CCC: Quantities in Chemical Reactions
CCC: Quantities in Chemical Reactions
Nazila, Meaghan, and Denise
Course: Grade 11 University Chemistry
Unit: Quantities in Chemical Reations
Table of ContentsMisconceptions…………………………………………………………………………………………… 2
Demo…………….……………………………………………………………………………………………. 4
Annotated References………………………………………………………………………………… 5
Safety……………………..…………………………………………………………………………………. 7
Activity 1: How many moles?...... 8
Activity 2: S’mores Stoichiometry……………………………………………………………….. 10
Activity 3: Bag O’Isotopes…………………………………………………………………………… 12
Activity 4: What Substance am I?...... 14
Activity 5: Explore learning limiting reagent………………………………………………… 16
Common Misconceptions/Teaching Strategies
The Mole:
Misconception: A mole is just a number of molecules.
Teaching Strategy: Should emphasize that 1 mole is equal to Avogadro’s constant, 6.02 x 1023. It is a unit of measure and can be considered a “counting word” (ie. it is has a number meaning). Can compare different counting words, like a couple (=2) or a dozen (=12), and then the mole (=6.02 x 1023). Like other counting words, the mole can be used to count anything. In chemistry, the mole is a standard number of particles, 6.02 x 1023 molecules, or atoms, or ions, etc. In fact, the mole is an SI Unit for an amount of any substance (Cerveliati et al. 1982).
Misconception: If two substances have equal masses, they have the same number of moles.
Teaching Strategy: Different hands-on activities should help. Can count out the same number of different candies, like M&M’s and Smarties. By comparing the masses, students will see that equal numbers of things don’t necessarily have equal mass. Also, can show students that equal mass does not mean equal numbers of things by counting how many peanut M&Ms are in a bag (49 g) versus regular chocolate M&Ms (50 g).
Chemical Proportions in Compounds:
Teaching Strategy: Molecular modelling kits or Lego can be useful in showing the difference between an empirical formula and a molecular formula. You can mass the atoms making up an empirical formula and relate it to the mass of atoms making up different molecular formulas (that are whole number multiples of the original empirical formula). E.g., compare mass of modelling kit / Lego atoms making up the empirical formula CH and the molecular formulas C2H2 and C6H6.
Stoichiometry:
Misconception: You can change the subscripts in chemical formulas to balance a chemical equation.
Teaching Strategy: Review the importance of subscripts in determining what a substance is, and that if subscripts are changed, the substance will change. Remind students that only the coefficients in front of the chemical formula should be changed when balancing equations. Remind students that if there is no coefficient in front of a chemical formula, it is 1.
Misconception: The number of moles of reactants must equal the number of moles of products (BouJaoude & Baraket 2000).
Teaching Strategy: Make sure that students realize that balanced equations need to satisfy the Law of Conservation of Mass (ie. same number of atoms of each element as reactants and products) NOT number of moles of reactant = number of moles of product. Can get students to balance equations and then calculate the number of moles to prove point.
Misconception: To calculate the molar mass of a chemical, you have to add the atomic weights of all the atoms in the chemical formula together, and then multiply/divide it by the coefficient in front of that chemical formula (BouJaoude & Baraket 2000).
Teaching Strategy: Molar mass is independent of the amount of a substance in the reaction (always g/mol, NOT g/ 2 mol or g/ 3 mol). E.g. Molar mass of water, H2O, is 18 g/ mol regardless of which chemical equation it is in. Therefore only need to look at chemical formula when calculating molar mass.
Misconception: The reactant with the smallest number of moles is the limiting reactant.
Teaching Strategy: To show students that it isn’t how many atoms/molecules you have of something, but the RATIO of reactants that is important in determining which is the limiting reactant. Hands-on activities can help, such as the S’mores activity from our Resources Workshop.
References:
Cerveliati, A., A. Montuschi, and D. Perugini. 1982. Investigation of Secondary School Students’ Understanding of the Mole Concept in Italy. Journal of Chemical Education, 59 (10): 852-856.
BouJaoude, S. and H. Baraket. 2000. Secondary school students’ difficulties with stoichiometry. School Science Review, 81 (296): 91-97.
Demo: Introduction to Limiting Reagent
Purpose: This simple demo will introduce students to the idea that one of the reactants is usually in short supply, so is the “limiting reagent” for a particular chemical reaction. The two different scenarios in the demo help students understand that the same reactant is not always the “limiting reagent”. You need to consider the ratio of reactants before deciding which one is the limiting factor.
Assumed Prior Knowledge: Students will need to have covered the Chemical Reaction Unit in Grade 11.
Materials:
-candle
-matches
-large glass beaker or jar
Safety Issues:
-ensure quick access to a fire extinguisher
-tie hair back with an elastic and tuck in loose clothing
-make sure match is completely out before disposing in garbage can!
Directions:
1) Set up candle, matches, and large beaker on workbench.
2) Ask students if can recall what limiting and excess reagents are.
Scenario 1
3) Light candle and place on workbench with open access to the room. Discuss a potential chemical equation for the combustion reaction of the candle with oxygen in the air (can provide chemical equation for wax, but students should be aware that oxygen is a reactant and that carbon dioxide and water are produced- as an extension, can have students balance this equation).
[Write on board]
C25H52 (Wax) + O2 → CO2 + H2O
Balanced equation:
C25H52 + 38 O2 → 25 CO2 + 26 H2O
4) Ask students if this reaction will continue forever, or if one of the reactants will run out (candle will be used up first). Since the candle gets used up first and there is plenty of oxygen left in the atmosphere (we can still breathe!), the candle is the “limiting reagent”.
Scenario 2
5) Pose scenario of covering candle with large beaker. How does this change the amount of reactants available to undergo combustion? (Oxygen supply is now limited). Ask students which of the reactants will be used up first and how will we be able to tell? (When run out of oxygen, flame will go out and some of the candle will be left over).
6) Cover candle with beaker and observe what happens (candle will go out). Ask if the candle or the oxygen is the “limiting reagent” now (oxygen is now the “limiting reagent”).
Summary
7) Bring student attention to the fact that in one case the candle was the limiting reagent, and in the other case oxygen was the limiting reagent- this shows that one particular substance is not always the limiting factor in a chemical reaction.
8) Secondly, bring up the point that you need to look at the ratio of reactants to see how many of each need to interact for the reaction to occur. Can get this information from the balanced chemical equation (ie. need 38 oxygen molecules to react with one molecule of wax in this combustion reaction).
Annotated References:
Celebrate Mole Day with help from the National Mole Day Foundation!
http://www.moleday.org/
This not-for-profit foundation has a website where you can learn this year’s Mole Day theme (“Molar Express”), learn more about Amadeo Avogadro (Mole Day History) and get great suggestions for Mole Day Projects (Mole piñata, anyone?). Links to other websites are limited and other parts of website are somewhat commercialized (but you can get a Mole-themed T-shirt if you like!)
ExploreLearning: Chemical Reactions Gizmo Collection
http://www.explorelearning.com/index.cfm?method=cResource.dspResourcesForCourse&CourseID=336
Can look at any Gizmo (interactive animation) on this great website for 5 minutes each Gizmo. Can sign up for free 30-day trial for better access. Chemistry section includes great animations, including the animation on limiting reactants in our Resources Workshop. See rest of website for a range of math and science gizmos for all grade levels.
Queen’s University Faculty of Education Resources: Stoichiometry
http://educ.queensu.ca/~science/main/concept/chem/c11/c11main.htm
Great list of different education resources for Stoichiometry, including Concept Development, Demos, Tips, and Labs/Activities. This is definitely worth exploring. Highlights include fun ideas like teaching balancing of equations using Oreo cookies or Lego. The “Comprehensive Stoichiometry Outline” link has a lot of good problems, “Another Stoichiometry Flow Chart” is an excellent graphical organizer, and “Balancing Equations- 3D Demo” is the idea of actually weighing molecule models of reactants and products to “balance” equations.
Chemistry: Ten Cool Sites
http://apps.exploratorium.edu/10cool/index.php?cmd=browse&category=4
Since 1995, this website aims to feature the best Chemistry websites it has found. A newer website it features is “Chalkbored”, which has links to lessons and even Powerpoint presentations! Also, a neat website is “What’s that stuff?” which is all about exploring the chemistry behind everyday items. For this unit, check out the “Chembalancer” website, which features interactive games to help students review how to balance equations.
Chemistry Lecture Notes
http://www.chemistrylecturenotes.com
This website has straightforward notes in a slide format that would really help students (and teachers) learn or review various topics in chemistry. The “Mole Calculations” lecture notes seemed to provide a good overview of the material. The “Stoichiometry” section may cover more than is needed under the new curriculum for this unit.
Mass Spectrometry Interactive Animation
http://www.colby.edu/chemistry/OChem/DEMOS/MassSpec.html
This interactive but basic animation will allow you to show students each part of the mass spectrometer and illustrates how heavier ions aren’t deflected to the same extent as lighter ions. Need Shockwave program installed on your computer to see this animation.
Happy Mole Day Video
http://www.youtube.com/watch?v=ReMe348Im2w
This trippy video is a fun introduction to Mole Day, on October 23! It has a fun review of the basic concepts around the mole including Avogadro’s constant, Mole ratios, Molar Mass, and volume of one mole of any gas at STP.
Safety Precautions and Tips
Chemical reactions are frequently used in this unit to demonstrate principles of limiting and excess, percent yield, as well as acid base titrations to determine mole ratios. The following safety precautions should be taken
Procedures
1. Have students identify the chemicals used and their individual safety warnings. Concentrated acids and bases can burn skin and clothes.
2. Use the fume hood when necessary, do not inhale vapours at any time
3. Research proper disposal for each chemical and make it CLEAR to students what they are to do with reaction products and excess chemicals.
4. Be mindful of Bunsen burners and hot plates and the inexperience students in grade 11 may have with such equipment.
5. Post emergency phone numbers by the phone
6. If making use of activities involving food, do not allow students to eat food used for activities.
7. Set up glass disposal as at least some glass breakage is bound to happen!
General Tips:
- Conduct a simple safety lab prior to assigning content related labs
- As the teacher, regulate chemicals, clean up, and equipment useage
- For classrooms which don’t have a separate lab area, make sure that books, bags, and other belongings are far removed from lab benches.
Activity #1
How many Moles?!
Teacher Information
Activity Rationale:
Because the Avogadro constant is a huge number students lose connection to the mole as solely a unit measurement. A mole’s magnitude can be more easily understood by visualizing the amount in everyday ordinary items. This activity presents the mole as a tangible and accessible means of expressing a quantity of atoms.
Rather than just being able to do the calculations, this activity will help students thoroughly understand the concept and practicality of mole. This activity should also help combat the misconception that moles of different substances have the same mass.
Explanation:
In this activity students will determine the number of moles of chalk used to draw a self portrait.
1. They will choose a piece of chalk and find the mass using an electronic balance
2. They will draw a self portrait and find the mass after the drawing
3. They will determine how many moles of chalk were used in the drawing. To do this they will need a periodic table to find the molar mass of chalk, CaCO3 (molar mass = 100.09g/mol).
4. Moles of chalk will = mass of chalk used in drawing/ molar mass (100.09g/mol).
Activity Extension:
This activity can be used as a demonstration or introduction into the Mole Scavenger Hunt. The Mole scavenger hunt should be adapted to your specific students and classroom but may involve such activities as:
- drink a can of mountain dew – how many moles of sugar were consumed?
- How much space is required to plan a mole of blades of grass?
- How many recycling bins are needed to recycle a mole of newspapers?
Name
How many moles?
Activity:
In this activity you will determine how many moles of chalk it takes to draw a picture of yourself! Make a guess first! How many moles will it take?
Instructions:
1. Choose a piece of chalk. Find its mass in grams using the electronic balance. Record the mass in the table provided.
2. On the chalkboard, draw a self portrait.
3. Find the mass of the piece of chalk again and record in the table.
Mass of Chalk (g)Before drawing:
After drawing:
4. How many grams of chalk were used in the portrait?
5. The chemical composition of chalk is CaCO3, using your periodic table, determine the molar masses of each element. What is the total molar mass of chalk?
Ca ______
C ______
O3 ______
Molar mass CaCO3 ______
6. Use the formula n = m/M to determine how many moles of chalk were used to draw your self portrait.