Download Gas and Vapor Phase Explosions

Victoria Smith
Chemistry Demonstrations
June, 2002
Hydrogen Explosion
In this reaction you will collect a flammable gas. This demonstration can be
integrated into different chemistry lessons. This can be used for various
discussions: metal reactivity, gas flammability, H2, and, replacement reactions.
MATERIALS
1 – 5–7” balloon
10–mL 6M HCL
1 Zn (granular)
1 – 125–mL Erlenmeyer flask
1 – pole or meter stick
PROCEDURE
1. Turn on a Bunsen burner with a small flame under the fume hood.
2. Mass 1.00g Zn (granular) into flask.
3. Add 10 mL of 6M HCL to the flask. Be careful the bottom of the flask will
get hot! The reaction is highly exothermic.
4. Quickly put a balloon over the top of the flask to collect the gas. Gentle
agitation speeds up the reaction.
5. Tie off the balloon when the inflation slows.
6. Tape balloon to a meter stick or pole.
7. Move balloon under fume hood and lower it over the flame.
BOOM!
2 HCl (aq) + Zn (s) ZnCl2 (aq) + H2 (g)
then
2 H2 (g) + O2 (g) 2 H2O (g)
WHAT DRIVES THE ENGINE OF YOUR CAR?
Procedure A: THE GAS PHASE COMBUSTION OF AN
AIR/FUEL MIXTURE
1. Find the polyethylene bottle with the screws sticking out of the sides.
2. Pour enough isopropyl alcohol into the bottle to create a pool about 1/8
inch deep. Place a cork into the opening (not real tight but not too loose
either).
3. Find the spark coil and turn it on by rotating the knob on its bottom
clockwise. You will hear it begin to make a sparking sound. CAUTION:
Do not touch the tip as you will get a shock from it!
4. Touch one of the screws in the polyethylene bottle with the tip of the
spark coil and stand back!
5. Turn off the spark coil and retrieve the cork, immediately put the cork
back in the bottle and try to ignite the alcohol a second time. What
happens?
6. This is a good experiment to do in a darkened room. If you can, turn off
the lights and cover the windows.
7. How did the spark coil ignite the isopropyl alcohol / air mixture?
8. What are the similarities between what you just did and what occurs in an
automobile engine?
9. What is the chemical reaction that you just initiated?
2 C3H8O(g) + 9 O2(9)
6 CO2(g) + 8 H2O(g) + HEAT
10. If the isopropyl alcohol did not ignite on the second try, explain why.
What was different about the second run relative to the first?
Procedure B:
SURFACE CATALYZED REACTION OF ISOPROPYL
ALCOHOL
1. Pour enough isopropyl alcohol into a beaker to create a pool about 1/4
inch deep. Heat the alcohol to around 50 °C on a hot plate. Use the digital
thermometer to monitor the temperature. Do not overheat the alcohol.
2. Light a Bunsen burner or propane torch, depending upon which is
available.
3. Find the glass rod with the cylindrical platinum gauze hanging from it.
4. Heat the platinum gauze in a flame until bright red. Then quickly lower it
into the beaker so that it is approximately 1 – 2 centimeters above the
surface of the alcohol. The glass rod can be laid across the rim of the
beaker.
5. If the alcohol catches fire, smother it by covering the beaker with the large
watch glass which is provided for this purpose.
6. What do you observe? Is a chemical reaction taking place? Where is it
occurring? Does the reaction give off heat?
7. Remove the platinum gauze from the beaker for a few seconds and then
return it. What happens? –
8. The reaction you are observing are:
CH3CHOHCH3(g) H2(g) + CH3COCH3(g)
(isopropyl alcohol)
(acetone)
2 H2(g) + O2(g) 2 H2O(l)
The first reaction (called a dehydrogenation reaction) requires heat to occur, thus
explaining the need to heat the platinum gauze to initiate the reaction. However,
the second reaction gives off an even larger quantity of heat which is sufficient to
sustain the first reaction and to maintain the gauze at a red–hot temperature.