C14.3 – Hydrocarbons

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1. Describe the properties of alkanes (exemplified by methane) as being
   generally unreactive, except in terms of burning.

Alkanes are the most un-reactive hydrocarbons, except in combustion.

2. State that the products of complete combustion of hydrocarbons, exemplified by methane, are carbon dioxide and water.

When there is enough oxygen to burn the alkane, it is called complete combustion. Carbon dioxide and water are released.
CH₄(g)+ 2O₂(g)——–> CO₂(g)+2H₂O(l)

(When there is not enough oxygen, incomplete combustion takes place and carbon monoxide is released).

3. Name cracking as a reaction which produces alkenes.

Cracking is the process by which alkenes are produced.

Alkenes are in much more demand than the other hydrocarbons, so these hydrocarbons are thermally decomposed to form alkenes in a process known as cracking. High temperatures are required to crack the bonding on these compounds and a catalyst to speed it up.

For example:
C₄H₁₀ ——–> C₂H₆ + C₂H₄
butane ——-> ethane + ethene

5. Recognise saturated and unsaturated hydrocarbons:
   • from molecular structures
   • by their reaction with aqueous bromine.

We mentioned in the previous topic, that alkanes are called saturated hydrocarbons because they have a single bonding of carbon atoms, and alkenes are called unsaturated hydrocarbons because of the double bonding of carbon atoms.

From a molecular diagram, you should be able to identify if the hydrocarbon is saturated or not by looking at how many bonds the carbon atoms have between them.

We can physically test if a hydrocarbon is saturated or not by putting a few drops of bromine water into it. Bromine  water is yellow/orange in colour. If the bromine water is decolourised (loses it’s colour) the hydrocarbon is unsaturated., becuase all alkenes decolourise bromine water. An alkane/saturated hydrocarbon would not decolourise the bromine water.

6. Describe the addition reactions of alkenes, exemplified by ethene, with bromine, hydrogen and steam.

Alkenes undergo addition reactions, where two reactants are added together to form only one product.

• The reaction of alkenes with bromine water:

C₂H₄ +  Br₂ ———-> C₂H₄Br₂
ethene + bromine———> 1, 2-dibromoethane

The bromine has added across the double bond. Bromine water does not react with saturated compounds (alkanes) because they do not have a double bond to ‘open up’.

• Hydrogen reacts with alkenes to form alkanes:

C₂H₄  +   H₂ ————> C₂H₆
ethene + hydrogen ———-> ethane

This is also called a hydrogenation reaction. It happens under the influence of a catalyst and high temperature

• Steam reacts with alkenes to form alcohols:

C₂H₄ + H₂O ——–>  C₂H₅OH
ethene + steam ———> ethanol

The steam is passed over a catalyst under high temperature and pressure. A lot of pure ethanol is made this way.

 

 

Notes submitted by Lintha

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