Alcohols as Fuels

Alcohols combust more readily (break bonds - lower bond enthalpy) than equivalent alkanes but release less energy since they are already partially oxidized.

Alcohol + oxygen → carbon dioxide + water

Alcohols are used a fuels:

• As a fuel for cars - either pure or blended with petrol.

• Methanol as fuel for competitive motor-sports including dragsters and monster trucks.

Much fuel ethanol is fermented from crops… crops that could otherwise be eaten, forcing up food prices. Is this ok?

Grain + yeast → alcohol (ethanol)

Oxidation state of carbon:

Carbon in alcohol (methanol) is -2. Carbon is the alkane (methane) is -4.

CH₄ → CO₂ (Has a movement of 8 electrons - carbons oxidation #)

CH₃OH → CO₂ (Has a movement of 6 electrons - release less energy)

Oxidation of alcohols

The most important reactions of the alcohols are their oxidation’s.

A range of compounds will oxidize them so the oxidizer is often represented as [O].

One oxidizing agent you need to know is potassium dichromate K₂Cr₂O₇.

• When using this, orange Cr (VI) is reduced to green Cr (III).

C₂H₅OH + Cr₂O₇^-2 → CH₃COOH + Cr³⁺

oxidation 3x(H₂O + C₂H₅OH → CH₃COOH + 4H⁺ + 4e^-)

reduction 2x(6e^- + 14H⁺ + Cr₂O₇^-2 → 2Cr³⁺ + 7H₂O)

16H⁺ + 2Cr₂O₇^-2 + 3C₂H₅OH → 11H₂O + 3CH₃COOH + 4Cr⁺³

Oxidation Reaction Schemes