Tests For Organic Compounds

The previous revision cards were about testing for inorganic compounds (things not built around a chain of carbon atoms).

However, the mystery substance in our exam could easily be organic.

In that case, here's what you have to do... 

Organic Compounds burn when heated:

1. Organic compounds burn in air, with a yellow-orange and/or blue flame. The greater the proportion of carbon in the compound, the more yellow and smoky the flame is. 

2. When there is plenty of air available, burning a hydrocarbon (an organic compound containing only hydrogen and carbon) produces carbon dioxide and water. If the amount of air is reduced then the carbon monoxide and carbon (soot) can also be produced.  

3. Solid organic compounds will char - i.e. their surface will get scorched with black marks of carbon.

Compounds with C=C bonds decolourise bromine water.

1. If your organic compound is unsaturated (i.e. it has double or triple bonds between carbon atoms), it will decolourise bromine water.

2. If your organic compound is saturated (i.e. there are no double or triple bonds), the bromine water will stay brown. 

3. Tou can do ths test on margarine, which has C=C bonds. Shake 1cm3 of bromine water with a small amount of melted margarine, and the bromine water decolourises.   

How to find the empirical formula of an organic compound: 

The empirical formula shows the ratios of all the elements in a substance. You can work out the empirical formula of an organic compound by burning a known mass of it completely in oxygen, and measuring the masses of all the products.

With a hydrocarbon, all the carbon ends up in CO2 and all the hydrogen ends up in water.

Just 3 steps...

1. Find the mass of each element in the compound.

- To find the mass of carbon in the compound, multiply the mass of CO2 produced by the proportion of C in CO2.

- Using relative atomic masses, the proportion of C in CO2 is 12 / 44 = 0.2727...

- And the proportion of H in H2O is 2 / 18 = 0.1111

Just 3 steps...

2. Divide these masses of C and H by the atomic masses of C and H (to find the number of moles).

Just 3 steps...

3. Divide both by the smallest one. This gives the simplest ratio of atoms of each element.   

Example: 0.4g of an organic hydrocarbon is burnt completely in oxygen. 1.1g of carbon dioxide and 0.9g of water are formed. What is the compounds empirical formula?

1. Find the mass of carbon in the compound: 1.1 × (12 / 44) = 0.3g. Do the same for hydrogen: 0.9 × (2 / 18) = 0.1g

2. The relative atomic mass of carbon is 12, so: 0.3 / 12 = 0.025 mol. The relative atomic mass of hydrogen is 1, so: 0.1 / 1 = 0.1 mol

3. Divide the biggest answer by the smallest one to get the ratio of carbon to hydrogen. The simplest whole number ratio of atoms of each element is 0.1 / 0.025 = 4 (meaning there is 1 carbon to 4 hydrogens). This gives an empirical formula for this compound of CH4.