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