AlcoholsIB MYP Chemistry: Revision notes
Section 1
What are alcohols?
Alcohols are organic compounds whose molecules contain the hydroxyl functional group, –OH. The functional group is the atom or group of atoms that decides how a compound reacts, so all alcohols have similar chemical properties.
Alcohols form a homologous series with the general formula CₙH₂ₙ₊₁OH. Each member differs from the next by one CH₂ group.
Section 2
Names and formulae
Alcohol names end in -ol and the first part shows the number of carbon atoms:
- 1 carbon: methanol CH₃OH
- 2: ethanol C₂H₅OH
- 3: propanol C₃H₇OH
- 4: butanol C₄H₉OH
In the displayed formula of ethanol, there are two carbon atoms joined by a single bond, five hydrogen atoms attached to the carbon atoms, and an –OH group on the end carbon atom, with every bond shown as a line.
Count the carbon atoms for the prefix (meth-, eth-, prop-, but-), then add -ol. C₃H₇OH is propanol, not propane.
Section 3
Combustion of alcohols
Alcohols burn in plenty of oxygen to form carbon dioxide and water and release energy:
alcohol + oxygen → carbon dioxide + water
Ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O Methanol: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O
Check ethanol: oxygen atoms on the left are 1 + 6 = 7, and on the right 4 + 3 = 7, so the equation is balanced. Longer alcohols release more energy per mole when they burn.
In limited oxygen, incomplete combustion produces carbon monoxide or soot.
Section 4
Uses of alcohols
As fuels: ethanol is burned in cars (alone or blended with petrol as bioethanol), and methanol is used in some racing engines. Ethanol made from plants is renewable.
As solvents: a solvent is a liquid that dissolves other substances. Ethanol dissolves many substances that water cannot, for example the oils in perfumes, and it evaporates quickly. It is used in perfumes, aftershaves, inks, paints and hand sanitisers.
Alcohols are flammable, so they must be kept away from flames. Methanol is toxic.
Section 5
Making ethanol by fermentation
Fermentation is the breakdown of sugar by the enzymes in yeast to make ethanol and carbon dioxide:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Conditions:
- Yeast (its enzymes act as the catalyst).
- A warm temperature, about 25–35 °C. Too cold and it is slow; too hot and the enzymes are destroyed.
- No oxygen (anaerobic conditions).
The mixture contains only about 15% ethanol, so it is separated from the water by distillation (ethanol boils at about 78 °C, water at 100 °C).
Fermentation does not need oxygen. If oxygen is present, yeast respires aerobically and makes carbon dioxide and water instead of ethanol.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Alcohols
- A teacher writes the formulae CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH on the board and tells her class that they are the first four members of a family of compounds called alcohols.Explain why methanol and ethanol have similar chemical properties.2 marks
- A bioethanol plant in Thailand makes ethanol from the sugar in sugar cane. The sugar solution is fermented in large tanks that are kept at about 30 °C.State two other conditions, apart from temperature, that are needed for fermentation to produce ethanol.2 marks
- A student in Cape Town compares the energy released when four alcohols burn. For each alcohol she burns 0.0050 mol in a spirit burner under a metal can containing 100 cm³ of water, and she measures the temperature rise of the water. The temperature rises were: methanol 6 °C, ethanol 11 °C, propanol 17 °C and butanol 22 °C.To make the comparison fair, the student kept some variables the same. Describe two variables that she should control and one safety precaution she should take.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).