C3bL8  Organic Compounds

Key Words

Alkane  - a hydrocarbon with no double bonds

Alkene - a hydrocarbon with at least one double bond.

Bromine water - water with bromine dissolved in it.

Empirical Formula - the lowest whole number ratio of carbon to hydrogen

Organic - carbon compounds.

Relative formula mass - the mass in grams of the same number of particles as there are in 12.0g carbon-12.

Saturated - there are no double bonds

Unsaturated - there is at least one double bond

Test Yourself

Homework
Physics GCSE
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Grade E

Organic chemistry is about carbon and its compounds.  Since there are so many carbon compounds, it's a whole branch of chemistry.  Indeed, there are organic chemists who know very little inorganic and analytical chemistry, and vice versa.

Hydrocarbons are organic compounds consisting of hydrogen and carbon only.  Saturated hydrocarbons are used a lot for fuels, which burn to give carbon dioxide and water:

butane + oxygen ® carbon dioxide + water

2C4H10(g) + 18O2(g) ® 4CO2(g) + 10H2O(g)

Ethane is a saturated hydrocarbon.  It has as many hydrogen atoms as it can possibly have.  Ethene, with its double bond, is unsaturated.

 

  

Ethane                                                                          Ethene

Ethene is an alkene, which are used widely in the chemical industry.  We test for any alkene by adding bromine water.  If the bromine water goes from brown to colourless, then we have an alkene

 

Grade C

The molecular formula for ethene is C2H4.  The empirical formula gives the lowest possible whole number ratio, CH2, one carbon atom to every 2 hydrogen atoms.

The ideal fuel should:

  • have a lot of energy to release;

  • non-polluting;

  • abundant.

Fossil fuels are running out, and cause a rise in the levels of greenhouse gases.  Biofuels absorb carbon dioxide, but need a lot of space to be grown.  This could be used for food crops or places for people to live.

Grade A

To work out the empirical formula, we need to know the ratio of moles.  If we know the relative formula masses of the products, and how much product (carbon dioxide and water) there are, we can divide the mass of each product by the relative formula mass to give us the moles.

  • Carbon dioxide, CO2 has a relative formula mass of 12 + (2 × 16) = 44 [Note the typo in the book; they say "12 + (2 + 16)"]
  • Water, H2O has a relative formula mass of (1 × 2) + 16 = 18

Suppose 3.6 g of hydrocarbon are burned  to give 9.9 g CO2 and 8.1 g water.  What would be the empirical formula?

Moles of CO2 = 9.9 ÷ 44 = 0.225 mol

Moles H2O = 8.1 ÷ 18 = 0.45 mol

Water has two hydrogen atoms, so we need to multiply the value for water by 2.  So we have 0.9 moles hydrogen.

We can work out the ratio:

carbon : hydrogen

0.225 :0.9

1 : 4

The empirical formula is CH4.

Note that the empirical formula is just a ratio.  It does not give us the molecular formula, nor the display formula.  In this particular case, it does not matter.  We simply can't have a molecule of C2H8.  The maximum number of hydrogen atoms for 2 carbon atoms is 6.