10/22/13-10/25/13 Scribe Post Author: Serene P
Tuesday, October 22
Today in class, we begun class by checking over our Stoichiometry #4 Partner Quizzes. The last question was a thinking problem that caused a little confusion. Ms. Friedmann then performed a demonstration to help us better understand this question.
We learned that the excess reactant of a reaction will not only contribute to the resulting products but will also retain its chemical identity as another product in its excess.
For a further extension on this type of problem by Kevin M's request, Ms. Friedmann then provided us with another limiting reactants problem that involved some thinking.
Here was the problem:
15.0 g of lead(II) nitrate reacts with 10.0 g of potassium iodide.
1. Which reactant is the limiting one?
2. How many grams of lead(II) iodide will be made?
3. How many grams of all four chemicals will be left in the container when the reaction is over?
4. How much of the excess reactant will be left over?
Here are the answers:
1.
15.0 g Pb(NO3)2 x 1 mole Pb(NO3)2 x 1 mole PbI2 x 461.00g PbI2 = 20.9g PbI2
331.22g Pb(NO3)2 1 mole Pb(NO3)2 1 mole of PbI2
10.0g KI x 1 mole KI x 1 mole PbI2 x 461.00g PbI2 = 13.9g PbI2
166.00gKI 2 mole KI 1 mole PbI2
Since 13.9g PbI2 is the smaller amount, KI must be the limiting reactant.
2.
The reaction will stop when the KI is used up, so 13.9g PbI2 will be made.
3.
Since KI is the limiting reactant, you can use that to figure out how many grams of Pb(NO3)2 will react and how many grams of KNO3 will be made.
10.0 g KI x 1 mole KI x 1 mole Pb(NO3)2 x 331.22g Pb(NO3)2 = 9.98g Pb(NO3)2 will be used up.
166.00g KI 2 moles KI 1 mole Pb(NO3)2
Since 15.0g Pb(NO3)2 were present at the beginning, 15.0g - 9.98g= 5.02g will be left.
10.0g KI x 1 mole KI x 2 moles KNO3 x 101.11g KNO3 = 6.09g KNO3 made
166.00g KI 2 moles KI 1 mole KNO3
Pb(NO3)2 + 2KI → PbI2 + 2KNO3
Pb(NO3)2 : 5.02g of excess will be left.
2KI : 0g left because this was limiting. It all gets used up.
PbI2 : 13.9g will be made.
2KNO3 : 6.09g will be made.
4.
15.0g - 9.98g = 5.02g Pb(NO3)2 will be left.
After working on this problem, Ms. Friedmann taught us the concept of Percent Yield.
Here are the notes:
What is percent yield?
Amount of product that you calculate you should get from a reaction “in theory”
Amount of product that you calculate you should get from a reaction “in theory”
THEORETICAL YIELD
Amount of product that a real chemist gets when they ACTUALLY do the reaction in the lab. : ACTUAL YIELD
Percent Yield = actual yield / theoretical yield x 100 = % yield
Example:
NaOH + HCl → H2O + NaCl salt
5g 5g
What is the limiting reactant? Use it to find how much NaCl (I should make. Theoretical.)
*Suppose I should get 8.47g NaCl (8.47 was a made up number because the real calculation was not done)
*I actually get 7.14g NaCl
7.14g x 100 = 84.3% yield NaCl
8.47g
Homework: Study for Unit 3 Test!
Wednesday, October 23
There was a study session beginning at 7 am today in our classroom led by Ms. Friedmann. The Unit 3 Review packet key and uploaded video were posted on moodle. In class, we took the Unit 3 exam for the whole period. Aside from that, it was Mole Day! Showing great spirit, Ms. Friedmann was dressed up in a mole costume during the study session! 
Thursday, October 24
To start class, Ms. Friedmann gave us a talk that failing is a necessary step in succeeding. Experiencing hard problems will help us be better prepared for future science courses and college. She said that the Unit 3 exams will be returned on Monday, and they will be part of quarter 1’s grades.
Next, Ms. Friedmann explained the next fun lab we were to do. It was termed Micro Mole Rockets Lab.
The purpose of this experiment was to generate hydrogen and oxygen and determine the optimum ratio for their combustion reaction to give water. The optimum ratio will be used to calculate the mole ratio for the reaction of hydrogen and oxygen in a balanced chemical equation. The goal is to find the most ‘powerful’ gas mixture and use it to launch a rocket across the room! We will be comparing the distances collected by each group to see who launched it the farthest.
The hydrogen gas is generated in this lab by the reaction of zinc metal and hydrochloric acid. Yeast will be used to catalyze the decomposition of hydrogen peroxide and generate oxygen gas. We will be using different ratios of hydrogen and oxygen to see which gas mixtures will produce the best ratio. We will conduct a ‘Pop test’ and judge each reaction by the loudness of the produced sound.
Today, we only did the first half of the lab. We did the “Calibrate Gas Collection Bulbs’ and ‘Collect and Test Hydrogen and Oxygen Gases’ portions of the lab packet.
In the “Calibrate Gas Collection Bulbs’ portion, we immersed the cut-off pipet bulb underwater in the 1000-ml beaker. We then squeezed the water out into an empty graduated cylinder to measure the total volume of water in the bulb. We found the total volume to be 6.1 mL. Then, we refilled the pipet bulb and squeezed out half of the total volume into an empty graduated cylinder. We then used a permanent marker to mark the water-level on the side of the bulb. Then, we divided each half into three equal parts.
For the ‘Collect and Test Hydrogen and Oxygen Gases’ portion of the lab, we added 3 M hydrochloric acid to the mossy zinc in one of the hydrogen gas generators. We then capped the tube with the gas delivery stopper. After that, we then added 3% hydrogen peroxide to the yeast suspension in one of the oxygen gas. Then, we capped the generator.
Soon, we cleaned up and continued this lab the next day.
Homework: Complete the prelab questions of this lab.
Friday, October 25
Today, Ms. Friedmann didn’t check in our homework. We went straight to completing the rest of our lab with our partners. We did the ‘Collect and Test Oxygen/Hydrogen Gas Mixtures’ and ‘Rocket Launches’ sections of the lab.
For the ‘Collect and Test Oxygen/Hydrogen Gas Mixtures’ portion, we followed certain steps. First, we filled the pipet bulb with water and placed it over the oxygen gas generator to collect oxygen. Then, depending on what ratio you were testing between oxygen and hydrogen gas, you would remove the pipet from the oxygen tube and place it over the hydrogen tube at a time when a specific amount of water was released from the tube. For example, if you were finding the 1:5 ratio of oxygen and hydrogen, you would quickly remove the bulb from the oxygen tube when the bulb was one-sixth full of gas and place it over the hydrogen gas generator. Then, you would continue collecting hydrogen until the bulb was filled with gas. Then. you should remove the bulb, cap it with a finger, and determine the relative loudness in the ‘poptest.’ You can scale the loudness between 0 to 10. (0=least reactive, 10=most explosive). You always have to collect oxygen first, followed by hydrogen. Then, you must record all results in the data table. After the 1:5 ratio, you should test the other volume ratios of 2:4, 3:3, 4:2, and 5:1.
For the ‘Rocket Launches’ portion of the lab, you should have collected the loudest gas mixture one more time and place the bulb on a rocket launch pad. Then, you were able to ignite it with a piezo sparker to see how far the micro mole rocket would travel. After this trial, you were to collect your best mixture again. Then, you should have left about 1 ml of water in the bulb and launch the micro mole rocket again.
When everyone completed their rocket launch, we entered our recorded distances onto the online spreadsheet, so we could compare who launch their rockets the farthest.
For the lab data, here is what my group collected:
Data Table
“Pop-test” Properties of H2 Gas alone
|
Yes
|
“Pop-test” Properties of O2 Gas alone
|
No
|
Pop-test Properties of O2: H2 Gas Mixtures
Oxygen:Hydrogen Mole Ratio
|
Pop Test Results
|
1:5
|
5
|
2:4
|
1
|
3:3
|
8
|
4:2
|
10
|
5:1
|
0
|
Observations and Notes:
*Scale from 0 to 10
*Our 4:2 ratio seemed to be the best ratio of hydrogen and oxygen.
-When we launched this, it traveled 16 feet(for our group)
*Prior to any water being inserted, the hydrogen gas generator seemed to have been reacting intensely. There was a lot of motion and activity that seemed to have been occurring within the container.
*Prior to any water being inserted, the oxygen gas generator was more passive than the hydrogen gas generator. However, some bubbles were still forming at the top of the substance in the container.
*Our 5:1 ratio was the least reactive mixture.
In our class, the distances traveled by the rockets ranged from 16 feet to 26 feet. After some calculations, I figured that our class average was 23 feet for the distances traveled by our rockets.
According to our rocket lab data for the distances traveled by the rockets, there were two teams who tied for having the greatest distance. With a distance of 26 ft, Pam & Georgia and Elayna & Jordan had the longest distances. Great job!
After cleaning up our stations, we completed this activity. Here is a video I found on Youtube that may help you better understand what we did for this lab. It’s a clip of two students from another school performing the same lab similarly. They explain their every step as they complete the lab. http://www.youtube.com/watch?v=PnHLSkBLCRM
Homework: Complete Lab Packet
This concludes this post. I apologize if combining several days of work into one blog caused any inconveniences for anyone.
The next scribe post author is Ekene N.
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