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Chemical Reactions: Actual and Theoretical Yields

 Dr. Walt Volland, Copyright 1999-2004, all rights reserved revised November 2, 2004

 

In this experiment you will examine the reaction between baking soda and vinegar. The baking soda is sodium bicarbonate, NaHCO3. Vinegar is a 5% solution of acetic acid, HC2H3O2, in water. The products of the reaction are water, H2O, carbon dioxide, CO2, and sodium acetate, NaC2H3O2. The reaction you are working with is shown here. You will collect the carbon dioxide gas and figure the mols of gas produced from the ideal gas equation and your data.

 

 

 

 

 

 

 

 

 

 

NaHCO3(s)

 

+

 

HC2H3O2(aq)

 

=>

 

H2O

 

+

 

CO2(g)

 

+

 

NaC2H3O2(aq)

 

 

 

 

 

 

 

 

 

You will measure the amount of baking soda used. You will collect and measure the quantity of carbon dioxide formed. You will be able to calculate the % yield for the reaction by comparing the actual mols of carbon dioxide collected with the predicted mols from the mol ratio in the equation.

This reaction can also be expressed using a net ionic equation. The reaction is really between the hydrogen ion, H+, of the acid and the bicarbonate ion, HCO31-, from the baking soda. The sodium cation and the acetate anion are spectator ions that do not take part in the actual chemical change.

HCO31- (aq)
+
H+(aq)
===>
H2O
+
CO2(g)

 

 

 

 

 

 

 

Please read entire procedure before you begin the experiment.

It is convenient to have a helper for this experiment; an extra pair of hands is very useful. This is particularly true when you need to steady the 2 liter bottle used to capture the CO2.

Record each measurement on the Report Sheet as soon as you make the measurement.

You will need to use the following.

  • 3 Ziploc bags, 1-gallon size
    •
  • Kitchen sink
    •
  • Empty 2-L soda bottle
    •
  • Long pin like a hat pin or corsage pin to make hole in bag
    •
  • Measuring spoon, 1 teaspoon size
    •
  • Baking soda
    •
  • Measuring cup, 1/2 Cup size
    •
  • Plastic wrap like Saran Wrap
    •
  • Large measuring cup marked in milliliters
    •
  • Regular vinegar, 5% acetic acid, 2 cups
    •

    Get Ready

    • Fill your sink about half-full with water.
    • Clear a work area.
    • Cut a piece of plastic wrap about 6 inches by 6 inches.
    • Lay out a 1 gallon plastic Ziploc bag on your work area.
    • Measure 1 teaspoon of baking soda and place the baking soda in the center of your piece of plastic wrap. Exactly 1 teaspoon of baking soda has a mass of 5.7 grams.
    • Fold the bottom and side edges of the piece of plastic wrap loosely over the baking soda. You do not want to wrap the baking soda tightly because it will be too difficult to unwrap it later.
    • Cut the neck off your 2-L plastic bottle to make an opening that is about 2 inches in diameter.
    • Find the total volume of your cut-off bottle in milliliters. Measure the water needed to fill the bottle by pouring in carefully-measured amounts of water. Fill your cut-off 2-L bottle to the very top.
    • Place your palm firmly over the opening of your 2-L bottle full of water and "upend" the bottle in the sink. Lower your hand and the open end of the bottle into the water in the sink. Make sure that the bottle's opening is below the water level in the sink before you remove your palm. This will keep the water in the bottle.
    • You will need to figure out how to support the plastic bottle so it stays where you put it and doesn't tip over. The bottle may stand upright if you cut a "square" or smooth enough edge. If possible, have your helper hold the bottle in place.

    Carry Out the Reaction
    • Measure 1/2 Cup of vinegar and pour it into the bottom of the 1-gallon size Ziploc bag.
    • Place your loosely-wrapped packet of baking soda in the upper part of this same Ziploc bag.
    • Eliminate as much air from the bag as possible and close it securely so nothing can escape, not even some gas. You will be able to lay the bag almost flat on your counter; the vinegar will be drawn into a pool in the bottom of the bag.
    • Tip the closed bag slightly so that all the vinegar runs into one corner at the bottom of the bag. The packet of baking soda should stay close to the "zipper".
    • Draw a small circle on the other bottom corner with a permanent marker. Lift the top layer of plastic a little bit away from the bottom layer. Poke a hole inside the circle in the top layer of bag(away from the vinegar pool). This will allow you to let CO2 bubble out into the 2-liter bottle after the reaction.
    • Grasp the corner with the hole in it and push that corner of the plastic bag up into the submerged opening in the 2-L bottle. The bag corner needs to be about 1/2 inch to 1 inch up into the bottle, but should not close up the opening in the bottle. You have to leave space for the water to exit from the bottle when the CO2 comes in.
    • Remember, your bottle opening must stay below the water level in the sink.
    • Manipulate the packet of baking soda inside the Ziploc bag so that it comes open and mixes with the vinegar. The vinegar and baking soda should generate bubbles of CO2. You may want to shake or rock the bag and contents. Be sure all the baking soda comes out of the plastic wrap and gets mixed with the vinegar.
    • Your Ziploc bag may puff up with CO2 gas from the reaction. The gas is supposed to be squeezed out through the corner opening and into the 2-liter bottle.
    Measure the Amount of CO2 Produced
    • Catch the bubbles of CO2 in your 2-L bottle. As the CO2 is trapped in the bottle, it will push the water out. This is known as "displacing the water." Be sure the plastic bag is not completely blocking or covering the opening of your bottle.
    • Force as much CO2 out of your Ziploc bag and into the bottle as you can. You want to finish the experiment with as little CO2 left in the bag as possible.
    • When you have all the CO2 out of the bag and trapped in your bottle, place your palm tightly against the opening of the bottle and turn the bottle "right-side-up". You want to trap any water that has not been pushed out of the bottle by the CO2 and keep this water in the bottle as you turn it over. You don't care that the CO2 will escape when you invert the bottle. You are going to measure the volume of CO2 by finding the volume of water "missing" from the bottle.
    • Pour the water that remained in your bottle into your large measuring cup; measure this water volume in milliliters. Subtract this number from the total volume of water your bottle will hold. This difference is the volume of CO2 that you trapped in the bottle.
    • Repeat this experiment twice more.

     Calculate the mols of carbon dioxide

    • Calculate the number of moles of NaHCO3 in 5.7g (1 teaspoon) of NaHCO3.
    • Use the balanced equation for the reaction between NaHCO3 and HC2H3O2 to calculate the number of moles of CO2 that could be produced from 5.7 g of NaHCO3. This is the theoretical yield of CO2.
    • Change the volume of CO2 trapped in your bottle from milliliters to liters. Then use the Ideal Gas Law, PV = nRT, to calculate number of moles, n, of CO2 trapped in your bottle. This is the actual yield of CO2. Assume that P = 1.0 atm and that T = 293 K. The gas constant, R, is 0.0821 liter-atm / mole-K.

     

    n = PV/RT = 1 atm ( V in liters) / (0.0821 liter-atm / mole-K)(293 K)

     

    • Calculate the % yield of CO2.
    • % = 100 x ( number of mols carbon dioxide made/ number of mols of carbon dioxide expected)

    Please put your name on your report sheet before you email it to

    your instructor. Paste the report in an email.

    For people in Blackboard you will normally submit an equiz for your report.

     

    Chemical Reactions Report Sheet

    Data

     

     

    Trial 1

     

    Trial 2

     

    Trial 3

     

    Water used to fill plastic bottle:

     

     

     

     

     

    1)

     

    mL

     

    mL

     

    mL

     

     

    2)

     

    mL

     

    mL

     

    mL

     

     

    3)

     

    mL

     

    mL

     

    mL

     

     

    4)

     

    mL

     

    mL

     

    mL

     

    Total volume of plastic bottle

     

    mL

     

    mL

     

    mL

     

    Volume of water left in bottle after CO2 was collected

     

    mL

     

    mL

     

    mL

     

    Mass of baking soda used

     

    g

     

    g

     

    g

    Calculations

     

     

    Trial 1

     

    Trial 2

     

    Trial 3

     

    Moles of baking soda, NaHCO3 used

     

    moles

     

    moles

     

    moles

     

     

    Moles of CO2 that can be made from this many moles of NaHCO3

    (theoretical yield of CO2) ** Show how

     

    moles

     

    moles

     

    moles

     

    Volume of CO2 trapped in bottle

     

    mL

     

    mL

     

    mL

     

    Volume of CO2 trapped in bottle

     

    L

     

    L

     

    L

     

    Moles of CO2 trapped in bottle

    (actual yield of CO2)

     

    moles

     

    moles

     

    moles

     

    % yield of CO2

     

    %

     

    %

     

    %

    **Calculation set-up for finding calculated or theoretical yield of CO2 for Trial 1.

    Percent yield average

     Trial 1

     Trial 2

     Trial 3

    Average Percent Yield
     % yield of CO2

     %

    %

    %
    %