Folks this hasn't come up that much in the past however it would be daft not to look over this at least.
CFC's
- Stands for chlorofluorocarbons - e.g. CClF3
- Are chemically inert and so were used a lot for aerosol propellants and refrigerants
- However they have found that CFC's cause depletion of ozone layer - thats why now banned in may countries
Depletion of Ozone layer
- Ozone destroyed by a chain reaction involving free redicals
- Free radicals formed when a covalent bond breaks evenly: Cl2 --> .Cl + .Cl
- As its a chain reaction a small number of free radicals will react with many ozone molecules
Stages of destruction of ozone are:
1) UV light splits CFC to form chlorine free radicals: CClF3 --> .Cl + .CF3
2) Chlorine free radicals react with ozone (O3) to make more radicals: .Cl + O3 --> .ClO + O2
3) This free radical then takes part in more reactions which also make more free radicals - a chain reaction!!
Overall reaction: 2 O3 --> 3 O2 - This therefore reduces ozone levels and allows harmful UV to reach us.
CFC's only removed slowly from stratosphere - so even though they're banned it will take many years for them to fully disappear.
Here are the key points you need to take away from this unit and are likely to come up in the exam.
What is hard water? - caused by calcium or magnesium ions
- Carbon dioxide dissolves in water to make it slightly acidic
- This acidic water then reacts with calcium carbonate in rocks
- A solution of calcium hydrogen carbonate is formed - this is temporary hardness
Removal of Temporary Hardness
- Temporary hardness can be removed by boiling the water - it decomposes it!
- This forms calcium carbonate - this is the limescale that forms on the filament of the kettle!
- Weak acids (so they dont damage metal) can then be used to remove limescale from the kettle
Permanent Hardness
- Caused by dissolved calcium sulfate - this cannot be removed by heating!!
Removal of permanent Hardness
1) Adding washing soda (sodium carbonate). Works by calcium ions in hard water reacting with carbonate ions in the soda. This forms insoluble calcium carbonate which is then rinsed out with the dirty water.
2) Ion exchange column (picture below): Works by sodium ions in the column swap with calcium ion in hard water. Calcium ions stay in the column and sodium ions (do not cause hard water) come out.
How can we measure water hardness? (Very likely to come up in exam!!)
Water hardness stops soap from lathering (getting foam!!). To test water hardness in the lab we can:
- Get different water samples in different test tubes
- Add drops of soap from pipette and count the drops added until a lather (foam) forms
- The water that needs the most drops to form a lather is the hardest
NOTE: Remember if you have tap water and boiled tap water the boiling will remove some hardness (the temporary hardness) but the permanent hardness still remains!
I was asked yesterday to remind how to do empirical formula questions. Here is an example and how to work it out.
Q.
Calculate the enpirical formula of a compound containing 38.70% Carbon, 9.67% Hydrogen and 51.60% oxygen.
A.
Step 1: Divide each percentage by the atomic mass (Mr) of each atom:
C = 38.70/12 = 3.225 H= 9.67/1 = 9.67 O = 51.60/16 = 3.225
STEP 2: Divide by the lowest number (in this case 3.225):
C= 3.225/3.225 = 1 H= 9.67/3.225 = 3 O= 3.225/3.225 = 1
STEP 3: Write out empirical fomula - in this case CH3O
The main things to take away for oils and fats (Unit C6g) are as follows:
- Saturated means it contains only single bonds
- Unsaturated means it contains at least one double bond
- Animal Fats are often saturated
- Unsaturated fats and oils often come from vegetables and plants - they are better for you as saturated fats may lead to heart disease.
The test for unsaturation is that if you add yellow bromine water to to the oil it will go colourless if it is unsaturated. It will stay yellow if it is saturated.
Making Margerine We can make margerine by adding hydrogen to an unsaturated oil (in the presence of a catalyst). This turns the oil into a soft solid as it becomes more saturated.
Oil and water do not mix. But is you add a stabiliser to the mixture you can get droplets of oil suspended in the water - this is called an emulsion
Making Soap - This is called Sapononification. You heat the oils with sodium hydroxide:
Fat + Sodium Hydroxide --> Soap + Glycerol
The addition of the hydroxide make this a hydrolysis reaction
I have created this blog in the hope that you will find it useful whilst you are revising for your GCSE chemistry exam.
On it I will try and put useful revision points relating to maybe something we didn't get to in class or something you all found difficult. You need to follow the blog to be able to post comments. Please make the most of this feature. If there is something you can't do or don't understand then ask the question and I will try to answer it for you. That way everyone will hopefully benefit from it.
Feedback on how useful this is for you would also be appreciated. Good luck with your revision - remember work hard now and you can then have a relaxing summer!
