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11.7Organic Chemistry

Carboxylic acids — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 11 parts Lesson 7 of 10 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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11.7 Carboxylic acids — Notes — IGCSE Chemistry notes: Describe the reactions of carboxylic acids with metals, bases and carbonates, their formation by oxidation and esterification.Functional group — –COOH, and the general formula is CnH2n+1COOH.Behaviour — they behave like other…

  • Salt names
  • Naming salts
  • Bacterial oxidation
  • Chemical oxidation
  • Fermentation
  • Hydration of ethene

Section 1 of 11Carboxylic acids

Carboxylic acids
Functional group
  • –COOH, and the general formula is CnH2n+1COOH.
Behaviour
  • Behaviour they behave like other acids and are weak acids.
Salts
  • Salts the salts formed end in -anoate, so ethanoic acid gives ethanoates.

Section 2 of 11Structure, naming and weak acid behaviour

Structure, naming and weak acid behaviour
Examples
  • Examples methanoic acid HCOOH, ethanoic acid CH3COOH and propanoic acid CH3CH2COOH.
Vinegar
  • Vinegar ethanoic acid gives vinegar its sharp smell and taste, and vinegar is about a 5% solution.
Acidity
  • Acidity the –COOH group can lose its hydrogen as H+ in water, so the solution is acidic.
Weak acid
  • Weak acid carboxylic acids ionise only partially in water: CH3COOH ⇌ CH3COO− + H+.
  • Compared with hydrochloric acid of the same concentration, ethanoic acid has a higher pH (about 3 compared with about 1).
  • It reacts more slowly with metals and conducts electricity less well.
  • Equal moles of each acid neutralise the same amount of base.

Section 3 of 11Reactions of ethanoic acid

ReagentProductsObservationExample
MetalSalt + hydrogenFizzing2CH3COOH + Mg → (CH3COO)2Mg + H2
Base (hydroxide)Salt + waterNoneCH3COOH + KOH → CH3COOK + H2O
CarbonateSalt + water + carbon dioxideFizzing2CH3COOH + K2CO3 → 2CH3COOK + H2O + CO2
  • Salt names
    • magnesium ethanoate and potassium ethanoate.

Section 4 of 11Observations in the reactions of carboxylic acids

ReagentObservation
Metal (Mg)Fizzing and the metal dissolves
Base (CuO)Black solid dissolves to a blue solution
Alkali (NaOH, KOH)No visible change
Carbonate (Na2CO3)Fizzing, and the gas turns limewater cloudy
  • Naming salts
    • the ending changes from -oic acid to -oate, such as magnesium ethanoate and potassium ethanoate.

Section 5 of 11Forming ethanoic acid (Extended)

Bacterial oxidation
  • Bacterial oxidation acetobacter in the air uses oxygen to oxidise ethanol, as when wine is left open or in vinegar production.
C2H5OH + O2 → CH3COOH + H2O.
Chemical oxidation
  • Chemical oxidation ethanol is heated under reflux with acidified potassium manganate(VII).
The condenser stops the volatile alcohol escaping.
The solution changes from purple to colourless.
CH3CH2OH + 2[O] → CH3COOH + H2O.
Bacterial oxidation gives a weak solution of vinegar, which is why an open bottle of wine turns sour.

Section 6 of 11Esterification (Extended)

Reaction
  • Reaction a carboxylic acid reacts with an alcohol, using an acid catalyst, to form an ester and water.
Catalyst
  • Catalyst concentrated sulfuric acid.
Equation
  • CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O.
Esters
  • Esters sweet-smelling oily liquids used as flavourings and perfumes.

Section 7 of 11Advantages and disadvantages of making ethanol (Extended)

1Fermentation
  • Fermentation uses renewable resources and simple equipment, but is slow and gives a dilute solution.
2Hydration of ethene
  • Hydration of ethene is fast and gives pure ethanol, but uses non-renewable crude oil and high temperature and pressure.

Section 8 of 11Uses of carboxylic acids

Uses of carboxylic acids
Vinegar
  • Vinegar a food preservative and flavouring.
Esters
  • Esters used in perfumes and flavourings.
Polymers
  • Polymers making polyesters and other chemicals.
Higher members (fatty acids)
  • Higher members (fatty acids) used to make soaps and margarine.
Natural acids
  • Natural acids citric acid in citrus fruit, lactic acid in sour milk and methanoic acid in ant stings.

Section 9 of 11More from the specification

More from the specification
Industrial steam reforming and fossil fuels
  • Industrial steam reforming and fossil fuels cracking raw petrochemical feedstocks from fossil fuels produces ethylene,
    • which is reacted with steam into carboxylic acids.
Homologous series
  • A family of compounds with the same functional group and general formula.
Substitution reaction
  • A reaction in which one atom or group is replaced by another.
Ultraviolet radiation
  • Radiation that provides the activation energy for alkane–halogen reactions.
Carboxylic Acid Functional Group and Weak Acidity
  • Carboxylic acids contain the terminal carboxyl group (-COOH) and behave as typical weak organic acids, partially dissociating in aqueous solution to establish an equilibrium (CH3COOH CH3COO- + H+).
Characteristic Acid Reactions of Ethanoic Acid
  • Ethanoic acid effervesces with reactive metals and carbonates (producing carbon dioxide, water, and ethanoate salts).
    • (yielding hydrogen and metal ethanoates)
Condensation Esterification Reactions
  • Heating a carboxylic acid with an alcohol in the presence of concentrated sulfuric acid catalyst produces sweet-smelling, volatile esters and water.
    • (e.g. ethanoic acid + ethanol → ethyl ethanoate + water)
Combustion
  • Burning in oxygen
Distillation
  • Separating a liquid from a solution by boiling and condensing the vapour.
Mixture
  • Two or more substances mixed but not chemically combined.
Fractional distillation
  • Separating a mixture of liquids by their boiling points.
Bromine water
  • An orange solution that is decolourised by alkenes.
Addition reaction
  • A reaction in which atoms add across a C=C bond to give one product.
Fossil fuel
  • A fuel formed from the remains of living things, such as coal, natural gas and petroleum.
Displayed formula
  • A formula showing all atoms and all bonds.
Alkene
  • A hydrocarbon with a C=C bond and a name ending in -ene.
Alkane
  • A saturated hydrocarbon with a name ending in -ane.
Molecule
  • Two or more atoms held together by covalent bonds.
Hydrocarbon
  • A compound containing hydrogen and carbon only.
Fraction
  • A group of hydrocarbons with similar chain length and boiling points.
Boiling
  • Liquid to gas throughout the liquid at the boiling point.

Section 10 of 11Key terms

TermMeaning
Carboxylic acidCompound containing –COOH, a weak acid.
EthanoateThe salt of ethanoic acid.
EsterCompound formed from an acid and an alcohol, with a sweet smell.
EsterificationAcid + alcohol ⇌ ester + water with an acid catalyst.
RefluxHeating with a condenser to stop volatile liquids escaping.
Weak acidan acid that ionises only partially in water.
Vinegaran ethanoic acid solution of about 5%.
Acetobacterbacteria that oxidise ethanol to ethanoic acid in air.

Section 11 of 11Exam technique

1
Saying carboxylic acids are strong acids — they are weak acids.
2
Forgetting hydrogen in the metal reaction and carbon dioxide in the carbonate reaction.
3
Naming the salt as an ethanoic salt — it is an ethanoate, such as sodium ethanoate.
4
Heating ethanol with potassium manganate(VII) without reflux — the condenser stops the alcohol escaping.
5
Saying weak acids are the same as dilute acids — weak refers to partial ionisation and dilute refers to concentration

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