Bleach

Lurking in my laundry cupboard is an opaque, plastic bottle containing bleach. I use it to whiten white clothes when they start to look a bit grey, but I also use it to clean blood stains, to remove mildew from walls and ceilings, and to remove the mould from bathroom tiles. The active ingredient in laundry bleach is sodium hypochlorite, NaOCl, and you will find a similar substance present in the "liquid chlorine" you add to swimming pools as a disinfectant.

Sodium hypochlorite is a very useful chemical, but it can also be dangerous if used incorrectly. 

The September 2018 issue of AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter for chemistry students and teachers, looks at the chemistry of bleach.

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Determining Hypochlorite in Bleach

The active ingredient in household bleach is sodium hypochlorite, but just how much hypochlorite will you find in a litre of bleach?
You can easily find out in your school lab, all it takes is a redox titration, which, ofcourse, you will find at AUS-e-TUTE!
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Exploding Pool Chemicals

Sydney Morning Herald, Sunday 12^th January 2014, "Sydney man severely injured mixing chlorine for pool"
In summary, the victim of this accident had a backyard pool which had turned green. He had been advised to add 2 kilograms of "chlorine" to his pool to turn it sparkling blue again. He went home, put 2 kg of the "chlorine" granules into a bucket, and added water. Within 30 seconds the mixture exploded. The man was taken to hospital in a critical condition.

Now, for some chemistry....

From the description of the "chlorine" it sounds like it was calcium hypochlorite, Ca(OCl)₂ , a white solid at room temperature and pressure that is commonly sold as "pool chlorine".
When added to water, calcium hypochlorite produces hypochlorous acid, HOCl, and calcium hydroxide, Ca(OH)₂(aq).

Word equation:	calcium hypochlorite	+	water	gives	hypochlorous acid	+	calcium hydroxide
Chemical equation:	Ca(OCl)2	+	2H2O	→	2HOCl	+	Ca(OH)2

Dissolving calcium hypochlorite in water produces energy. The heat (or enthalpy) of solution is about 630 kJ mol^-1
We can now calculate how much energy was released when the man added water to 2 kg of calcium hypochlorite in the bucket.
First we need to calculate how many moles of calcium hypochlorite were in the bucket:
moles (Ca(OCl)₂ ) = mass ÷ molar mass
            and mass (Ca(OCl)₂) = 2 kg = 2 x 1000 = 2,000 g
            and molar mass (Ca(OCl)₂) = 40 + 2(16 + 35.5) = 143 g mol^-1
moles (Ca(OCl)₂ )  = 2000 ÷ 143 ≈ 14 mol
Now,  from the heat (enthalpy) of solution, we know that 1 mole of calcium hypochlorite dissolves in water to produce 630 kJ of energy.
We can calculate how much energy is released when 14 mol of calcium hypochlorite dissolves in water:
energy released = moles x energy in kJ mol^-1
energy released  = 14 x 630 = 8820 kJ
which is equivalent to exploding about 100 g of TNT in the bucket !

Reference:
http://www.smh.com.au/nsw/sydney-man-severely-injured-mixing-chlorine-for-pool-20140111-30nl3.html

Suggested Reading:
Defining Enthalpy Change 
Energy Profiles 
Strength of Acids
Strength of Bases 
pH of Aqueous Salt Solutions 


Suggested Study Questions:

1.  What is the name given to the type of reaction that produced energy?
2.  What name is given to the type of reaction that absorbs energy in order to produce products?
3. Sketch an energy profile diagram for the reaction between calcium hypochlorite and water.
4. Sketch an energy profile diagram for the explosion of TNT.
5. For 2 kg of calcium hypochlorite completely dissolving in water, calculate the moles of calcium hydroxide that would be produced.
6. For 2 kg of calcium hypochlorite dissolving in water, calculate the mass of hypochlorous acid that would be produced.
7. Assume the man used a 30 L of water in a 40 L bucket, what is the concentration of calcium hydroxide expected to be?
8. Hypochlorous acid is considered to be a very weak acid. What does this mean? 
9. Calcium hydroxide is considered to be a strong base. What does this mean?
10. What is the approximate pH of the solution when 2 kg of calcium hypochlorite is dissolved in  30 L of water?
11. What safety precautions do you think pool owners should take when mixing calcium hypochlorite granules with water?
12. Design a safety poster that could be displayed in a pool shop clearly showing the safest way to mix calcium hypochlorite granules with water. 

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 Cl_2(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 -101^oC and its boiling point is about -34^oC. So, chlorine will be a liquid at temperatures between -34^oC and -101^oC.
For comparison, your refrigerator is probably set to maintain a temperature of about 4^oC while the freezer has a temperature of around 0^oC, 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, Cl_2(g), through an aqueous solution of sodium hydroxide, NaOH_(aq) at room tmeperature:
Cl_2(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) → Cl_2(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 25^oC
   • chlorine gas is cooled to 4^oC
4. Sodium hydroxide has a melting point of 319^oC and a boiling point of 1390^oC 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 -192^oC, boiling point -110⁰C)
   • potassium chloride (melting point 772^oC, boiling point 1407^oC)
   • sulfur dioxide (melting point -75^oC, boiling point -10^oC)
   • ethanol (melting point -114^oC, boiling point 78^oC)