The methane rocket

Students will need to duck and cover as a plastic bottle goes flying overhead. Mixing oxygen and methane in a 2:1 ratio will create a powerful rocket when ignited, that will shoot across your learning space. 

This experiment should take 10 minutes.

Equipment

Apparatus

• Eye protection
• Ear protection
• A carbonated soft drink bottle of between 300 cm³ and 500 cm³ capacity, made of PET.
• A rubber bung to fit the bottle.
• A large trough or washing up bowl
• Measuring cylinder, 500 cm³
• Rubber tubing to fit the gas tap

Chemicals

• Oxygen cylinder or other source of oxygen, see CLEAPSS instructions on how to generate oxygen gas.
  

Health, safety and technical notes 

• As with all experimental work in schools, you must carry out your own risk assessment in accordance with your employer’s health and safety policy. See also the RSC’s standard health and safety guidance.
• Oxygen is OXIDISING and may cause or intensify fire; oxygen cylinders contain GAS UNDER PRESSURE and may explode if heated. Be aware that combustible substances burn much more fiercely in air which has been only slightly enriched with oxygen, see CLEAPSS Hazcard HC069.
• Methane is EXTREMELY FLAMMABLE and an asphyxiant, see CLEAPSS safety sheet HC045a.
• Do not use a bottle any larger than 500 cm³, ensure it is made from PET (this will be stated on the label or the bottle) and do not use the bottle more than once.
• Both demonstrator and audience should wear eye protection.
• The demonstrator should wear ear protectors, the audience should be at least 4 m away and advised to cover their ears with their hands.
• Do not use a larger bottle than specified, and use each bottle for one demonstration only.

Procedure 

1. Select a suitable place to fire the bottle – a corridor might be a better choice than a laboratory.
2. Prepare a launch pad – open a fairly heavy paperback book in the middle and place it, covers down, on a table, rest the bottle in the centre.
3. Fill a plastic carbonated drinks bottle of between 300 cm³ and 500 cm³ capacity with water and pour the water into a measuring cylinder to determine its total volume.
4. Pour one-third of the bottle’s volume of water back into the bottle and mark the level with a waterproof pen. 
5. Completely fill the bottle and invert it in a trough or washing-up bowl of water.
6. Place the end of a rubber tube connected to the gas tap under the neck of the bottle, and fill the bottle to the marked level with methane (natural gas) from the gas tap.
7. Fill the rest of the bottle with oxygen from the chosen source. The bottle now contains a 2 : 1 mixture of oxygen and methane by volume. This is the stoichiometric mixture.
8. Stopper the bottle with a rubber bung and place the bottle on the launch pad. Check the aiming of the rocket and ensure that none of the audience is near the flight path.
9. Wear eye and ear protection and advise the audience to cover their ears.
10. Remove the bung and ignite the gas mixture by applying a lighted splint to the neck of the bottle. The rocket will take off with a loud bang and fly for several metres.
11. If a second flight is to be done, use a new bottle.

Notes

• Remember to turn your gas sources on for a few seconds to allow air in the tube to be displaced before starting to fill the bottle.
• After firing, the rocket can be recovered and shown to the audience to point out that it is covered on the inside by condensation – droplets of water formed in the reaction. 
• The reaction is:
  CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) ΔH = -890 kJ mol^–1
• The gases react in a 2:1 ratio, and the reaction is strongly exothermic.
• Note that there are three moles of gas on both sides of the equation, so all the force that propels the rocket comes from the expansion of the gases as they are heated by the energy given out by the reaction, rather than by the production of extra molecules of gas.
• Please note: You can get students to balance the equation and try to work out for themselves what the ratio should be, then fill the bottle and launch.
• The best bang will be from a stoichiometric mix.