Combustion of hydrogen gas
Hydrogen gas is produced through the reaction of "mossy" metallic zinc and hydrochloric acid. The resulting hydrogen may be captured and burned with great effect!
Ingredients: zinc, concentrated hydrochloric acid, a test tube or glass soda pop bottle, balloons, rubber gloves, a meter long dowel rod or yard stick, a candle, matches
Procedure: A minimal recipe follows.
1. Add hydrochloric acid to the glass bottle until it is one third full.
2. Add zinc to bottle. Use mossy zinc as it has a larger surface area than zinc plate.
3. Pour concentrated hydrochloric acid into the bottle to cover the zinc.
4. Place a balloon over the bottle's top. Hold the balloon on bottle's neck as it fills with gas. When it's full, tie it off.
Understanding: This reaction of hydrochloric acid with zinc is an example of a redox reaction where the electrically neutral metallic zinc reacts with the positively charged hydrogen ions in the hydrochloric acid
Zn(s) + 2 H+(aq) + 2 Cl-(aq) → ZnCl2(s) + H2(g)
As a result, the hydrogen is reduced (in charge) and the zinc is oxidized. The oxidized and positively charged zinc ion is then strongly attracted to negatively charged chloride ions. The white precipitate that you find in your soda bottle following the reaction is zinc chloride.At the level of atoms and molecules, two hydrogen ions migrate through the aqueous solution to the surface of the zinc metal. Electrons from the zinc atom mug the hydrogen ions, reducing the charge and forming molecular dihydrogen. The molecule of hydrogen then leaves the solution to join the air above and fill the balloon.
Once your balloon is full of hydrogen gas, tie it off and tape it to the end of the dowel rod. Slowly lower the balloon over the flame of the burning candle. Kaboom! There will a great ball of burning gas and the sound of rapid burning that is less a pop than a loud hiss. The reaction that occurs is the highly exothermic rearrangement
2 H2(g) + O2(g) → 2 H2O(g)
Look for the product of this reaction. Do you find any liquid water below the point of the explosion? If not, why not? That's due to the fact that the great heat of the reaction has left the water as vapor in the air.Why don't we hear a great explosion? If we were to mix oxygen into the balloon with the hydrogen, in the ratio of two parts hydrogen to one part oxygen, we would have not only heard a great explosion, but felt it. In the case of your burning balloon, think of where the reaction can occur. For the hydrogen to react, it must contact oxygen from the air. That oxygen is found only at the surface of the burning ball of hydrogen. The ball of hydrogen burns slowly from the outside in. However, if we first mix the hydrogen and oxygen, the reaction occurs rapidly and throughout the balloon producing great heat, fast expansion, and shock waves!
Hiss, pop, boom!
Question: If you can obtain pure oxygen gas, there is a wonderful variation of this experiment that can be done. It is strongly suggested that you tape the balloon to a hard surface and ignite the balloon with a candle attached to a long pole - six feet would not be too long!Begin by mixing only a bit of oxygen to the hydrogen balloon. When the balloon burns, you should hear a distinct pop. Now increase the amount of oxygen so that the ratio of hydrogen:oxygen is 1:1. When the balloon burns, you will hear a loud bang. What will happen if you increase the amount of oxygen so the hydrogen:oxygen ratio is 2:1?