Carboxylic acids

O Level / IGCSE Chemistry · Organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Carboxylic acids contain the –COOH group. Ethanoic acid, CH3COOH, is the acid in vinegar. They are weak acids, so they are only partly dissociated in water, but they still do the usual reactions of an acid with metals, bases and carbonates. The salts are called ethanoates, and only the hydrogen of –COOH is replaced.

Ethanoic acid is made by oxidising ethanol. In the laboratory this is done by warming ethanol with acidified aqueous potassium manganate(VII). In vinegar production, bacteria use oxygen from the air to do the same job.

A carboxylic acid reacts with an alcohol, with an acid catalyst, to form an ester and water.

Rules to remember

Common mistake

Students think a weak acid does not react with carbonates or metals. It reacts in the same way as a strong acid, only more slowly, because it has a lower concentration of H+ ions.

Worked example

5.3 g of sodium carbonate reacts with excess ethanoic acid: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2. What volume of carbon dioxide forms at room temperature and pressure? (C 12, O 16, Na 23; molar gas volume 24 dm3)

  1. Mr of Na2CO3 = (2 × 23) + 12 + (3 × 16) = 106.
  2. Moles of Na2CO3 = 5.3 ÷ 106 = 0.050 mol.
  3. 1 mol of Na2CO3 gives 1 mol of CO2, so moles of CO2 = 0.050 mol.
  4. Volume of CO2 = 0.050 × 24 = 1.2 dm3.

Answer: 1.2 dm3 of carbon dioxide

Practice questions

Try each one, then open the answer.

1. What is the formula of sodium ethanoate?

  1. A
    C2H5ONa
  2. B
    HCOONa
  3. C
    CH3COONa
  4. D
    CH3CH2COONa
Show answer

Answer: C. Ethanoic acid is CH3COOH; the H of –COOH is replaced by Na. CH3CH2COONa is sodium propanoate.

2. Wine left open to the air slowly turns into vinegar. What is happening?

  1. A
    the ethanol is fermented back into glucose by the yeast
  2. B
    the ethanol is reduced to ethane by the air
  3. C
    the ethanol is cracked into smaller molecules by sunlight
  4. D
    bacteria oxidise the ethanol to ethanoic acid
Show answer

Answer: D. Vinegar is a dilute solution of ethanoic acid, made by bacterial oxidation of ethanol using oxygen from the air.

3. Ethanoic acid reacts with magnesium to form magnesium ethanoate, (CH3COO)2Mg, and hydrogen. What mass of magnesium ethanoate is formed when 2.4 g of magnesium reacts with excess ethanoic acid? (H 1, C 12, O 16, Mg 24)

  1. A
    7.1 g
  2. B
    28.4 g
  3. C
    142 g
  4. D
    14.2 g
Show answer

Answer: D. 2CH3COOH + Mg → (CH3COO)2Mg + H2. 2.4 g of Mg is 0.10 mol, which makes 0.10 mol of salt. Mr of (CH3COO)2Mg = 2 × 59 + 24 = 142, so the mass is 14.2 g.

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