AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The structures of carboxylic acids can be represented in the following…
Full specification wording
Carboxylic acids: The structures of carboxylic acids can be represented in the following forms: CHCOOH or Students should be able to: describe what happens when any of the first four carboxylic acids react with carbonates, dissolve in water, react with alcohols (HT only) explain why carboxylic acids are weak acids in terms of ionisation and pH (see Strong and weak acids (HT only) (page 48)).
Carboxylic acids can be shown as displayed formulae, structural formulae or molecular formulae such as CH₃COOH.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Carboxylic acids can be shown as displayed formulae, structural formulae or molecular formulae such as CH₃COOH. Their reactions follow from the –COOH group. With carbonates they fizz, producing a salt, water and carbon dioxide; for example, ethanoic acid plus sodium carbonate gives sodium ethanoate, water and carbon dioxide. In water they dissolve and ionise only slightly, releasing a small number of H⁺ ions, so their pH is higher than that of a strong acid of the same concentration. At higher tier, carboxylic acids react with alcohols to form esters, typically with an acid catalyst. The limited ionisation explains both the weak acidity and the relatively high pH.
What earns the marks
- Represent carboxylic acids using displayed, structural or molecular formulae such as CH₃COOH.
- Describe the reaction with a carbonate as producing a salt, water and carbon dioxide, with effervescence.
- Name the salt formed from the acid and the metal in the carbonate, for example sodium ethanoate.
- Describe dissolution in water as forming an acidic solution that ionises only slightly.
- Explain weak acidity in terms of partial ionisation giving few H⁺ ions and a higher pH than a strong acid.
- At higher tier, describe reaction with an alcohol to form an ester, usually with an acid catalyst.
Common mistakes and how to put them right
Saying carboxylic acids fully ionise in water.
Put it right by stating that ionisation is partial, so only a small proportion of molecules release H⁺ ions.
Predicting hydrogen gas when a carboxylic acid reacts with a carbonate.
Put it right by naming carbon dioxide, water and a salt as the products.
Confusing the ester product with the alcohol or acid reactant.
Put it right by identifying the ester formed from the acid and the alcohol, for example ethyl ethanoate from ethanoic acid and ethanol.
Exam tips
- For carbonate reactions, write the general word equation acid + carbonate → salt + water + carbon dioxide before substituting names.
- When explaining weak acidity, compare the extent of ionisation and the resulting pH with a strong acid of the same concentration.
- At higher tier, link the alcohol and acid names directly to the ester name, remembering the acid provides the '-oate' ending.
Try the lesson
The start of the Carboxylic acids: The structures of carboxylic acids can be represented in the following forms: CHCOOH or Students should be able to: describe what happens when any of the first four carboxylic acids react with carbonates, dissolve in water, react with alcohols (HT only) explain why carboxylic acids are weak acids in terms of ionisation and pH (see Strong and weak acids (HT only) (page 48)). lesson, made from checked teaching notes for this specification point.
Have a go first
Which ion is released when a carboxylic acid such as ethanoic acid dissolves in water?
Listen
Carboxylic acids can be shown as displayed formulae, structural formulae or molecular formulae such as CH₃COOH. Their reactions follow from the –COOH group. With carbonates they fizz, producing a salt, water and carbon dioxide; for example, ethanoic acid plus sodium carbonate gives sodium ethanoate, water and carbon dioxide. In water they dissolve and ionise only slightly, releasing a small number of H⁺ ions, so their pH is higher than that of a strong acid of the same concentration. At higher tier, carboxylic acids react with alcohols to form esters, typically with an acid catalyst. The limited ionisation explains both the weak acidity and the relatively high pH. A strong answer earns marks for these points. Represent carboxylic acids using displayed, structural or molecular formulae such as CH₃COOH. Describe the reaction with a carbonate as producing a salt, water and carbon dioxide, with effervescence. Name the salt formed from the acid and the metal in the carbonate, for example sodium ethanoate. Describe dissolution in water as forming an acidic solution that ionises only slightly. A common slip: Saying carboxylic acids fully ionise in water. Instead, stating that ionisation is partial, so only a small proportion of molecules release H⁺ ions. Exam tip: For carbonate reactions, write the general word equation acid + carbonate → salt + water + carbon dioxide before substituting names.
Key words
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Representing organic structures
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Acid reactions with carbonates
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Weak acids, ionisation and pH
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Esterification with alcohols
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See every definition
- Representing organic structures
- Carboxylic acids can be shown as displayed formulae, structural formulae or molecular formulae such as CH₃COOH.
- Acid reactions with carbonates
- Carboxylic acids fizz with carbonates, producing a salt, water and carbon dioxide, for example sodium ethanoate from ethanoic acid and sodium carbonate.
- Weak acids, ionisation and pH
- Carboxylic acids dissolve in water and ionise only slightly, releasing few H⁺ ions, so their pH is higher than that of a strong acid of the same concentration.
- Esterification with alcohols
- At higher tier, carboxylic acids react with alcohols to form esters, usually with an acid catalyst.
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Test your understanding of Carboxylic acids: The structures of carboxylic acids can be represented in the following forms: CHCOOH or Students should be able to: describe what happens when any of the first four carboxylic acids react with carbonates, dissolve in water, react with alcohols (HT only) explain why carboxylic acids are weak acids in terms of ionisation and pH (see Strong and weak acids (HT only) (page 48)).
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