The reaction of ethyne with chlorine

Lumps of calcium carbide added to dilute sulfuric acid in a beaker produce ethyne gas. As this gas is released, addition of household bleach to the acid produces chlorine gas as well. When the two gases meet they react spontaneously, giving bursts of yellow flame and leaving a black sooty deposit of carbon.

Read our standard health & safety guidance

Lesson organisation

This is a quick teacher demonstration, taking 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 esily visible.

Apparatus and Chemicals

The teacher will need:

Eye protection
Safety screen
Access to a fume cupboard

Beaker, 250 cm³
Dropping pipette
Tongs

Calcium carbide, CaC₂ (Highly flammable), one fresh, pea-sized lump (see note 1)
Sodium chlorate(I) solution (Household bleach), 5% (0.7 mol dm^-3) (Irritant), 5 cm³, (see note 2)
Sulfuric acid, 2 mol dm^–3 (Corrosive), 50 cm³

Technical Notes

Calcium carbide (calcium dicarbide) (Highly flammable) Refer to CLEAPSS Hazcard 19B
Sodium chlorate(I) solution (Household bleach), 5% (0.7 mol dm^-3) (Irritant) Refer to CLEAPSS Hazcard 89 and Recipe Card 62
Sulfuric acid, 2 mol dm^–3 (Corrosive) Refer to CLEAPSS Hazcard 98A and Recipe Card 69

Small quantities of two hazardous gases are produced in this experiment:

Ethyne (Acetylene) (Extremely flammable) Refer to CLEAPSS Hazcard 45C
Chlorine (Toxic, Dangerous for environment) Refer to CLEAPSS Hazcard 22A

1 Calcium carbide reacts with moisture in the air, leaving a residua 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.

2 Avoid using household bleaches containing detergents or thickening agents. Sodium chlorate(I) solution purchased for chemical suppliers is more concentrated (about 1.5 mol dm^-3) and is classed as Corrosive. It should be diluted.

Procedure

HEALTH & SAFETY: This demonstration should be performed in the fume cupboard. Wear eye protection. Place a safety screen between the demonstration and the class.

a Place 50 cm³ of 2 mol dm^–3 sulfuric acid in a 250 cm³ beaker.

b Add one pea-sized lump of calcium carbide. This will sink and react to give off bubbles of ethyne gas.

c Now add about 1 cm³ of domestic bleach to the acid using a dropping pipette. Chlorine gas is evolved.

d 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.

e More bleach or calcium carbide may be added as appropriate to continue the reaction.

f 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)
or
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 build up 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.

Health & Safety checked, August 2008