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4.6Organic Chemistry

Carboxylic acids — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 18 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

Carboxylic Acids: Properties and Reactions — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the structure, naming and reactions of carboxylic acids, including why they are weak acids.Carboxylic acids contain the carboxyl group –COOH and have the general formula CnH2n+1COOH.

  • bases and alkalis
  • carbonates
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Section 1 of 8Structure and naming

Carboxylic acids contain the carboxyl group –COOH and have the general formula CnH2n+1COOH.

Structure and naming
-oic acid
The names end in -oic acid: methanoic acid HCOOH, ethanoic acid CH3COOH, propanoic acid CH3CH2COOH and butanoic acid CH3CH2CH2COOH.
The carbon atom of the carboxyl group is counted in the chain, so ethanoic acid has two carbons.
Ethanoic acid is the acid in vinegar, which contains about 5% ethanoic acid.
Carboxylic acids are formed by oxidation of alcohols, either with an oxidising agent (acidified potassium dichromate(VI)) or by bacterial action on ethanol in air.

Section 2 of 8Properties: weak acids

Carboxylic acids dissolve in water to form acidic solutions with pH about 3 for dilute solutions.

Weak acids
They are weak acids: only a small fraction of the molecules ionise, CH3COOH ⇌ CH3COO- + H+, and the reaction is reversible.
Compared with a strong acid of the same concentration,
  • such as hydrochloric acid, a carboxylic acid has a higher pH, reacts more slowly with metals and carbonates and conducts electricity less well,
  • but it will react with the same total amount of base because it has the same number of acidic hydrogens.
Property0.1 mol/dm3 HCl0.1 mol/dm3 ethanoic acid
pHAbout 1About 3
IonisationCompletePartial
Rate with magnesiumFastSlower
Moles NaOH to neutralise 25 cm3SameSame

Section 3 of 8Reactions

Metals
With metals (magnesium): 2CH3COOH + Mg → (CH3COO)2Mg + H2, giving magnesium ethanoate and hydrogen.
Bases and alkalis
With bases and alkalis: CH3COOH + NaOH → CH3COONa + H2O, giving sodium ethanoate.
Carbonates
With carbonates: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2, with effervescence.
  • This test distinguishes carboxylic acids from alcohols, which do not react.
Alcohols
With alcohols in the presence of concentrated sulfuric acid as catalyst, they form esters (see esters lesson).

The salts are named by changing -oic acid to -oate: ethanoate, propanoate.

Section 4 of 8Uses

Ethanoic acid is used as vinegar for food preservation and flavour, for making esters, cellulose acetate, and in making polymers. Citric and ascorbic acids are natural carboxylic acids in fruits. Larger carboxylic acids (fatty acids) are the basis of fats, oils and soap.

Section 5 of 8Worked examples and practical observations

Compare ethanoic acid with hydrochloric acid in a practical: add magnesium ribbon to equal volumes of 1.0 mol/dm3 solutions.

Worked examples and practical observations
The hydrochloric acid gives fast fizzing, a bigger temperature rise and the test tube becomes warm; the ethanoic acid gives slow bubbling.
Add universal indicator: HCl gives red (pH 0–1), ethanoic acid gives orange (pH about 2–3).
The final volume of hydrogen collected from excess acid and a fixed mass of magnesium is the same for both acids,
  • because the same amount of acidic hydrogen is available;
    • the weak acid just supplies it more slowly as the equilibrium shifts to replace used hydrogen ions.

Worked titration25.0 cm3 of vinegar (ethanoic acid) needs 20.0 cm3 of 0.50 mol/dm3 NaOH.

1
Moles NaOH = 0.50 × 0.0200 = 0.0100; the acid reacts 1:1, so 0.0100 mol in 25.0 cm3, a concentration of 0.40 mol/dm3.
2
With Mr 60 this is 24 g/dm3, about 2.4% by mass.
3
Name the products of the neutralisation: sodium ethanoate and water.
4
Distinguish between acids and esters: carboxylic acids are acidic and effervesce with carbonates; esters are neutral, with fruity smells.

Section 6 of 8Worked examples

  1. Write the equation for ethanoic acid with sodium carbonate. [2]2 marks

  2. Explain why ethanoic acid is a weak acid. [2]2 marks

  3. Name the salt formed when propanoic acid reacts with magnesium. [1]1 mark

  4. Describe a test to tell ethanol from ethanoic acid. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Carboxylic acidCompound with –COOH
Weak acidOnly partially ionised in water
EthanoateSalt of ethanoic acid
Carboxyl groupThe –COOH functional group
VinegarDilute aqueous ethanoic acid
OxidationConverting ethanol to ethanoic acid

Section 8 of 8Exam technique

1
Counting carbons wrongly (ignoring the COOH carbon).
2
Thinking weak means dilute.
3
Naming the salt as -ate instead of -oate.
4
Writing the formula as CH3COH.

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