Carboxylic acidsIB MYP Chemistry: Revision notes
Section 1
What are carboxylic acids?
Carboxylic acids are organic compounds that contain the carboxyl functional group, –COOH. They form a homologous series with the general formula CₙH₂ₙ₊₁COOH.
Their names end in -oic acid, and the first part shows the number of carbon atoms, including the carbon in the –COOH group:
- 1 carbon: methanoic acid HCOOH
- 2: ethanoic acid CH₃COOH
- 3: propanoic acid C₂H₅COOH
- 4: butanoic acid C₃H₇COOH
Count the carbon atom in the –COOH when naming: CH₃COOH has 2 carbons, so it is ethanoic acid (not methanoic).
Section 2
Weak acids
When an acid dissolves in water it releases hydrogen ions (H⁺). A strong acid, such as hydrochloric acid, is completely ionised. A weak acid, such as ethanoic acid, is only partially ionised: only a small fraction of its molecules release H⁺ ions, and the rest stay as whole molecules.
For the same concentration, a weak acid has a lower concentration of H⁺ ions, so it has a higher pH and reacts more slowly. For example, 0.10 mol dm⁻³ hydrochloric acid has a pH of about 1, and 0.10 mol dm⁻³ ethanoic acid has a pH of about 3.
Weak does not mean dilute. 'Weak' describes how much the acid ionises; 'dilute' describes how much acid is dissolved in the water.
Section 3
Reactions of carboxylic acids
Carboxylic acids react like other acids and form salts that end in -oate.
- With a metal: acid + metal → salt + hydrogen. Mg + 2CH₃COOH → (CH₃COO)₂Mg + H₂ (magnesium ethanoate). Test hydrogen with a lighted splint: squeaky pop.
- With a carbonate: acid + carbonate → salt + water + carbon dioxide. CaCO₃ + 2CH₃COOH → (CH₃COO)₂Ca + H₂O + CO₂. The mixture fizzes and the gas turns limewater milky.
- With a base: acid + base → salt + water. CH₃COOH + NaOH → CH₃COONa + H₂O (sodium ethanoate).
The reactions are slower than with a strong acid of the same concentration.
Section 4
Making esters
A carboxylic acid reacts with an alcohol to form an ester and water. The reaction needs a concentrated sulfuric acid catalyst and gentle heating:
carboxylic acid + alcohol ⇌ ester + water
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
ethanoic acid + ethanol ⇌ ethyl ethanoate + water
The name of the ester comes from the alcohol first (ethyl) and then the acid (ethanoate). Esters have sweet, fruity smells and are used in perfumes and food flavourings.
Section 5
Ethanoic acid and vinegar
Vinegar is a dilute solution of ethanoic acid, about 5% by volume. It is made when ethanol in wine or beer reacts with oxygen. It is used for flavouring food, in salad dressings and for pickling. The acidity stops bacteria and mould from growing.
Pure ethanoic acid is corrosive and has a sharp smell, so it must be handled with care. Dilute vinegar is safe to eat because ethanoic acid is a weak acid.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Carboxylic acids
- A teacher writes the formulae HCOOH, CH₃COOH, C₂H₅COOH and C₃H₇COOH on the board. They are the first four members of a family of organic compounds.Explain why ethanoic acid is a weak acid, whereas hydrochloric acid of the same concentration is a strong acid.2 marks
- A student investigates how ethanoic acid reacts with different substances: magnesium ribbon, calcium carbonate and sodium hydroxide solution.Write the word equation for the reaction between ethanoic acid and sodium hydroxide.2 marks
- A student compares ethanoic acid and hydrochloric acid, each with a concentration of 0.10 mol dm⁻³. She measures the pH of each acid with a pH meter. She then times how long a 2 cm strip of magnesium ribbon takes to dissolve completely in 50 cm³ of each acid at 20 °C. Hydrochloric acid had a pH of 1.0 and the magnesium dissolved in 40 s. Ethanoic acid had a pH of 2.9 and the magnesium dissolved in 210 s.State a testable hypothesis that the student could have been investigating, and include a scientific reason for it.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).