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Unit 2 – Matter and Chemical Bonds

Lab Investigation: Combustion of Acetylene

Written By: Jerry Liu

Question: What ratio of acetylene to air is needed for complete combustion to occur?

Purpose: To find the ratio needed for a complete combustion of acetylene with oxygen.

Hypothesis: If acetylene is combusted with a 1 to 9 ratio with air, then a complete combustion will

occur.

Materials: Chemicals:

- Bunsen burner - Calcium Carbide

- Flint Lighter

- Forceps

- Grease pencil

- Wooden splint

- Test tubes (x4)

- 1000ml beaker

- Water

Procedure:

1. Marked four test tubes using a grease pencil with the following volumes: 1/2, 1/3, 1/5, 1/10.

2. Filled a 1000ml beaker with water.

3. Filled each of the test tubes full of water and inverted them into the beaker without letting the

water leak from the test tubes.

4. With the use of the forceps, a small piece of calcium carbide was placed into the beaker.

- Bubbles were created at this step. The bubbles are acetylene gas.

5. Took the inverted test tubes one by one and held them over the bubbles to collect the

acetylene.

6. Each test tube was filled with acetylene gas up to the marked volumes.

7. With proper procedures, the Bunsen burner was lit.

8. Took the test tube that was filled half way with acetylene from the beaker; the water was slowly

let out and the air replaced the water.

9. The opening of the test tube was covered and the contents within were mixed by turning the

test tube up and down.

10. The test tube was held horizontally and a burning splint was inserted into the test tube.

11. Observations of the reaction and products were recorded.

12. Steps 7,8,9, and 10 were repeated for the remaining test tubes.

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Observations:

Level of acetylene Level of Air Description of Reaction and Product

½ ½ When the acetylene and air mixture was ignited by introducing the burning splint, the fire kept burning for a

while.

A white smoke was produced.

A Yellow smoke was produced.

Lots of black soot.

An “burnt wood” smell is released.

1/3 2/3 When the acetylene and air mixture was ignited by introducing the burning splint, the fire kept burning for a

shorter period of time compared to the previous reaction.

Lots of black soot.

1/5 4/5 When the acetylene and air mixture was ignited by introducing the burning splint, the fire went out quickly

compared to the previous reactions.

Only a little bit of black soot was created.

1/10 9/10 When the burning splint was introduced to the acetylene and air mixture, no visible reaction could be observed other than

a loud “pop”.

Discussion:

1. In which of the test tube(s) did complete combustion occur? How could you tell?

A complete combustion occurred within test tube #4 with a 1 to 9 ratio. This could be

told because it was a very clean combustion as there was no soot created.

2. In which test tube(s) did incomplete combustion occur? What other products were formed?

In test tubes # 1 – 3, an incomplete combustion occurred. There was lots of black soot

created when the mixture finished reacting.

3. A) Write a balanced chemical equation for the complete combustion of acetylene gas.

2C2H2 (g) + 5O2 (g) -> 4CO2 (g) + 2H2O (g)

B) Write a balanced chemical equation for the incomplete combustion of acetylene gas.

C2H2 (g) + O2 (g) -> CO (g) + CO2 (g) + H2O (g) + soot

4. When welders first light an acetylene torch, the flame is yellow coloured and sooty. Why? What

needs to be done in order to get the proper welding flame?

When welders first light an acetylene torch, an incomplete combustion reaction is

occurring. When an incomplete combustion is reacting, the flame will be yellow and sooty. In

order to get to the proper welding flame, the welder must increase the oxygen to acetylene

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ratio to around 1 part acetylene and 9 parts oxygen; because then, a complete combustion

reaction will occur.

5. Seasonal furnace maintenance is essential to make sure your furnace is running efficiently and

safely. A furnace that is not running at peak performance can be deadly due to the production of

a toxic gas. Name this toxic gas and describe its effect on a red blood cell.

When a furnace is not running at its peak performance, the fuel that it is burning will not

be cleanly burned (therefor not a complete combustion). When this happens, a deadly gas called

carbon monoxide is produced. Carbon monoxide will bond to your red blood cells very easily and

it will prevent your red blood cells from bonding with oxygen. When that happens, your body

will slowly suffocate as your red blood cells are all bonded with carbon monoxide and no longer

transport oxygen through your body.

Conclusion:

Through the completion of the lab and the results recorded from the observations, the

acetylene to air ratio necessary for complete combustion is 1 to 9.