The main points you should understand by the end of this section is that:
- Organic compounds burn or char when we heat them
- We can detect carbon-carbon bonds using bromine water. [You should already know this from earlier topics in C1 and C2]
- We can work out the empirical formula of an organic compound from the ratio of the products when it burns [Higher]
Testing for organic substances!
So according to the AQA book, a scientist called Jakob Berzelius did some experiments to classify chemicals into two groups - organic and inorganic.
- Organic - Chemicals that burned or charred (went black with soot - i.e. carbon) on heating. They mainly came from living things.
- Inorganic - Other substances melted or vaporised on heating but returned to their original state when cooled.
However, the modern take on the definition of organic is substances containing mainly carbon.
Detecting the carbon-carbon double bonds
Unsaturated hydrocarbons (opposite of saturated which are full of hydrogen and therefore only have single carbon-carbon bonds) react with bromine water to produce a colourless compound.
Unsaturated hydrocarbon (colourless) + Bromine water (orange-yellow) -> Products (colourless)
Saturated compound (colourless) + Bromine water (orange-yellow) -> Products (orange-yellow)
Determining the number of carbon-carbon double bon
In the way that bromine is use to detect the double carbon-carbon bonds; iodine is ised to determine the number of carbon-carbon double bonds present.
The oil is titrated against an iodine solution
The 'iodine number' is based on the number of molecules of iodine needed to react with each carbon-carbon double bond present in one molecule of fat.
To work out how much of each product you end up with after combustion, you must follow two rules.
Number of Carbons present in the reactant = Number of Carbon Dioxide molecules
Number of Hydrogen present in the reactant divided by 2 = Number of Water molecules
Examples Include :
- Propane C3H8 + 5O2 -> 3CO2 + 4H2O
My step-by-step way of calculating the empirical formula from the amounts of the product formed from burning it!
Step 1: Work out the Mr of Carbon Dioxide and water
Step 2: Work out the moles of each substance by dividing the mass by the Mr
Step 3 : Work out the moles of Carbon and Hydrogen - this means the moles for carbon is the same as carbon dioxide [which was worked out above], but the moles of water has to be doubled to find out the moles of Hydrogen, i.e it's H20 :)
Step 4 : Divide the moles of hydrogen by the moles of carbon