Uses of carboxylic acids
NCERT Class 12 Chemistry Chapter 8 Section 8.10
Quick answer
The main uses of carboxylic acids are in industry, in food and as starting materials for polymers, perfumes and soaps. Methanoic (formic) acid is used in the rubber, textile, dyeing, leather and electroplating industries, and ethanoic (acetic) acid as a solvent and as vinegar, a 5–8% solution of acetic acid in water. Hexanedioic acid is used to make nylon-6,6, esters of benzoic acid in perfumery, sodium benzoate as a food preservative, and higher fatty acids to make soaps and detergents.
With your textbook
NCERT Chemistry Part II (Class 12), Chapter 8 “Aldehydes, Ketones and Carboxylic Acids”, Section 8.10 “Uses of Carboxylic Acids”, page 254.
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What are carboxylic acids used for?
Uses of carboxylic acids
NCERT gives the uses of carboxylic acids in one short paragraph (page 254): methanoic acid in the rubber, textile, dyeing, leather and electroplating industries; ethanoic acid as a solvent and as vinegar; hexanedioic acid for nylon-6,6; esters of benzoic acid in perfumery; and, in NCERT’s words:
Sodium benzoate is used as a food preservative. Higher fatty acids are used for the manufacture of soaps and detergents.
NCERT Chemistry Part II (Class 12), Section 8.10, page 254
In plain words: formic and acetic acids have industrial uses, and dilute acetic acid is the acid in vinegar, a six-carbon diacid builds a nylon, benzoic acid serves as a preservative (as its sodium salt) and in perfumes (as its esters), and the long-chain fatty acids end up as soap.
Methanoic acid in industry
- Methanoic acid is used in the rubber, textile, dyeing, leather and electroplating industries (NCERT page 254).
- Its common name, formic acid, recalls where it was first obtained: from red ants (Latin formica, ant) (NCERT page 244).
Vinegar, nylon-6,6 and food preservatives
- Ethanoic acid is used as a solvent and, as vinegar, in the food industry (NCERT page 254). Its common name, acetic acid, comes from the Latin acetum, vinegar (NCERT page 244).
- Acetic acid is vinegar’s main acid, while water is its largest constituent; NCERT Class 11 calls acetic acid its main constituent (Unit 6, page 189), and NCERT Class 10 Science describes vinegar as a 5–8% solution of acetic acid in water, used widely as a preservative in pickles (Chapter 4, page 73).
- Hexanedioic acid, HOOC(CH2)4COOH, common name adipic acid (NCERT Table 8.3, page 244), reacts with hexane-1,6-diamine in the manufacture of nylon-6,6; the two sixes refer to the six-carbon diamine and six-carbon diacid (NCERT page 254).
- Esters of benzoic acid are used in perfumery, and sodium benzoate, C6H5COONa, is used as a food preservative (NCERT page 254). Sodium benzoate is the salt that benzoic acid gives with NaOH, or with NaHCO3, with effervescence (reasoned from NCERT page 249).
| Acid | Use (NCERT page 254) |
|---|---|
| methanoic acid | rubber, textile, dyeing, leather, electroplating |
| ethanoic acid | solvent; vinegar |
| hexanedioic acid | nylon-6,6 |
| benzoic acid | esters in perfumery; sodium benzoate as a food preservative |
| higher fatty acids | soaps and detergents |
Soaps and detergents from fatty acids
- Higher fatty acids are used to make soaps and detergents (NCERT page 254). NCERT highlights common C12–C18 fatty acids, a range rather than a strict definition, and notes that they occur in natural fats as esters of glycerol (page 244).
- A soap is the sodium or potassium salt of a long-chain carboxylic acid (NCERT Class 10 Science, Chapter 4, page 73). Its ionic end interacts with water while its carbon chain interacts with oil, which is how soap helps disperse oily dirt in water (Chapter 4, page 75). Synthetic detergents may instead contain sulfate or sulfonate head groups; they are not all carboxylate salts.
NCERT’s 5–8% vinegar range is a textbook description, not a specification for every product. Read the labelled acidity and whether the percentage is by mass or by volume before a calculation.
Solved examples
Example 1: Acetic acid in vinegar
A vinegar contains 6.0% acetic acid by mass, within NCERT’s 5–8%. How many moles of acetic acid are in 150 g of it? (C = 12, H = 1, O = 16.)
Answer: The mass percentage gives 0.060 × 150 = 9.0 g of CH3COOH. M = 24 + 4 + 32 = 60 g mol−1, so the amount is 9.0/60 = 0.15 mol (reasoned from NCERT Class 10 Science, Chapter 4, page 73).
Example 2: Hydrogen ions in a vinegar sample
The pH of a sample of vinegar is 3.76. What is its hydrogen ion concentration? (NCERT Class 11, Exercise 6.42.)
Answer: [H+] = 10−pH = 10−3.76 = 1.74 × 10−4 M (reasoned from the pH scale, NCERT Class 11, Unit 6). The pH alone does not determine total acetic-acid concentration, because buffers and other acids can change it. For comparison only, assuming an unbuffered aqueous acetic-acid solution, Example 1’s 6.0% vinegar is about 1.0 mol L−1 acetic acid (taking its density as 1.0 g mL−1), so with Ka = 1.74 × 10−5 (NCERT Class 11, Unit 6, page 195) its [H+] ≈ √(Kac) = 4.2 × 10−3 M and its pH is about 2.4 (reasoned). Inferring total acidity in real vinegar requires composition information or a titration; pH measures hydrogen-ion activity (approximated here by concentration).
Example 3: Making sodium benzoate
What mass of sodium benzoate can 6.1 g of benzoic acid give with excess sodium hydrogencarbonate? (C = 12, H = 1, O = 16, Na = 23.)
Answer: C6H5COOH + NaHCO3 → C6H5COONa + H2O + CO2. M(C6H5COOH) = 84 + 6 + 32 = 122 g mol−1, so 6.1 g is 0.050 mol; M(C6H5COONa) = 84 + 5 + 32 + 23 = 144 g mol−1, so the mass is 0.050 × 144 = 7.2 g (reasoned from NCERT pages 249 and 254).
Common mistakes to avoid
Common mistake
Calling vinegar pure acetic acid
Vinegar is a dilute solution, 5–8% acetic acid in water by NCERT Class 10’s description; pure acetic acid freezes at 290 K (NCERT Class 12 Unit 1, page 19).
Common mistake
Calling the fatty acid itself a soap
A soap is the sodium or potassium salt of a long-chain carboxylic acid, not the free acid.
Quick revision summary
- Methanoic (formic) acid: rubber, textile, dyeing, leather and electroplating industries.
- Ethanoic (acetic) acid: solvent; vinegar, a 5–8% solution of acetic acid in water.
- Hexanedioic (adipic) acid: nylon-6,6. Esters of benzoic acid: perfumery. Sodium benzoate: food preservative.
- Common higher fatty acids, including C12–C18 members: soaps and detergents; a soap is the sodium or potassium salt of a long-chain carboxylic acid.
Formula sheet
- Vinegar: 5–8% acetic acid in water
- Hexanedioic acid: HOOC(CH)COOH, for nylon-6,6
- RCOOH + NaHCO → RCOONa + HO + CO
- Soap: RCOONa or RCOOK, R a long carbon chain
Questions students ask
How does sodium benzoate preserve food, and how is it different from benzoic acid?
Sodium benzoate, C6H5COONa, is the sodium salt of benzoic acid (NCERT page 254), and it preserves food by turning back into the acid, a point beyond the NCERT text: in acidic foods such as pickles, jams, juices and soft drinks the benzoate ion takes up H+, and the benzoic acid formed can inhibit susceptible microorganisms (Wikipedia, “Sodium benzoate”). The salt is used because it dissolves far better in water, while benzoic acid is nearly insoluble in cold water (NCERT page 249). Benzoic acid passes into a microbe’s cells and makes them more acidic inside, which slows their fermentation of glucose (Wikipedia).
What is the difference between sodium benzoate and potassium sorbate?
Sodium benzoate and potassium sorbate are both salts of carboxylic acids used as food preservatives, but from different acids: aromatic benzoic acid, C6H5COOH, and sorbic acid, CH3CH=CHCH=CHCOOH, IUPAC name (2E,4E)-hexa-2,4-dienoic acid, a point beyond the NCERT text (Wikipedia, “Sorbic acid”). Each is sold as the salt, which dissolves more readily, while the un-ionised acid is what acts on microbes (Wikipedia). Sorbic acid, pKa 4.76 (Wikipedia), is weaker than benzoic acid, 4.19 (NCERT page 250), so more of it stays un-ionised, about 37% against 13% at pH 5.0 (reasoned), using the Henderson–Hasselbalch equilibrium. This fraction alone does not establish preservative performance: organism, concentration, food composition and formulation also matter.
Why can sodium benzoate and vitamin C together form benzene in drinks?
Sodium benzoate and vitamin C can together give benzene in some drinks, a point beyond the NCERT text: in its 2005–07 survey the US FDA found that some drinks with both a benzoate salt and ascorbic acid develop benzene at parts per billion, and that warmth and light make more form (US FDA, “Questions and Answers on the Occurrence of Benzene in Soft Drinks and Other Beverages”). Benzene, C6H6, is the benzoate ion less its carboxyl group, lost as CO2, the decarboxylation sodium benzoate also undergoes, under very different conditions, with soda lime (NCERT Class 11, Unit 9, page 321; reasoned). The FDA’s statement concerned its historical survey, not every drink or formulation: ten tested products exceeded 5 ppb, and manufacturers reformulated affected products still in production; retested reformulations were below 1.5 ppb. The reaction in a drink involves different chemistry from hot soda-lime decarboxylation; the structural carbon count does not establish a shared mechanism.
What is formic acid used for in the rubber, leather and textile industries?
Formic acid is used in the rubber industry, one of the industries NCERT lists for it (page 254), as a coagulant that makes the rubber in natural latex separate out, a point beyond the NCERT text (Wikipedia, “Formic acid”). In leather making it is used for tanning, and in the textile industry for dyeing and the final processing of cloth, in each case as an acid (Wikipedia). Manufacturer BASF also lists use in animal-feed and silage preservation. These are applications, not a claim about current market share or effectiveness against every microorganism.
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Name the carboxylic acid used (a) as vinegar, (b) for nylon-6,6, (c) in electroplating, and the salt used as a food preservative.
Show answerHide answer
(a) Ethanoic acid; (b) hexanedioic acid; (c) methanoic acid; sodium benzoate is the food preservative (NCERT page 254).
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Q2. What is a soap, chemically, and which acids is it made from?
Show answerHide answer
The sodium or potassium salt of a long-chain carboxylic acid (NCERT Class 10 Science, Chapter 4, page 73). NCERT Class 12 says higher fatty acids are used to make soaps and detergents (page 254).