9.26C–9.28C, 9.33C–9.34CAlcohols
Chemistry only — not on the Combined Science specification
Alcohols contain the functional group –OH. Methanol, ethanol, propanol and butanol are the first four members of a homologous series of alcohols.
Alcohols can be represented as a formula such as CH₃CH₂OH, or as a displayed structural formula in which the –OH group is drawn out. You should be able to recognise alcohols from their names or from given formulae — the ending -ol in a name, or an –OH group in a formula. You do not need to know the names of individual alcohols other than methanol, ethanol, propanol and butanol.
You should be able to describe what happens when any of the first four alcohols react with sodium, burn in air, are added to water, and react with an oxidising agent, and recall the main uses of these alcohols.
The main uses: ethanol is the alcohol in alcoholic drinks, and is also used as a fuel — added to petrol — and methanol is used as a feedstock in the chemical industry. Both ethanol and methanol are widely used as solvents, because they dissolve substances that water will not.
You are not expected to write balanced chemical equations for the reactions of alcohols other than for combustion reactions. For example, ethanol burns as C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Aqueous solutions of ethanol are produced when sugar solutions are fermented using yeast, and you should know the conditions used.
The conditions are a warm temperature of about 25–35 °C, an aqueous solution of sugar, yeast to provide the enzymes, and the absence of oxygen — anaerobic conditions. Too cold and the reaction is very slow; too hot and the enzymes in the yeast are denatured, so fermentation stops altogether.
Common mistake
Fermentation produces an aqueous solution of ethanol, not pure ethanol. To obtain concentrated ethanol the solution has to be separated afterwards by fractional distillation.
- Alcohol
- an organic compound containing the functional group –OH; the first four are methanol, ethanol, propanol and butanol
9.29C–9.31CCarboxylic acids
Chemistry only — not on the Combined Science specification
Carboxylic acids have the functional group –COOH. The first four members of a homologous series of carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid.
The structures of carboxylic acids can be represented as a formula such as CH₃COOH, or as a displayed structural formula showing the carbon double bonded to one oxygen and single bonded to an –OH.
You should be able to recognise carboxylic acids from their names or from given formulae — the ending -anoic acid, or a –COOH group. You do not need to know the names of individual carboxylic acids other than those four.
You should be able to describe what happens when any of the first four carboxylic acids react with carbonates, dissolve in water, and react with alcohols. You are not expected to write balanced chemical equations for the reactions of carboxylic acids.
- Carboxylic acid
- an organic compound containing the functional group –COOH; the first four are methanoic, ethanoic, propanoic and butanoic acid
9.30CWhy carboxylic acids are weak acidsHigher tier
Chemistry only — not on the Combined Science specification
Higher tier students should be able to explain why carboxylic acids are weak acids in terms of ionisation and pH — the ideas met under strong and weak acids.
The explanation: a carboxylic acid is only partially ionised in aqueous solution, so only a small proportion of its molecules release hydrogen ions. A strong acid such as hydrochloric acid is completely ionised, so every molecule releases its hydrogen ion. At the same concentration, the carboxylic acid therefore has a much lower hydrogen ion concentration, and so a higher pH.
Worked example
Solutions of hydrochloric acid and ethanoic acid have the same concentration, but the ethanoic acid has a pH three units higher. Explain.
- Both solutions contain the same amount of acid in the same volume — they are equally concentrated.
- Hydrochloric acid is a strong acid and is completely ionised, so almost all of it has released hydrogen ions.
- Ethanoic acid is a weak acid and is only partially ionised, so only a small fraction of its molecules have released hydrogen ions.
- A pH three units higher means a hydrogen ion concentration 1000 times lower, because each pH unit is a factor of 10.
Answer: Equal concentration but unequal ionisation: the ethanoic acid releases about a thousandth as many hydrogen ions, so its pH is three units higher. Concentration and strength are separate ideas.
Before you practise
- The alcohols are methanol, ethanol, propanol and butanol, and their functional group is –OH.
- Alcohols burn, react with sodium to give hydrogen, dissolve in water to give a neutral solution, and are oxidised to carboxylic acids.
- Ethanol is made by fermenting a sugar solution with yeast, warm and without air — about 30 °C and anaerobic.
- Fermentation stops at around 15% alcohol, so fractional distillation is used to concentrate it.
- The carboxylic acids are methanoic, ethanoic, propanoic and butanoic, and their functional group is –COOH.
- They behave as typical acids: with carbonates they give a salt, water and carbon dioxide.
- They are weak acids, so they only partially ionise — a solution has a higher pH than a strong acid of the same concentration.
- Members of a homologous series react alike because they share a functional group.