Lab 5 Instructor Notes

Lab 5: Limiting Reactants

Remind Students:

  • Goggles MUST be worn whenever ANY group has chemical at their station.
  • Review safety
    • Metals react with water.
    • Acid is caustic.
    • Methanol is flammable.
    • Hot glassware looks like cold glassware.
  • Make sure they secure all equipment in bin when moving between the cart and their stations.
  • All glassware must be cleaned and dry (in their bin) before leaving lab.

When reviewing the protocol explain:

  • Limiting reactants cannot be determined by mass, appearance, or balanced equation. They will have to use stoichiometry.
  • Molar mass of calcium chloride dihydrate must include mass of both waters.
  • Explain the equation in the pre-lab IS balanced. (They will not understand that there are indeed 6 mol of water on both sides).
  • You may want to do one sample calculation (using different numbers).
  • Remind students to not touch the metal.
  • Write on the board “Beaker A” and “Beaker B” and list what goes into each.
  • Get as close to the indicated masses as possible.
  • Only need to use the analytical balance for the 0.05 g of Al. (need 2 SF).
  • Remind them the observations are just to indicate what a chemical reaction looks like and not to indicate limiting reactants.
  • If Al is left, can add HCl in 1 mL increments. Stir gently to initiate the acid reacting with the Al but to prevent the Cu from getting mixed up. (Will have to wait for Cu to settle before decanting)
  • They should NOT add more than 5 mL HCl. There will still be Al left in one of the beakers. It will be okay, they can either pick it out later with tweezers or leave it and let it contribute to error.
  • Explain how to decant. Decant after each addition of water.
  • The methanol just ensures the product will dry in a small amount of time. .
  • Once they have decanted the methanol THEN they can go to the hood
    • (NOT BEFORE). We do not want the acid bottles or methanol near a heat source.
  • Only heat on LOW heat until the product is dry (~10-15 minutes).
  • Hot glassware should be set on wire gauze or paper towels to prevent breakage. Hot glassware will also mess with the calibration of the scale.
  • Once beaker has cooled, either remove the aluminum with tweezers (if they want) or weigh the product.
  • Talk them through the post-lab questions. What does their percent yield mean? How could they have gotten more or less than 100% yield? Stoichiometry lets them go from reactant to reactant, from reactant to product, or anywhere!