Organic Compunds - Testing and analysis AQA - C3

A quick overview of testing for organic substances followed by combustion analysis :)

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Key Points

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]
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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.

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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)

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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.

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Combustion Analysis

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
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Empirical Formula

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

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Comments

cherryducks

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Why thank you Amber.. =]

Amber Hornsby

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I tried :)

Well I made some for myself and I shall now enjoy using them :)

But thought I might as well let others use them :)

Alice Manterfield

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lovely =) like the creator =) xxxx

Ashleigh

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thankyou very much! :D

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