Saturday, January 11, 2014

Exploding Pool Chemicals

Sydney Morning Herald, Sunday 12th 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 !


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. 

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