Chemistry WJEC 3 - Organic Chemistry

Alkanes are saturated hydrocarbons. This means that all of their bonds are used up and there is no opportunity to allow other atoms to join onto the molecule. Alkanes have the general formula of, C_nH_2n+2 where C is carbon, H is hydrogen and n is any interger.  

Methane – CH₄  

Ethane – C₂H₆

Propane – C₃H₈

Butane – C₄H₁₀

All alkenes are made up of carbon and hydrogen, and all contain at least one carbon to carbon double bond, represented by C=C. Alkenes are said to be unsaturated because their double bond can open up and let other atoms join on. They have the general formula C_nH_2n.

Ethene  - C₂H₄

Butene – C₄H₈

Pentene – C₅H₁₀

All alcohols contain the functional group (-OH).

Methanol – CH₃OH

Ethanol – C₂H₅OH

Propanol – C₃H₇OH

Enzymes are catalysts produced by living cells. In a enzyme catalysed reactions the more the temperature rises, the more the enzyme action increases because they gain more kinetic energy for a collision.

During fermentation, carbohydrates from plants are broken down into sugars and then converted into ethanol by the action of enzymes from yeast. The carbohydrates come from crops (sugar cane etc.). The key step is anaerobic respiration.

Glucose             Ethanol + Carbon Dioxide

C₆H₁₂O₆               2C₂H₅OH + 2CO₂

The rate of reaction is affected by temperature. It is slow at low temperatures, but the enzymes are denatured if the temperature is too high. A compromise temperature of 35 degrees is used.
Air is kept out to stop the oxidation of ethanol to ethanoic acid.
Once the fermenting solution contains about 15% ethanol the enzymes are unable to function and fermentation stops.

Why is Ethanol Used as a Car Fuel?

Ethanol releases ~ 70% of the energy as the same volume if petrol, however; ethanol is cheaper, carbon neutral and renewable.

Why Isn’t Ethanol Used as a Fuel in the UK?

The UK has a somewhat dreary and cold climate, whereas sugar cane (where ethanol comes from naturally) requires a hot and sunny climate to be grown. Also, there is not enough land available to grow enough sugar cane to produce fuel for the whole county.

The Fire triangle is a diagram which lists the three ingredients needed for most fires. These include

• Oxygen
• Heat 
• Fuel

Microbial oxidisation occurs, whereby microbes in the air respire in the wine, thus using up oxygen and thus converting the ethanol to ethanoic acid, the acid found in vinegar.

Ethanol + Oxygen                    Ethanoic Acid + Water

C₂H₅OH + O₂                     CH₃COOH + H₂O