The reaction of ethyne with chlorine

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Illustrate the spontaneous reaction of ethyne and chlorine, with its bursts of yellow flame and black sooty deposit

In this demonstration, students observe what happens when lumps of calcium carbide are added to dilute sulfuric acid in a beaker to produce ethyne gas. As this gas is released, the addition of household bleach to the acid produces chlorine gas as well. Students then watch as the two gases meet and react spontaneously, producing mini-explosions with yellow flames and leaving a black sooty deposit of carbon.

The experiment should take no longer than about 5 minutes. It should be performed in a fume cupboard or hood to avoid exposure to chlorine in particular. In a slightly darkened room the flashes of flame that accompany the mini-explosions are more easily visible.

Equipment

Apparatus

Eye protection
Safety screen
Access to a fume cupboard
Beaker, 250 cm³
Dropping pipette
Tongs

Chemicals

Calcium carbide, CaC₂ (HIGHLY FLAMMABLE), one fresh, pea-sized lump (see note 4 below)
Sodium chlorate(l) solution (sodium hypochlorite, household bleach), 5% (0.7 M) (IRRITANT), 5 cm³ (see note 5 below)
Sulfuric acid, 2 M (CORROSIVE), 50 cm³

Health, safety and technical notes

Read our standard health and safety guidance.
Wear eye protection throughout. Place a safety screen between the demonstration and the class.
The demonstration should be performed in the fume cupboard.
Calcium carbide (calcium dicarbide) (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC019C. Calcium carbide reacts with moisture in the air, leaving a residue of calcium hydroxide. In selecting calcium carbide lumps for this experiment, avoid pieces coated with significant white powdery deposit, indicating they have deteriorated in storage. Ensure that calcium carbide is always kept in a securely capped container in a dry environment.
Sodium chlorate(I) solution (sodium hypochlorite, household bleach), 5% (0.7 M) (IRRITANT) – see CLEAPSS Hazcard HC089 and CLEAPSS Recipe Book RB081. Avoid using household bleaches containing detergents or thickening agents. Sodium chlorate(I) solution (sodium hypochlorite) purchased for chemical suppliers is more concentrated (about 1.5 mol dm^-3) and is classed as Corrosive. It should be diluted.
Sulfuric acid, 2 M (CORROSIVE) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.
Small quantities of two hazardous gases are produced in this experiment:
- Ethyne (Acetylene) (EXTREMELY FLAMMABLE) – see CLEAPSS Hazcard HC045c.
- Chlorine (TOXIC, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC022a.

Procedure

Place 50 cm³ of 2 M sulfuric acid in a 250 cm³ beaker.
Add one pea-sized lump of calcium carbide. This will sink and react to give off bubbles of ethyne gas.
Now add about 1 cm³ of domestic bleach to the acid using a dropping pipette. Chlorine gas is evolved.
Within a few seconds (or possibly immediately) the two gases will react with explosive ‘pops’, mostly at the surface, giving a yellow flame and black sooty smoke. Intermittent flames will continue for about a minute.
More bleach or calcium carbide may be added as appropriate to continue the reaction.
If necessary, quench the reaction by pouring into a large bowl of cold water in the fume cupboard.

Teaching notes

The reaction of calcium carbide with water to form ethyne in is still used occasionally by cavers in ‘carbide lamps’ to produce a bright yellow flame by burning the gas:

CaC₂(s) + 2H₂O(aq) → C₂H₂(g) + Ca(OH)₂(aq)

In the presence of dilute sulfuric acid this becomes:

CaC₂(s) + H₂SO₄(aq) → C₂H₂(g) + CaSO₄(s)

Chlorine is produced from bleach (a mixture of sodium chlorate(I) and sodium chloride) by the reaction:

ClO^– (aq) + Cl^–(aq) + 2H⁺(aq) →Cl₂(g) + H₂O(l)

The reaction of ethyne with chlorine can be simple addition across the double bond:

C₂H₂(g) + Cl₂(g) →CHCl=CHCl(l)

C₂H₂(g) + 2Cl₂(g) →CHCl₂–CHCl₂(l)

However, under the conditions of this experiment, chlorine reacts explosively with ethyne, removing hydrogen from the hydrocarbon and depositing soot, in what is probably a free radical reaction:

C₂H₂(g) + Cl₂(g) →2C(s) + 2HCl(g)

The intermittent reaction seems to be caused by a buildup of ethyne followed by reaction in which it is all used up. More ethyne is evolved and it again builds up to a critical value and so on.