What units are used to measure energy?
Well, that depends on who you are and what you are doing!
If you are a chemistry student, your preferred unit should be joules, but it is likely that you will have to convert between joules and kilojoules quite often.
You might also come across calories and kilocalories as a unit of measurement of energy, and you will then need to convert between these units, and between these and joules and kilojoules.
AUS-e-TUTE has set of resources to help you learn how do these conversions, and to practice your conversion skills.
There is currently a free-to-view tutorial available for evaluation purposes at:
http://www.ausetute.com.au/energyconv.html
Showing posts with label heat. Show all posts
Showing posts with label heat. Show all posts
Wednesday, November 26, 2014
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).
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:
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.
Friday, June 7, 2013
AUS-e-NEWS June 2013
Have you ever noticed that the price of E10 fuel, also known as gasohol, is less than the price of petrol (gasoline) ?
Just today, when I went to put fuel into my car, the price
for standard unleaded petrol (ULP) was 130.6 cents per litre while the price of
E10 fuel was 128.4 cents per litre.
Will using E10 instead of ULP save me money?
Read the June 2013 edition of AUS-e-NEWS to find out!
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