Stoichiometry & Chemical Reactions
Objectives:
1. Students will convert data from one unit of measure to another using dimensional analysis and stoichiometry. 2. Students will interpret data from data tables, including the periodic table. 3. Students will demonstrate an understanding of the concepts of mole and Avogadro’s number by setting up and mathematically manipulating conversion factors.
Part 1: The Chemistry of Cookies
Introduction:
The word stoichiometry derives from two Greek words: stoicheion (meaning “element”) and metron (meaning “measure”). Chemists use stoichiometry as a tool to calculate mass and quantity relationships between reactants and products in a reaction. Stoichiometry relies on the balanced chemical equation and serves as a mathematical proof of the conservation of mass.
In chemical reactions, atoms react in fixed proportions, as represented by the balanced chemical equations. However, it is uncommon to have only a small number of atoms interacting in isolation. Even a very small sample of a reactant, for example 1 gram, contains a large number of atoms. For convenience, chemists have defined the following relationships for atoms:
Molar Mass (from the periodic table) = the mass (in grams) of one mole of each element 1 mole of any element = 6.022 x 1023 atoms
Today we will be using stoichiometry to convert unrealistic molar measurements to more functional measurements for a chocolate chip cookie mug recipe. Make your conversions wisely if you want to create something you’ll enjoy eating at the end of today’s class!
60-Second Chocolate Chip Cookie Mug Recipe
Ingredients Recipe Directions
6.78 x 10-2 moles melted butter 1. Very lightly spray the inside of a mug with 3.6 x 10-2 moles granulated sugar cooking spray 2.585 x 10-2 moles brown sugar 2. Add the melted butter, egg yolk, sugars, and 8.8995 x 10-3 moles salt salt 1.045 x 10-1 moles egg yolk 3. Mix 2.61 x 10-1 moles flour 4. Add the flour 4.73 x 10-1 moles chocolate chips 5. Mix 6. Add the chocolate chips 7. Mix 8. Microwave for 1 minute at 80% power 9. Enjoy with homemade ice cream! (See Part 2)
Data Tables
Molecular Formula Table
Note: Most of these substances have extremely complex formulas. I have greatly simplified your work by listing a representative formula only.
Ingredient Formula Brown Sugar C12H22O11 Butter C9H14O6 White (Cane) Sugar C12H22O11 Chocolate C4H8O4 Flour C4H8O5 Table Salt NaCl Egg Yolk C6H12O3N2
Unit Conversions Table
Use the following conversions to get from grams to a standard cooking unit of measure.
Ingredient Mass (g) 1 egg yolk 18.6 1 cup flour 141.95 1 teaspoon salt 4.16 1 cup butter 236.59 1 cup white (cane) sugar 198.73 1 cup brown sugar 141.46 1 tablespoon chocolate chips 28.35
Other Important Information Table
1 cup = 16 tablespoons
Name______Date______
Student Data Sheet
Instructions: Use stoichiometry to convert the 60-second chocolate chip cookie mug recipe from moles to the standard cooking measurements listed for each ingredient in the table below. Refer to the data given in the Molecular Formula Table, Unit Conversions Table, Other Important Information Table, and Periodic Table. Be sure to show all of your conversions in the space provided for full credit. You can staple additional paper if needed.
Butter (convert to tablespoons)
Granulated Sugar (convert to tablespoons)
Brown Sugar (convert to tablespoons)
Salt (convert to teaspoons)
Egg yolk (convert to number of egg yolks)
Flour (convert to cups)
Chocolate Chips (convert to tablespoons)
Instructions: Fill in the blanks for the recipe with the appropriate numeric value from your calculations above. Make sure your calculated value matches the units listed in the recipe.
______tablespoon(s) melted butter
______tablespoon(s) granulated sugar
______tablespoon(s) brown sugar
______teaspoon(s) salt
______egg yolk
______cup(s) flour
______tablespoon(s) chocolate chip
Part 2: The Ice Cream Reaction
What’s a cookie without ice cream? Let’s use chemistry to make some…
Materials 1 cup half & half or milk ½ teaspoon vanilla 2 tablespoons sugar 4 cups crushed ice ½ cup rock salt 2 quart size zip-top plastic bags 1 gallon size zip-top freezer bag
Instructions Pour the first three ingredients into a quart-size zip-top bag. Squeeze out air and seal the bag tightly. Place inside the second quart-size bag, and seal. Place the double-bagged ingredients inside the gallon-size freezer bag. Fill the freezer bag with ice, pour in the rock salt, squeeze out any extra air, and seal. The salt will begin to melt the ice because salt lowers the freezing point of water. Now comes the fun part: Gently shake the bag, making sure the ice is evenly spread out. Continue to gently shake and knead the bag in your hands. The energy from shaking and kneading—and the heat transferred from your hands—causes the ice to melt further. o Melting ice isn’t as cold as frozen ice, right? Not necessarily! Remember that your ice is mixed with salt. As the melting ice combines with the salt, the salt-water solution has a lower freezing point than water alone, so the melted ice-salt mixture is actually colder than the original ice! Continue to shake the bag for 10 minutes. (It’s a long time… but it’s worth it!)
Fun Facts Salt makes ice melt. That’s why people sprinkle it on icy roads and driveways in cold climates. How does salt do this? It actually lowers the freezing point of the ice. Water normally freezes at 32 degrees Fahrenheit, or 0 degrees Celsius. o Salt water is harder to freeze than plain water. You have to make it colder than 32 degrees Fahrenheit in order to freeze it. That’s one of the reasons why a fresh water pond will freeze before one that’s mixed with salt water from a nearby ocean. If you put two ice cube trays in the freezer, one with plain water and the other with a salt water solution, the plain water will freeze first. o The salt ions break apart and interfere with the crystal lattice of the water molecule bonds. The salt-ice water that is formed is actually below freezing. It is super-cooled. Not all types of salt work the same. The larger the salt crystals, the more time it takes to dissolve. This keeps it colder, longer. As salt melts, the compound (NaCl) breaks into two parts—Na and Cl. These particles then disrupt the arrangement of the ice crystals. This process changes the state of matter. For example, the liquid (milk mixture) turns into a solid (ice-cream). As the liquid gets colder it expands. (Have you ever put a soda bottle in the freezer and forgotten about it? The liquid expands and then explodes!) The molecules in the liquid slow down and eventually freeze in place.