Microscale Gas Chemistry

HENRY CAVENDISH is credited with having first isolated and studied hydrogen as a true chemical element in 1766.  He produced the gas, which he called inflammable air by reacting hydrochloric acid with metals.  Although others, including John Mayow and Robert Boyle also knew of the reaction between metals and acid, Cavendish is given credit for the discovery because he was the first to systematically study the properties of the gas.  In the late 18th century, Antoine Lavoisier recognized the substance as an element and later named it hydrogen from the two Greek words meaning “water-forming”.

    Hydrogen is a colorless, odorless, and tasteless gas that is insoluble in water.  Very little molecular hydrogen, H2, can be found in nature.  The most familiar compound of hydrogen is water which is 2/18 or 11% hydrogen by mass but 2/3 hydrogen byatom count.  Hydrogen makes up over 92% of all the atoms of the Universe.  Our sun consists of 30% by mass hydrogen. 

    Hydrogen is the lightest of all known molecular substances.  Even helium has a molar mass that is twice as large.  As a result, the density of hydrogen is very small (0.0824 g/L at 25 ºC and 1 atm) and is over 14 times smaller than that of air.  Another result of having such a small molar mass is that hydrogen’s melting point (-259.14 ºC or 14.0 K) and boiling point (-252.5 ºC or 20.7 K) are extremely low. 

    The most readily used source of hydrogen is natural gas.  Methane is combined with steam at 1000 ºC to produce hydrogen and carbon monoxide.  The mixture of CO and H2 is called synthesis gas or just syn gas. 

CH4(g) + H2O(g)    CO(g) + 3 H2(g)        ΔHºrxn = +206 kJ

    The major industrial use of hydrogen in terms of mass used is in the synthesis of ammonia by the Haber process.  Over 16 million tons of ammonia are produced annually in the USA and most of this is used for fertilizer. 

3 H2(g) + N2(g)   2 NH3(g)

    Hydrogen is also used to hydrogenate oils in the food industry.  Vegetable oils contain numerous double bonds and are called polyunsaturated oils.  These double bonds react with hydrogen in the presence of a catalyst.  The product is margarine which has properties of a solid and is called a saturated fat.

 

    Hydrogen is used in the manufacture of methanol used in many industrial applications including the plastics and adhesives industries.  It is also used to prepare metals from their oxides at elevated temperatures.  The process is expensive so that only precious metals are produced this way. 

MOx + x H2(g)    M(s) + x H2O(g)

Suitability
    The following experiments are included here:

Part 1. Experiments with Hydrogen

Experiment 1. Traditional test for hydrogen
Experiment 2. Hydrogen forms explosive mixtures with air
Experiment 3. Reversible conversion of copper metal and copper(II) oxide
Experiment 4. Reduction of iron(III) oxide with hydrogen

Part 2. Demonstrations and Advanced Experiments with Hydrogen

Experiment 5. Effusion of hydrogen is faster than air
Experiment 6. Hydrogen burns with a gentle flame
Experiment 7. Disappearing/reappearing candle flame
Experiment 8. Calcium and calcium hydride produce hydrogen in reactions with water
Experiment 9. Deuterium isotope effect

    The first four experiments are designed for use by all levels of students (from middle school age through university level).  Very little prior knowledge is required. Students with more experience will understand the experiments at a higher level than beginning students, but will also enjoy these experiments. 

    The experiments in Part 2 are intended to be classroom demonstrations conducted by the teacher, however, most could be use as student laboratory experiments.  In Experiment 5, observations are made over an extended period of time (a week or more).  Experiment 6 is potentially dangerous if misused, so it should be done by the teacher for safety reasons.  Experiment 9 is a demonstration because D2O is expensive.

    For use in high school settings, Part 1 of these experiments can be conducted at about the time that chemical formulas and reactions are being introduced.  As a laboratory activity, these experiments are appropriate when discussing chemical compounds, chemical formulas, chemical reactions, the mole, as well as a variety of topics including physical and chemical changes.

    Experiments in Part 2 are best performed as classroom demonstrations.  They address particular concepts usually encountered later in the first year chemistry course.  Experiment 5 should be done with the gas laws to demonstrate effusion.  Experiment 6 demonstrates chemical reactivity of hydrogen. Experiment 7 is a very impressive classroom demonstration; a candle flame appears to be extinguished as a syringe full of hydrogen is lowered over the burning candle.  As the syringe is raised, the candle re-ignites.  The process can be repeated several times.  Experiment 8 works well with a discussion of the chemical reactivity of alkaline earth metals and/or metal hydrides.  Experiment 9 is the only advanced topic experiment and is used to demonstrate the deuterium isotope effect. 

Background skills required
    Students should be able to:

generate a gas
measure quantities of liquid reagents
use a balance
use a Bunsen burner

Time required
    Students should be able to generate hydrogen and perform the four experiments in Part 1 in a single 45 minute laboratory period.

Preparation of hydrogen in a “gas bag”
    Large samples of H2(g) can be prepared conveniently in 1 L food storage bag.

Student Instructions
    For classroom use by teachers, one copy per student in the class may be made free of charge and without further permission.  Student instructions and questions only (without teaching tips, suitability information, etc.) can be downloaded free of charge as a Microsoft Word document from the website. Download now.

Answers to the questions, lists of materials and chemicals, and additional reference information.
    This page is fairly similar in content to Chapter 3 in our book Microscale Gas Chemistry.   Our 500-page book includes all of the information included at this website and much more!  Answers to all of the questions, chapter-by-chapter lists of the equipment and chemicals needed to conduct the experiments as classroom demonstrations or laboratory activities for the entire class, construction instructions for various pieces of equipment, information for the preparation of stock solutions, ordering information, and a complete index — are all available in the book, but not at the website.  The book, Microscale Gas Chemistry, can be ordered from Educational Innovations (Part Number BK-590) from  their website.

General Safety Precautions
    Always wear safety glasses.  Gases in syringes may be under pressure and could spray liquid chemicals.  Follow the instructions and only use the quantities suggested.  CAUTION! Hydrogen forms explosive mixtures with air.

Toxicity
    Hydrogen is relatively non-toxic; however, it is a simple asphyxiant if inhaled in very large quantities.  We will not be generating large quantities of hydrogen. 

Equipment (Vendors and Part Numbers)
    Microscale Gas Chemistry Kit:

two 60 mL plastic syringes with a LuerLOK fitting
two Latex LuerLOK syringe caps
two plastic vial caps
one 15 cm length of Latex tubing
one 3 cm length of Latex tubing
one small bottle of silicone oil
one plastic pipet
one clear plastic beverage cup (250 mL/9 oz)
one small plastic weighing dish
one small test tube (12 x 100 mm)
one medium test tube (18 x 150 mm)
one birthday candle

Chemicals (needed for each syringe full of hydrogen produced)

0.05 g solid Mg turnings or ribbon (approximately 6 cm ribbon = 0.05 g)
3 - 5 mL 2 M HCl(aq)

This quantity of magnesium will produce approximately 50 mL of H2.  The production of H2 is very fast and it typically takes less than 30 seconds to fill a syringe.  The reaction is:

Mg(s) + 2 HCl(aq)    H2(g) + MgCl2(aq)

 
Generating hydrogen gas samples
    Samples of hydrogen are generated by the In-Syringe Method.  A summary of these steps is provided here:

1. Wear your safety glasses!

2. Lubricate the seal.  
    Lubricate the black rubber seal of the plunger with silicone oil. 
     
3. Measure out 0.05 g Mg.   
    Place the magnesium directly into the vial cap.

     

4. Fill the syringe barrel with water.
    Fill the barrel completely with water.  Place your finger over the hole to form a seal.    

 

5. Float the vial cap
    Float the vial cap containing the solid reagent on the water surface.   

   

6. Lower the cap by flotation
    Release the seal made by finger to lower the cap into the syringe barrel without spilling its contents.    

 

7. Install the plunger
    Install the plunger while maintaining the syringe in a vertical position.  The plunger has a plastic “rib” near the rubber seal that snaps past the “catch” — a small ridge just inside the mouth of the syringe.  Usually it takes a firm push to move the rib past the catch.  After that, the plunger should move smoothly.

     
8. Draw 3 - 5 mL 2 M HCl(aq) into syringe
    Pour the 2 M HCl(aq) into a small weighing dish.  Draw 3 – 5 mL of the solution into the syringe. 

     

9. Install syringe cap
    Push the syringe into the syringe cap. 

     

10. Generate the gas
    Shake the device up and down in order to mix the reagents.  Gently help the plunger move up the barrel.   

    

11. Remove cap to stop gas collection
    Remove the syringe cap with the syringe held “cap-up” as shown.  Assume contents are under positive pressure. 

12. Discharge reagents
    Discharge the liquid reagent into the plastic cup.  Immediately cap the syringe to prevent loss of gas.   

    

Wash away contaminants
    Hydrogen-filled syringes must be washed in order to remove traces of unwanted chemicals from the inside surfaces of the syringe before the gases can be used in experiments.  Follow the procedure summarized here.

1. Remove the syringe cap,
2. draw 5 mL water into the syringe,
3. cap the syringe,
4. shake syringe to wash inside surfaces,
5. remove cap,
6. discharge water only, and finally
7. recap the syringe.
8. Repeat?

Repeat these Washing Steps if necessary.
(All traces of the reactants should be washed away.)


Disposal of hydrogen samples
    Unwanted hydrogen samples can be safely discharged into the room.

Teaching tips

1. The generation of hydrogen is a rapid reaction and should be practiced before showing your students.

2. Hydrogen is the only gas for which diffusion is a major concern.  When the syringe cap is off, keep the opening directed downward to minimize loss of hydrogen through the hole.

3. If magnesium ribbon is being used, measure the mass of a 25 cm length.  Determine the mass per cm and use that to cut pieces of desired mass instead of using a balance. 

Introductory Questions

1. What is the formula for hydrogen gas?

2. Why should you hold the syringe with the opening down when the cap is off? 

3. What was the volume of hydrogen you actually obtained in your syringe?

Questions

4. Is the reaction exothermic or endothermic?

5. How accurately can you read the syringe? 

6. What about the position of the plunger before the reaction started (due to the volume of air and solution already present before the reaction started)?  Should this volume be subtracted?

Advanced Questions

7. Determine the number of moles of magnesium you used to prepare your hydrogen.

8. Use the molar concentration of HCl and the volume used to determine the number of moles of HCl you used to make hydrogen gas.

9. Write the balanced chemical equation for the reaction occurring in your syringe.

10. Which is the limiting reactant, Mg(s) or HCl(aq)?

11. Using the ideal gas law and your answer to the previous question, what volume of gas is predicted? (Assume the temperature is 25 ºC and standard pressure)

EXPERIMENT 1. TRADITIONAL TEST FOR HYDROGEN

Equipment

Microscale Gas Chemistry Kit
Match or lighter
a coin

Chemicals

H2(g), 20 mL     

Applications, Topics, Purpose
    chemical properties of gases, combustion reactions, gas density, characterization of gases, energy changes

Instructions
    Affix a candle to a coin.  Melt a drop of wax from a candle onto a coin and immediately push the base of a candle into hot wax so that the candle stands vertically.

Part 1.  Fill the test tube with water and place it upside down in the cup of water.  Light the candle.  Connect the long piece of tubing (15 cm) to the fitting on the syringe.   Displace the water with H2 as shown in the figure.  Remove the test tube from the water, keeping it open-end downward, and move the open end near a lit candle.  A BARK! sound will confirm the presence of hydrogen.

Part 2.  Repeat the experiment using a test tube half-filled with air and half-filled with hydrogen.  To do this, start with a test tube half-filled with air and half-filled with water and then displace the water with hydrogen. A much louder BARK! will result. 

Teaching tips
    Show students how to invert water-filled test tubes without loss of water.  Filling the cup to nearly full makes it easier.  Students with small fingers should use their thumb to cover the test tube while inverting it in water.

Introductory Questions

1. Hydrogen is much lighter than air.  What other gas is also lighter than air?  Hint: Think of party balloons. 

2. What is the purpose of collecting the gas under water — by displacing the water in a test tube as was done in this experiment?

3. Hydrogen is an element with the atomic symbol H.  It exists in nature as a molecular substance with the formula H2.  Oxygen does exactly the same thing.  Write the atomic symbol for oxygen and write the formula of the molecular substance it forms. 

Questions

4. Why was the test tube containing hydrogen gas stored upside down in the water?

5. Suppose you had two unlabeled test tubes, one that contained hydrogen and the other carbon dioxide.  Suggest an experiment you could do to determine what gas was in each tube.

6. Why was the hydrogen-air mixture considerably louder than that of pure hydrogen?

7. Sketch what would happen to the hydrogen gas if you were to rotate the test tube so that the open end was directed upward. 

Advanced Questions

8. What familiar product formed when you ignited the H2(g)?

9. Write and balance the equation for the reaction that takes place when hydrogen burns (explodes) in air?

EXPERIMENT 2. HYDROGEN FORMS EXPLOSIVE MIXTURES WITH AIR

Equipment

Microscale Gas Chemistry Kit
Match or lighter

Chemicals

H2(g), 50 mL
3% dish soap solution

Suitability
    middle school lab, high school lab, university lab, and classroom demonstration

Applications, Topics, Purpose
    combustion reactions, kinetics, chemical properties of gases, activation energy

Instructions
    Part 1.  Fill a small plastic weighing dish with 3% dish soap. Generate hydrogen using quantities listed above.  Discharge the aqueous solution and equip the syringe fitting with a 3 cm length of tubing.  (If you must store the syringe for even a few minutes, use the syringe cap.)  Place the free end of the tubing into the soap solution and slowly discharge some of the hydrogen.  Once a rounded mound of bubbles has been produced, remove the tubing and ignite the bubbles.  A mild BARK! will occur as the hydrogen undergoes a comparatively gentle “explosion”. 

    Part 2.  With a syringe half-full of hydrogen, draw in an equal volume of air and repeat the experiment.  This time, when the mixture is ignited, a louder, more definite explosion occurs. 

Teaching tips

1. The hydrogen and air mixture produced in the syringe is an explosive mixture.

2. 3% dish soap solution  is prepared my adding 3 g dish soap to enough water to make 100 mL.

Introductory Questions

1. What difference did you notice between Parts 1 and 2 in the experiment?  Explain.

2. What is the purpose of the soap solution?

3. Why should extreme care be exercised when working with hydrogen gas?

Questions

4. What are the two major gases found in the air?  Which one is reacting with hydrogen in this experiment?

5. If one bubble of hydrogen and one bubble of air were side-by-side and ignited, would the bang be louder or not as loud than if the same amount of gas were contained in a single bubble?

6. Would the explosion be louder if oxygen were used instead of air — with the same amount of hydrogen?  Air is 21% oxygen.  

Advanced Questions

7. Write the balanced chemical equation for the reaction occurring in this experiment.

8. Suggest an experiment to determine what mixture produces the loudest bang from a hydrogen/air mixture using the soap bubble solution.

EXPERIMENT 3. REVERSIBLE CONVERSION OF COPPER METAL AND COPPER(II) OXIDE

Equipment

Microscale Gas Chemistry Kit
small Bunsen burner
glass Pasteur pipet
ring stand and clamp
match or lighter   

Chemicals

copper wool such as a ChoreBoy kitchen scrubbing pad
hydrogen, H2(g), 50 mL

wooden stick such as a kabob skewer — must be able to fit into the pipet

Suitability
    middle school lab, high school lab, university lab, and classroom demonstration

Applications, Topics, Purpose
    Preparation of metals, preparation of an ionic substance, types of solids, oxidation/reduction

Instructions
    Use a wooden stick to position a 0.50-g plug of copper wool into a glass pipet.  Clamp the pipet in a horizontal position.  

      

    Part 1. Heat the Cu/pipet for 30 s and then slowly pass 60 mL air through the pipet while continuing to hold the pipet in the flame.  The copper will quickly turn black.  The reaction is:

2 Cu(s) + O2(g)    2 CuO(s)

    Part 2.  Connect the H2-filled syringe to the pipet.  Heat the CuO/pipet and then slowly pass the hydrogen gas through the pipet while continuing to hold the pipet in the flame.  Water droplets should appear inside the stem of the pipet. The reaction is:

2 CuO(s) + H2(g)   2 Cu(s) + H2O(l)

Teaching tips

1. Tell students that hot glass looks like cold glass!  Burns from touching hot glass are treated with aloe vera. 

2. The terms oxidation and reduction are omitted from this experiment for use at introductory levels.  Add them to your discussion if appropriate.

3. One purpose of this experiment is to draw attention to the four fundamental types of substances — metals, ionic compounds, molecular compounds and network covalent compounds.  Students will work with all four in this experiment.  (SiO2, the primary component of the glass pipet, is a network-covalent compound.)

4. You may wish to construct the pipet devices for the students.  These can be stored and used again. 

Introductory Questions

1. Give the formulas for (a) copper(II) oxide, (b) elemental copper, (c) molecular hydrogen and (d)water, all of which were encountered in this experiment.  

2. What did you observe that indicates to you that a reaction has taken place?

3. Do you think this reaction could be repeated over and over without consuming the copper — or will the copper eventually be used up?

Questions

4. How do you know that the conversion of copper to copper(II) oxide and then copper(II) oxide to copper is reversible?

5. Why is it necessary to heat the Cu in order for it to react? 

Advanced Questions

6. The four fundamental types of substances are: metals, ionic compounds, molecular compounds and network covalent compounds. What fundamental type of substances id each of the following?  (a) hydrogen; (b) copper; (c) copper(II) oxide; (d) water; and (e) silicon dioxide (the predominant component of the glass pipet)

7. Three of the four fundamental types of substances are almost always solids under standard conditions.  Which type can be solid, liquid or gas under standard conditions?

8. These reactions can be discussed in terms of oxidation and reduction chemistry.  Determine the oxidizing agent and reducing agent in each of the two reactions given in the instructions.

Gently heat the Cu wool	The Cu oxidizes to black CuO as air is passed through the pipet.
Switch the syringe and pass H2 through the pipet.  The H2 reduced black CuO to shiny metallic copper.	Water vapor is noted condensing inside the pipet stem and on a test tube held near the opening.

EXPERIMENT 4. REDUCTION OF IRON(III) OXIDE WITH HYDROGEN

Equipment

Microscale Gas Chemistry Kit
glass disposable pipet
Bunsen burner
Matches or lighter
Magnet
“000” steel wool    

Chemicals

H2 (g), 40 mL
Fe2O3(s), 0.25 g

Suitability
    middle school lab, high school lab, university lab, and classroom demonstration

Applications, Topics, Purpose
    magnetic properties of metals, preparation of a metal from an ore, oxidation/reduction. 

Instructions
     Prepare the pipet device as shown in the figure using 0.25 g Fe2O3.  Test the Fe2O3 by holding a magnet above it through the glass.  Heat the Fe2O3 for 30 s and then slowly pass H2 gas through the pipet while continuing to hold the pipet in the flame.  The powder will quickly darken and water droplets should appear inside the stem of the pipet.  Remove the heat and allow the pipet to cool.  Hold a magnet above the mound of solid metallic powdered iron.  The reaction is:

Fe2O3(s) + 3 H2(g)   2 Fe(s) + 3 H2O(g)

Teaching tips

1. Tell students that hot glass looks like cold glass!  Burns from touching hot glass are treated with aloe vera. 

2. If an analytical balance is available, one can determine the mass of the Fe2O3(s) used and the mass of the pipet/ Fe2O3(s) before and after the reaction. The difference is the mass of oxygen lost during the reaction.  A very close comparison between the predicted and observed mass loss is possible. 

Introductory Questions

1. What does the “2” and “3” mean in the formula Fe2O3?

2. What did you observe that indicates to you that a reaction has taken place?

3. How does the magnet tell us that a reaction has taken place?  Why will this test not work for every reaction? 

4. Substances that are attracted to a magnet are called ferromagnetic.  Is Fe2O3 ferromagnetic?  Is Fe?

Questions

5. Write the chemical reaction that has taken place in sentence form: “Iron(III) oxide and …)”

6. Calculate the rate of hydrogen flow in mL/min.

Advanced Questions
7. Why is it necessary to heat the Fe2O3 in order for it to react?

8. Calculate the number of moles of Fe2O3 used in the pipet.  Calculate the number of moles of H2 passed through the pipet.  Which reagent was in excess?

9. As the reaction between oxygen and iron took place, a bright white glow may have been observed.  Does this indicate an exothermic or endothermic reaction?

Clean-up and storage
    At the end of the experiments, wipe excess lubricant off of rubber seal on the plunger.  Clean all syringe parts with soap and water.  Use plenty of soap to remove oil from the rubber seal.  This extends its life.  It may be necessary to use a 3 cm diameter brush to clean the inside of the barrel.  Rinse all parts with water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.

EXPERIMENT 5. EFFUSION OF HYDROGEN IS FASTER THAN AIR

Equipment

Microscale Gas Chemistry Kit
Balloon, 25 cm diameter
rubber bands, two heavy duty
scissors

Chemicals

H2(g), 60 mL      

Suitability
    classroom demonstration

Applications, Topics, Purpose
    gas density, effusion/diffusion, kinetic molecular theory of gases
 

Instructions
    Cut the end off of a large balloon as shown. Generate H2(g), discharge the aqueous reagents and then cap the syringe. With the syringe cap directed upward as in the figure above (so that hydrogen does not escape), remove the plunger and secure the balloon over the syringe opening with a rubber band.  Gather the balloon so that the rubber is taut over the syringe opening.  Use a second rubber band as well.  The rubber bands must be wound quite tightly in order to prevent hydrogen loss.

    Clamp the syringe in position with balloon end up (opposite of that shown in the figure).  Within 30 minutes there should be visible evidence that the balloon is being pushed into the syringe barrel.  By the next day, the balloon is drawn in by a very noticeable amount.  After 3-5 days, the balloon will be extensively drawn into the syringe barrel.  The experiment can be continued for weeks.

Teaching tips

1. No chemical reaction occurs with this experiment. 

2. Noticeable results of the experiment are not immediate.  If there is no evidence that the balloon being pulled into the syringe after several hours, it is likely that the balloon has not been tightly sealed to the syringe. 

3. This experiment shoud be done as a demonstration because of the time involved. 

Questions

1. Why is the balloon being pushed into the syringe?  What is pushing the balloon inward?

2. What does the word effusion mean?  What is the difference between diffusion and effusion?  If you do not know, look these terms up in a dictionary.

3. Predict what would happen if this experiment were repeated with a syringe filled with CO2(g) instead of hydrogen.

EXPERIMENT 6. HYDROGEN BURNS WITH A GENTLE FLAME

Equipment

Microscale Gas Chemistry Kit
Glass disposable pipet
Matches or lighter
Food storage bag, 1 L / 1quart
hemostat (or pinch clamp)
ring stand and clamp

Chemicals

Chemicals to generate hydrogen     

Suitability
    classroom demonstration

Applications, Topics, Purpose
    explosive mixtures, combustion, simple reactions


Instructions
    Like most gases, hydrogen can be generated inside the “gas bag” which consists of a length of tubing inserted into a food storage bag.  See Chapter 5 for construction details. Assemble the apparatus as shown in the figure.  Use a 15 cm length of tubing to connect the gas bag to the pipet.

    Use the gas bag to discharge hydrogen at a constant, controlled rate in order to sustain a small flame.  Keep the gas bag away from flames.  Open the hemostat (or pinch clamp) and ignite the gas issuing from the pipet.  Gently press down on the gas bag to control and sustain the flame.  To stop the combustion, pinch the tubing shut.

    The flame is “gentle” because of the lack of oxygen in the fuel. The flame is yellow because of the sodium in the glass; hydrogen normally burns with a blue flame.

 Teaching tips

1. Do this experiment as a demonstration to reduce the chance for an explosion.  Follow the instructions exactly. 

2. Hydrogen forms explosive mixtures with air.

Questions

1. From Experiment 2, you learned that hydrogen and air form explosive mixtures.   How does the design of this experiment prevent an explosion?

2. Write and balance the chemical reaction that takes place in both this and Experiment. 2

3. In Experiment 2, we deliberately created an explosive mixture of hydrogen and air and then ignited the mixture.  Why are we so concerned about the same sort of explosive mixture in this experiment and have designed the experiment so that an explosion does not take place?

Advanced Questions

4. When hydrogen burns, we see a flame.  When hydrogen/air mixtures explode we do not see a flame.  Why?

5. What would happen if one were to hold a piece of glass above the flame at a safe distance (so it is not heated by the flame.

EXPERIMENT 7. DISAPPEARING/REAPPEARING CANDLE FLAME

Equipment

Microscale Gas Chemistry Kit

Chemicals

magnesium turnings or ribbon, 0.10 g
hydrochloric acid, HCl(aq), 5 mL of 2 M

Suitability
    classroom demonstration (all levels — including middle school)     

Applications, Topics, Purpose
    explosive mixtures, combustion, simple reactions

Instructions
    Affix a candle to a coin with a drop of molten wax.  Generate hydrogen using 0.10 g magnesium turnings or ribbon and 5 mL of 2 M HCl(aq) instead of the usual quantities.  This will generate more than a syringe full of hydrogen.  As the plunger reaches the top of the syringe barrel and is almost ready to pop out of the barrel, stop the reaction by discharging the aqueous solution into the plastic cup. Immediately cap the syringe with the syringe cap.  Light the candle and turn off the room lights.  With the cap directed upward, remove the plunger from the syringe barrel and immediately lower the syringe carefully but quickly over the top of the candle flame as shown in the figure.  As soon as the flame goes out, raise the syringe.  The candle should re-ignite.  This experiment takes some practice. Once perfected, it makes an interesting magic trick. 

Teaching tips

1. There are several possible explanations.  Most likely, the wick is glowing hot enough to re-ignite the hydrogen once enough air is available again.  In pure hydrogen, the candle does not burn.  Hydrogen needs oxygen to burn.

2. A QuickTime video of this experiment is available.  This is a good way to see how it is done.

1. Discuss possible explanations with other students.  Agree upon the most plausible explanation. 

2. What, if anything, is wrong with other explanations that have been proposed?

3. Are all flames visible?  Can a substance burn without a visible flame?

EXPERIMENT 8. CALCIUM AND CALCIUM HYDRIDE PRODUCE HYDROGEN IN REACTIONS WITH WATER

Equipment

Microscale Gas Chemistry Kit

Chemicals

Calcium metal, free of oxide, < 1 g
Calcium hydride, CaH2, < 1 g

Suitability

college prep high school lab, university lab, and classroom demonstration     

Applications, Topics, Purpose
    Chemical reactions, oxidation of metals, oxidation-reduction reactions, Part A: reactivity of alkaline earth metals, Part B: hydrides, reactivity of hydrides

Instructions
    Part A. A sample of fresh calcium, free of oxide coating, should be used.  The exact mass (between 0.03 - 0.04 g) is determined using an analytical balance.  The calcium is placed in a dry vial cap and lowered into the syringe.  The liquid reagent is 20 mL of distilled water.  Record the initial volume mark on the syringe barrel.  Use the outer rubber ring on the plunger as a convenient reference as shown in the figure.  Perform the reaction by shaking the syringe.  The solution will bubble as hydrogen is produced and will eventually become cloudy due to Ca(OH)2(s) which is only sparingly soluble in water.  The reaction is:   

Ca(s) + 2 H2O(l)  Ca(OH)2(s) + H2(g)

    Record the final volume mark and determine the volume of hydrogen by subtracting the volume of air originally present.  The volume is quite close to what is expected from stoichiometry and ideal gas calculations.  An ideal gas laboratory experiment could be developed that utilizes this procedure.

    The 20 mL of solution now contains suspended calcium hydroxide solid and a small amount of dissolved calcium hydroxide.  A variety of experiments can be performed with the solution.  Add a few drops to an indicator solution to show that the pH is high.  Discharge 10 mL of the solution into a plastic cup.  Add just enough 1 M HCl(aq) to the cloudy suspension to produce a clear solution of aqueous calcium chloride:

Ca(OH)2(s) + 2 HCl(aq)  CaCl2(aq) + 2 H2O(l)

As another variant, the calcium hydroxide suspension can be allowed to settle in the syringe where contact with air is avoided.  After a few hours, the suspended Ca(OH)2(s) has settled producing a clear solution of limewater, Ca(OH)2(aq).  Limewater has a number of uses as a reagent; it is used to test for CO2(g) and is used with alum to enhance settling of turbid water. 

Part B. Repeat the experiment using between 0.03 - 0.04 g CaH2 instead of calcium metal.  The reaction is:

CaH2(s) + 2 H2O(l)  Ca(OH)2(s) + 2 H2(g)

Teaching tips

1. If the calcium metal is not fresh, make sure the oxide coating has been scraped off. 

2. Scraps of calcium and calcium oxide should be destroyed by reaction with water or any acidic solution.  Do not place scraps of calcium metal or calcium hydride in the trash!  A fire may result.  Add small amounts of unwanted calcium metal and/or calcium hydride to a large quantity of water in order to destroy them. 

Questions

1. Describe the appearance of the calcium metal before the reaction.

2. Describe the appearance of the calcium hydride before the reaction.

3. What is the formula of calcium hydride?  Is calcium hydride ionic, covalent, metallic, or network covalent?

4. Describe your observations of the reaction between calcium metal and water (and between calcium hydride and water if Part B was done). 

5.  What class of reaction describes this (these) reaction(s)?

6.  How might the product gas be tested?

EXPERIMENT 9. DEUTERIUM ISOTOPE EFFECTS
(This experiment is based on one published by Binder and Eliason, Journal of Chemical Education, 63, 536 (1986)

Equipment

Microscale Gas Chemistry Kit

   
Chemicals

Bromphenol blue solution, < 2 mL
5 M H2SO4 Solution, < 2 mL
3.33 g D2O
Magnesium turnings, < 1 g

Suitability
    university lab and classroom demonstration

Applications, Topics, Purpose
    chemical kinetics, isotopes and chemical reactivity

Instructions
    The rates of chemical reactions are often dependent on the isotopes of the elements involved.  This occurs because compounds containing heavier isotopes have slower average molecular speeds in both the gas phase and in solution.  This phenomenon can be demonstrated as follows.

    You will need two syringe assemblies for this experiment.  They should be labeled "H2O" and "D2O."  In two plastic weighing dishes measure out equimolar amounts of reagents as per the table:

Place 0.20 g of magnesium turnings in each of two vial caps.  Draw all of the solutions into their respective syringes and install the syringe caps.  Hold both syringes in the same hand and vigorously shake the syringes to mix the solutions.  Within 2-3 minutes the color in the syringes begins to turn blue as the acid is consumed.  The color change occurs in the H2O syringe well before it does in the D2O syringe.

The chemical equation for the reaction of magnesium with sulfuric acid is:

Mg(s) + H2SO4(aq)  MgSO4(aq) + H2(g)

In the syringe with the D2O(l), hydrogen ion exchange between H⁺ and D⁺ is nearly instantaneous.  Because the ratio of D/H is approximately 3:1 we can think of sulfuric acid as having the formula D2SO4 and thus the reaction is:

Mg(s) + D2SO4(aq)  MgSO4(aq) + D2(g)

Teaching tips

1. The rate of a chemical reaction depends to a small extent on the isotope used.  Heavier isotopes move slower than lighter isotopes of the same element.   When these isotopes are part of a compound, the differences can be extremely small.  However, in the case of H2O vs. D2O, the difference in molar mass is 18 g/mol vs. 20 g/mol — a difference of over 10%.  The reaction with D2O should be noticeably slower.  This phenomenon is called the deuterium isotope effect. 

2. The bromphenol blue indicator will be yellow at the start of the experiment when the pH is lower and change to a blue color (a higher pH) when the experiment is complete.

Questions

1. What does the word isotope mean? 

2. What is the difference between H and D?  Write their isotope symbols using   notation, where A is the atomic mass number, Z is the atomic number and X is the element symbol.

3. Hydrogen has a third isotope,   called tritium.  Tritium is extremely rare and radioactive.  How would the rate of reactions involving tritium compare to those involving deuterium and regular hydrogen?  Rank the three of these isotopes in order of rate of reaction from slowest to faster.

    At the end of the experiments, wipe excess lubricant off of rubber diaphragm. Clean all syringe parts (including the diaphragm), caps and tubing with soap and water.  Use plenty of soap to remove oil from the rubber seal.  This extends the life of the plunger.  It may be necessary to use a 3-cm diameter brush to clean the inside of the barrel.  Rinse all parts with distilled water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.