Limiting Reactant

Experiment Objectives

• Apply dimensional analysis and the concept of limiting reactants in an example about baking cookies
• Investigate a chemical reaction involving aluminum and copper metals
• Use both observations and stoichiometry to determine a limiting reactant
• Calculate theoretical yields and percent yield

Safety Considerations

• Wear your safety goggles at all times during the experiment.
• HCl is a strong acid and is corrosive to skin and eyes and toxic to inhale.  Handle HCl solutions carefully.
• Methanol is flammable.  Do not keep containers of methanol near a heat source such as a hot plate.

Experimental Procedure

Record all measurements and calculations into your lab notebook.  Then, complete the questions, data, and calculations on your worksheet for today's experiment.

Task 1

Today, we will work with copper and aluminum. As an element, copper is a shiny brown metal (like a penny) whose surface will slowly change color (like the statue of liberty). As an element, aluminum is a shiny silvery metal. Its surface also reacts, but without the dramatic color change. The reaction we will perform today is between \(\ce{Al + CuCl2*2H2O}\). You will need a balanced chemical equation for the reaction to be able to analyze your results (some background reading). You should be able to figure out the products from your observations.  You can watch a video on how to decant before you start the experiment.

1. Label a 250 mL beaker as “A.” Weigh the beaker and record your measurement in the data and observations table.
2. Using an analytical balance and disposable plastic spatulas and weigh boats, weigh approximately 0.50 g \(\ce{CuCl2*2H2O}\) and record the exact mass. Be careful using the copper chloride salt.  It can damage other metals, so clean up any spills right away. 
3. Using the same analytical balance (it's good practice to use the same balance during an experiment to minimize uncertainty), weigh approximately 0.25 g Aluminum foil and record the exact mass. You can tear off small pieces of aluminum foil until you get to the total mass you need. Do not fold or crumple the foil. 
4. Place the Al and \(\ce{CuCl2*2H2O}\) into beaker “A”. Make sure that the aluminum foil is unfolded so that it will completely react.
5. Label a second 250 mL beaker “B”. Weigh and record.
6. Using a balance and a new disposable weigh boat, weigh out 0.70 g of \(\ce{CuCl2*2H2O}\) and 0.05 g of aluminum foil.
7. Place the Al and \(\ce{CuCl2*2H2O}\) into beaker “B”. Again, make sure that the aluminum foil is unfolded so that it will completely react.
8. Even if you don't see any spills, clean off the balance by using the brush to sweep anything out of the corners or cracks and use a damp paper towel to wipe it clean.
9. Look at the contents of each beaker. Record the color of substances and any other observations (bubbling, heat formation, etc.) that are visible before the reaction beings in the data table.
10. Using a graduated cylinder, measure 50.0 ml of deionized water and add to each beaker. When water is added to the beakers, the \(\ce{CuCl2*2H2O}\) will dissolve and the reaction will proceed.
11. Stir the substances in the beakers occasionally with the stirring rod and break up the aluminum foil as it reacts. The reaction should take about 30 minutes to complete.

WHILE YOU ARE WAITING FOR REACTION TO FINISH, COMPLETE TASK 2.

12. After waiting at least 30 minutes, now record any color changes or any other observations after the reaction into your notebook.
13. When the reaction is complete and you no longer notice bubbles forming, if there is excess aluminum foil still observed in the beakers, add 6 M HCl in 1 mL portions under the hood until the foil is completely reacted and no longer visible (but do not add more than 5 mL). Stir to dissolve. If there is any remaining aluminum foil, carefully remove the pieces leaving the copper behind in the beaker.
14. Allow the solid Cu to settle in both beakers. Decant (pour off the liquid) the solution from the beakers into a waste container.  Be careful not to lose any of the copper.
15. Wash the copper solid with 15 mL of deionized water. Let solid settle. Decant again (be careful to pour as much water off as possible without losing any of the copper solid). Repeat this decanting step once more with another 15 mL of deionized water.
16. Wash the copper solid with 10 mL of methanol. Let solid settle. Decant the methanol into the waste container.
17. Under the hood, heat the beakers on a hot plate at a low setting until dry. Avoid heating at high temperatures for longer periods of time which may cause the unwanted oxidation of the copper product.  If you see any "smoke," carefully but quickly take your beaker off the hot plate to cool it down, and turn down the temperature of your hot plate.
18. When the product appears dry, carefully place the beakers on wire gauze or paper towels.
19. After cooling, weigh the beakers and their contents. Record this in your notebook.

Task 2

Now we'll consider an example showing the importance of limiting reactants in baking.  On your lab bench you'll find three ingredients (the "reactants") needed to bake peanut butter cookies. Look at the provided recipe to see how much of each ingredient is needed for a batch of cookies.  Next consider how much of each ingredient you have by looking at the information provided on the packaging.  For the brown sugar and peanut butter, you'll need to convert the amounts into units of cups using dimensional analysis.  You will need to look up conversion factors to use in your calculations.  The quantity of eggs needed should be an exact number, or a count.  Then determine which ingredient is the limiting reactant.

1. Individually:  First write down your ideas and initial calculations to determine the limiting reactant by yourself and record into your lab notebook.
2. In a Pair:  Once you have completed your calculations, find someone at the same lab bench who is also done and compare your calculations.  Talk through any discrepancies, asking the instructor for guidance if needed.
3. In a group at your lab bench:  Since you have the same starting ingredients at each lab bench, your final answers should all be the same at the same lab bench.  Compare your answers and your work among everyone at the same bench.  Do you have the same answer for what the limiting reactant is?  You should!  Did you all do the same calculations?  Not necessarily, as there are multiple different but correct ways to perform unit conversions and limiting reactant calculations!  Again, ask your instructor if there are any discrepancies you can't resolve.

Once you are done working out all your calculations in your notebook, fill in the worksheet (individually) to complete the Task 2 questions.  

Acknowledgements: This experiment was adapted from Lumen Learning 2014, Chesapeake Campus. https://courses.lumenlearning.com/chemistry1labs/