heat" and you might remember q as the quantity of heat, since it is measured in joules.
Are there other sources of error, perhaps? Did the thermometer finish rising/falling, or perhaps too much heat was lost because we did not use a lid? Any other ideas?
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So here's more math - Enthalpy Stoichiometry - and here's a teacher explaining: (the first 8 minutes will help you on this worksheet, so after 8 minutes please stop the video and do the worksheet >>answers at the end of the blog. Correct me if I'm wrong.) Also, watch with pencil and paper in hand, and do the math along with the teacher.
Finish watching the video, and then work on this worksheet Enthalpy Calorimetry, problems 1,3,5 only. All these problems assume no heat is lost to the calorimeter, so the simple equation to use is q=mcAT (I think) also remembering that deltaH=-q (sorry, the blog has no Greek letters to use, that I know about)
(you may notice a discrepance in the signs of delta H)
If you have not already done so, finish the homework page from Monday (Specific Heat worksheet) For Review:
Read textbook 10.6, work the self-check and answer questions 22-28 and 29-31. ( Also review key terms and summary on page 317). My answers will be below...soon. Also, could you email me to let me know how you are doing on this latest bit of math (Q=mc(delta)T and the new stuff). And then back to Hank:
************************************Weird Historical Twist************* In the late 1940s, German theoretical physicist Arnold Sommerfeld, having previously written a series of books in physics: mechanics (1943), electrodynamics (1948), optics (1950), etc., was asked why he had never written a book on thermodynamics? The following is his humorous and frequently quoted answer: [8]
“Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you anymore.”
In an odd twist of fate to this quote, in April of 1951, while in the midst of writing a book on thermodynamics (Thermodynamics and Statistical Mechanics), and having been nominated 81 times for the Nobel Prize (more than any other physicist), but not yet having won, Sommerfeld was killed from injuries after a traffic accident while walking his grandchildren. The book was published post-humorously the following year. [9]
Homework is to complete the Energy Diagram lab from class, as well as the two math worksheets also handed out. The first worksheet is Temperature scales (converting between Fahrenheit, Celsius and Kelvin scales - if you need help, look in textbook 5.7 or online); the second worksheet is about specific heat capacity calculations. The answers are linked at the bottom of the page. As you know, there may be errors. These worksheets should be finished by next Monday, but be sure to get going now so you can ask questions on Thursday if needed.
The textbook sections to cover is chapter 10.4-10.5.
Q = mcΔT
Q is the energy in joules
m is mass in grams
c is the specific heat (from the chart) and the unit is J/g°C (the textbook uses s instead of c)
ΔT is the change in temperature in °C. It could be positive or negative.
Textbook Reading: Chapter 10.1-10.3 + answer the focus questions on page 293.
A very helpful Explanation of the Energy Diagram here. Copy the exothermic reaction graph, and make an endothermic graph, as directed at the end of the explanation.
...to the amount of homework.
1. Limiting Reagent worksheet (last page of the handout)
2. 28 minute video - World of Chemistry13. The Driving Forces Endothermic and exothermic reactions are investigated and the role of entropy is revealed.
Click the VoD button, and keep trying. Take notes.
I got up at 4am and finished it at 8am, so I know it is tough. If you dig in for an hour and a half of concentrated attention, that would satisfy me, and we can finish next week. If you get on a roll, keep going. However, it's hard to make progress if we expect to only "get it" in class, so give it a good shot.
To make it a little easier, here is an app for calculating the molar mass: molar mass calculator.
And the answer sheet has been done by me, so if you find my mistakes, please send me a note.
By the time I had finished, I developed a system: (Here is a form to do the homework out- print out 5 pages- don't worry, it takes very little ink. Percent Yield workpage and Answer sheet
Write the balanced equation.
Figure out the molar masses that will be needed; write in a box on the side. Under that write the actual yield, which is given in the problem.
For Questions 2, 3, 4,9 and 10, you need to find the limiting reactant. Maybe you should do those last. Example 9.8 + the self-check 9.7 is the best example to follow (on page 275 and A20 in your text.) (oops, I had better check my work. I have some fixing to do on those.)
Write the equation: on the left , the given in grams, and on the right, what you are trying to find:
3.74 g Na x ___________x____________x__________=[ ] g Na2O2
Fill in all the molar masses and ratios. [grams>moles>>mole ratio>grams]
Calculate to find the theoretical yield.
Using the actual yield (given) and the theoretical yield (worked), calculate the percent yield.
Think: Does this seem right? Be alert for calculator problems.
If you want to try another video tutor, here's Brightstorm, which is a little different than my way:
