003 Limiting Reagent

Expt 003 -- Limiting Reagent - Explosions

An inexpensive calibrated plastic sparking apparatus (known as a eudiometer) is assembled. Combining volumes can be determined for the ratio of H₂ to O₂ and the ratio of H₂ to air. This experiment duplicates one of the landmark experiments in chemistry with an explosive demonstration of limiting reagents and mole ratios.

Chemical Concepts

Hydrogen reacts with oxygen in a very quick and exothermic manner.
Additional energy is often required to get reactant particles to collide hard enough to initiate a reaction; this extra energy is known as the activation energy. Electric sparks add energy to the molecules.
Gas volumes in this experiment are directly related to the moles present. The combining volumes can be used to determine the ratio of the combining moles of material.

PV = nRT
The pressure and temperature are the same for the hydrogen and oxygen gases. Assume that hydrogen is 1 and oxygen is 2.
(PV₁)/ (PV₂) = (n₁RT)/(n₂RT)
or
(V₁)/ (V₂) = (n₁)/(n₂)
The molar ratio of reactants consumed depends on the chemical equation. Either reagent in a reaction may limit the extent of the reaction.

Safety

Goggles are a must. Flying plastic can damage eyes. Even students should wear goggles while you demonstrate.
NEVER use a glass tube for this experiment.
Hydrogen and oxygen form an explosive mixture. Mix only very small amounts. If too much is used, the tube empties completely and air is drawn in, invalidating the results. As a rule of thumb, keep the total volume of the gas mixture to less than one-fourth the volume of the entire tube.
Set up a FIRE EXTINGUISHER nearby.

Procedure

Set up the apparatus. Add colored water to a plastic cup. Pour the water into the sparking tube. Fill completely. Place a finger over the end. Use care to exclude all air. Invert the tube. Submerge the end of the tube covered by your finger under the surface of the colored water, and remove your finger. Secure in the buret clamp.

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Hydrogen

Fill a modified zipper plastic bag with hydrogen. (See Lab Hints for source.) Empty a large plastic syringe (fully depress the plunger), then connect it to the hose on the modified bag and fill the syringe with hydrogen. Disconnect the syringe, push out the hydrogen, reconnect the syringe to the tube and refill it with hydrogen. This gas rinsing ensures a relatively pure sample of hydrogen in the syringe.
Connect the plastic syringe to a "J" shaped plastic delivery tube made from the stem of a thin-stem plastic pipet. Cut the tip of a tapered plastic pipet to use as an adapter between the diameter of the "J" and the diameter of the syringe. Tape or use hot glue to attach the "J" to the plastic tip adapter that you cut. Connect to flexible tubing, and use a pinch clamp on the tubing to regulate the flow of the gas.

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Repeat this technique to fill a second syringe with oxygen. (See Lab Hints for source.)
Place the bent tube of the oxygen syringe in the water, open the pinch clamp, and expel a few bubbles out. This should flush out any air that was in the J-tube. Then move the tip of the J-tube under the open end of the sparking tube, and slowly squeeze 2 mL of oxygen into the tube. Use the marks on the sparking tube to measure the volume added. (Note: the bubbles take time to rise, so it is easy to overshoot the mark!)
Use the same technique to deliver 2 mL of hydrogen to the sparking tube.
Holding the tube securely near the base, place the piezoelectric sparking mechanism across the top wires and spark the gas mixture. (See Lab Hints.)

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Note the final volume of the gas remaining in the tube.
Repeat the experiment for several different ratios of hydrogen and oxygen. Add 2 mL of oxygen to each, but vary the amount of hydrogen. Record the data for each experiment.
Ask students to predict the final volume for different mixtures. Let students spark the reaction as a reward. Be certain that students wear goggles.
Ask students to predict which gas and how much gas to add to completely react the leftover gas from one of the trials.

Variation

Prepare 3 or 4 sparking tubes. Assign different ratios to different group of students. Let each group spark and record their data as a demonstration for the class. More data points can be collected in the same amount of time this way. Use only ONE sparking mechanism, and carefully supervise the students. Keep oxygen constant at 2 mL and vary hydrogen.
Or prepare a total of seven sparking tubes and line them up. Add the following mixtures:
- tube #1 #2 #3 #4 #5 #6 #7
- mL H₂ 0 1 2 3 4 5 6
- mL O₂ 6 5 4 3 2 1 0
Spark them in order, and observe the pattern made by the resulting water levels.

Air

Use 5 mL of air. Use a syringe of air, or admit air to the sparking tube by wiggling your finger with the tube inverted.

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Add an approximately equal volume of hydrogen gas. Note the volume after the explosion.

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Repeat with several different ratios of air and hydrogen. Record the volumes of hydrogen, air, and remaining gas for each explosion.
Ask students to predict the volume of gas remaining for some of the mixtures.

Questions

For the data at 2 mL of oxygen, plot the amount of hydrogen added against the amount of gas leftover.
Indicate the point on your graph where no gas is in excess.
Use your graph to determine the combining ratio of hydrogen and oxygen.
In the experiment with equal volumes of hydrogen and oxygen, which gas is the limiting reagent?
5.2 mL of air and 5.6 mL of H₂ are mixed in the tube for a total initial volume of 10.8 mL. After sparking, the volume decreases to 7.4 mL. What is the percentage of oxygen in the air sample?

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 003 Molar Volume of a Gas

Watch the movies, and note any evidence of a chemical reaction. Use the data below to answer the questions.

Hydrogen

Oxygen

Gas in Excess

1

2

1.5

2

2

1.0

3

2

0.5

4

2

0.0

5

2

1.0

Curriculum-

Use when studying the mole or limiting reagents. Understanding gas laws is helpful but not necessary to draw conclusions about the molar ratio or limiting reagents. The experiment can be a follow up to the hands on "Micro Rocket " (BBExperiment 050). The air variation allows a relatively quick and accurate determination for the percentage of oxygen in a sample of air (See Question and Answer #5).

Activity-

Demonstration - Student or Teacher

The experiment works as a demonstration by the teacher with active participation by students. In any event the activity requires careful teacher supervision. Each group of students may be enlisted to prepare and spark one of the reaction mixtures. With several sparking chambers and student help, you can collect more data points in a short time. All students record all of the data for analysis.

Safety-

Goggles are a must. Flying plastic can damage eyes.
NEVER use a glass tube for this experiment.
Hydrogen and oxygen form an explosive mixture. Never mix large amounts of the two.
If you prepare hydrogen and oxygen from generators, prepare only small amounts and store for a short time in plastic bags. Use a hood for the generators if possible.

Time-

Teacher Preparation: 15 minutes (Building the apparatus the first time requires an additional 15 minutes.)

Class Time: 15-20 minutes

Materials-

20 g of zinc (mossy zinc)
200 mL of 3% H₂O₂ -- (Available from drug stores; use as delivered. Buy fresh.)
200 mL of 1.0 M HCl -- (1 liter -- Add 83 mL of 12 M HCl, hydrochloric acid solution to approximately 550 mL of water. After the solution equilibrates, dilute to 1000 mL with water.)
1 mL dry yeast or 5 mL of 0.1 M Fe(NO₃)₃ (50 mL--Dissolve 2.020 g of Fe(NO₃)₃•9H₂O, ferric nitrate,Iron(III) nitrate in 27.5 ml of water and 2 mL* of 3 M HNO₃. Dilute to 50 mL with water.)
2 250-mL Erlenmeyer flasks with one-holed stoppers to fit.
2 4-cm fire-polished glass tubes inserted half-way into the stoppers
2 1-qt. zipper plastic bags
2 plastic syringes, 25 cc or larger
2 thin-stem plastic pipet tips bent to a "J" by heating briefly.
4 2-cm lengths of rubber tubing
2 pinch clamps
plastic rigid tube about 30-40 cm long and 12-15 mm in diameter (1/2" rigid aquarium tubing works well. So do plastic thermometer cases.) See Lab Hints.
a hot glue gun and glue
20 cm of 24 AWG speaker hook-up wire (side-by-side solid wire)
a piezoelectric sparking mechanism (modified Charcoal Lighter such as the Scripto Aim 'n Flame^® Torch lighter -- widely available in grocery stores for $3.00 - $4.00) See Lab Hints for modifications.

Disposal-

Decant the acid from the zinc. Rinse the zinc. Neutralize the acid solution before flushing down the sink.
Other solutions generated during the experiment may be disposed of safely at the sink with a water flush.

Lab Hints-

Apparatus

Cut a 10-cm length of the speaker wire, and split it at one end. Pull the strands apart half-way to form a "Y". Strip about 2 cm of insulation off each of the two single strands on top, Insert the bottom (unsplit, unstripped) end into the top of a plastic tube. Use a hot glue gun to seal the wire in place. Allow it to cool a bit. Seal the top of the tube around the wire with hot glue. Allow it to cool.

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Calibrate the tube with water measured in a plastic pipet. After the first three marks, you may measure the rest with a ruler. Mark the zero point with a ruler.

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Plastic Bag Modification

To make the plastic bag storage tanks, cut 1-cm off the bottom corner of a zipper plastic bag, open it up and wedge the #4 stopper into the hole in the corner. Wrap some electrician's tape tightly around the bag and stopper to make an airtight seal. Insert a short glass tube half-way into the stopper, then place a short length of rubber hose over the glass tubing. Expel the air from the bag, then use a pinch clamp to close off the hose.

Obtaining Gases in the modified plastic bag

Fill the plastic bag storage tanks, one with oxygen, the other with hydrogen, from cylinders (try the shop) or from a simple generator.

Hydrogen Gas

Place 20 g of zinc in the 250-mL Erlenmeyer flask, add the 200 mL of 1.0 M HCl, cover with a one holed stopper (fitted with a short glass tube).
Allow the reaction to run 30-40 seconds to flush out any air in the flask, then connect the hose of the plastic bag to the glass tube of the generator, and immediately open the pinch clamp.
When the bag is half-filled, remove it from the generator, squeeze it empty and place it right back on the generator. This flushing out procedure ensures a relatively pure sample of hydrogen in the bag. When the bag is almost full, close the pinch clamp and remove the bag from the generator.

Oxygen Gas

In a second Erlenmeyer flask, repeat the same technique described above, only use 1 mL of yeast (or 5 mL of 0.1 M Fe(NO₃)₃) and 200 mL of 3% hydrogen peroxide in place of the zinc and 1 M HCl.

Construct the piezoelectric sparking mechanism.

Obtain a piezoelectric lighter from a hardware store. Modify by attaching a wire to the end to separate the sparking gap.
Use a 10-cm length of speaker wire. Again, divide one end half-way and strip 1-2 cm of insulation off the separated ends. Insert one branch of the wire into the hole in the end of the lighter. Align the other branch with the outside of the lighter. Wrap electric tape around the tip to hold the outer wire in place. Now split the other end and strip 1-2 cm of insulation off the ends. With the two exposed ends positioned 1-2 mm apart, test the sparker to confirm that arcing occurs.

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Observations-

When the mixtures are sparked, almost faster than the eye can see, the water level shoots down and then shoots up, to a level higher than where it was originally. The quick drop is due to the exothermic reaction of hydrogen and oxygen combining to produce hot steam. The subsequent quick rise is due to the immediate condensing of that steam into liquid water, with a volume several orders of magnitude smaller than the hot steam. The gas left in the tube after the reaction represents the volume of the left over reactant.
For the variation involving seven tubes in a row, the water levels form a nice bar graph indicating how much reaction took place. This graph peaks at the stoichiometric ratio of
- 4 mL H₂ to 2 mL O₂ (2:1) confirming the balanced equation:
- 2 H₂ + O₂ -> 2 H₂O.

Data Table-

Oxygen constant:

Hydrogen

Oxygen

Gas in Excess

1

2

1.5

2

2

1.0

3

2

0.5

4

2

0.0

5

2

1.0

A plot of hydrogen vs. the excess is a straight line to zero and a second straight line as oxygen becomes the limiting reagent. See Figure below:

Ratios changing:

Hydrogen	Oxygen	Gas in Excess	Gas Reacted
0	6	6.0	0.0
1	5	4.5	1.5
2	4	3.0	3.0
3	3	1.5	4.5
4	2	0.0	6.0
5	1	3.0	3.0
6	0	6.0	0.0

Water in the tubes displays a physical bar graph similar to the one displayed below:

Answers-

Q1. For the data at 2 mL of oxygen, plot the amount of hydrogen added against the amount of gas leftover. A1. See Figure above
Q.2. Indicate the point on your graph where no gas is in excess. A2. The point where the excess gas is zero. It is also the intercept of the hydrogen axis.
Q3. Use your graph to determine the combining ratio of hydrogen and oxygen. A3. 4 volumes of hydrogen reacted with 2 volumes of oxygen. The ratio is 2 hydrogen to 1 oxygen.
Q4. In the experiment with equal volumes of hydrogen and oxygen, which gas is the limiting reagent? A4. Hydrogen is the limiting reagent. All hydrogen is consumed in the reaction with oxygen.
Q5. 5.2 mL of air and 5.6 mL of H₂ are mixed in the tube for a total initial volume of 10.8 mL. After sparking, the volume decreases to 7.4 mL. What is the percentage of oxygen in the air sample? A5. 10.8 -7.4 = 3.4 mL gas reacted H₂ reacted = 2 x (O₂ reacted) from chemical equation 3.4 = 2 x (O₂ reacted) + O₂ reacted = 3 O₂ reacted O₂ reacted = 1.1 mL O₂ ( 1.1/5.2 ) = 0.21 = 21%

Key Words 1-

moles, limiting reagent, molar volume, combining ratios, gas laws, water vapor, vapor pressure