Friday, November 25, 2011

Liquid Chlorine?

What is wrong with this picture?

Is it possible for an ordinary plastic bottle with a screw cap to contain liquid chlorine?
Probably not!

Chlorine exists as a diatomic yellow-green gas at room temperature and pressure, that is, chlorine exists as Cl2(g).
In order to produce liquid chlorine we could:
  • lower the temperature of the bottle to change the gas into a liquid at atmospheric pressure.
  • raise the pressure within the bottle to change the gas into a liquid at room temperature.
  • lower the temperature and raise the pressure at the same time.
At 1 atmosphere pressure, the melting point of chlorine is about -101oC and its boiling point is about -34oC. So, chlorine will be a liquid at temperatures between -34oC and -101oC.
For comparison, your refrigerator is probably set to maintain a temperature of about 4oC while the freezer has a temperature of around 0oC, not cold enough to liquefy chlorine! A plastic bottle sitting on the shelf in your garage is not going to be cold enough to store chlorine as a liquid!

Gaseous chlorine could also be changed into a liquid by applying pressure. At room temperature this can be achieved with a pressure about 8 times that of atmospheric pressure, which is highly unlikely to occur in our plastic bottle with the screw cap.

So, the fluid in the plastic bottle labelled "liquid chlorine" is not chlorine. What is it?
It is most likely to be an aqueous solution of sodium hypochlorite, NaClO(aq).
Aqueous solutions of sodium hypochlorite are produced by bubbling chlorine gas, Cl2(g), through an aqueous solution of sodium hydroxide, NaOH(aq) at room tmeperature:
Cl2(g) + 2NaOH(aq) → NaClO(aq) + NaCl(aq) + H2O(l)

When the aqueous sodium hypochlorite solution is mixed with dilute acid, chlorine gas is released:
2H+(aq) + OCl-(aq) + Cl-(aq) → Cl2(g) + H2O(l)

The chlorine gas that is released can kill bacteria and other microbes, so aqueous solutions of hypochlorites are often used as disinfectants.

Further Reading
Chemical and Physical Changes
Kinetic Theory of Gases

Suggested Study Questions:
  1. Identify each of the changes below as either a chemical change or a physical change:
    • freezing water in a freezer
    • boiling water in a kettle
    • cooling chlorine gas to make chlorine liquid
    • boiling liquid chlorine to make chlorine gas
    • bubbling liquid chlorine though aqueous sodium hydroxide solution to form a solution of sodium hypochlorite
    • bubbling chlorine gas through water to make hypochlorous acid
  2. Name each of the physical changes above.
  3. Use the kinetic theory of matter to explain what happens to chlorine molecules when:
    • chlorine gas is cooled to produce liquid chlorine at 1 atm pressure
    • chlorine gas is subjected to a pressure of more than 8 atmospheres at 25oC
    • chlorine gas is cooled to 4oC
  4. Sodium hydroxide has a melting point of 319oC and a boiling point of 1390oC at 1 atm pressure. Describe how you could produce sodium hydroxide liquid.
  5. Which of the following pure substances could be kept in an ordinary plastic bottle with a screw cap on a shelf in your garage?
    • ozone (melting point -192oC, boiling point -1100C)
    • potassium chloride (melting point 772oC, boiling point 1407oC)
    • sulfur dioxide (melting point -75oC, boiling point -10oC)
    • ethanol (melting point -114oC, boiling point 78oC)

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