Cambridge Notes, Past Papers, Revision Questions

Organic Chemistry – Carboxylic Acids

Ethanoic Acid (Acetic Acid) – \$CH_3COOH\$

Ethanoic acid is a weak, monoprotic acid. Think of it as a gentle “acidic” friend that can react with metals, bases, and carbonates to make new compounds and sometimes release gas bubbles.

(a) Reaction with Metals

When ethanoic acid meets a metal, it behaves like a weak acid and gives up a proton to the metal. The metal becomes a cation, and the acid’s conjugate base (acetate) stays behind. Hydrogen gas is released – the bubbles you see are \$H_2\$.

Sodium (Na): \$CH3COOH + 2Na \;\longrightarrow\; 2NaCH3COO + H_2\$
Salt produced: sodium acetate – \$NaCH_3COO\$

Potassium (K): \$CH3COOH + 2K \;\longrightarrow\; 2KCH3COO + H_2\$
Salt produced: potassium acetate – \$KCH_3COO\$

Calcium (Ca): \$2CH3COOH + Ca \;\longrightarrow\; Ca(CH3COO)2 + H2\$
Salt produced: calcium acetate – \$Ca(CH3COO)2\$

Magnesium (Mg): \$2CH3COOH + Mg \;\longrightarrow\; Mg(CH3COO)2 + H2\$
Salt produced: magnesium acetate – \$Mg(CH3COO)2\$

Exam tip: Remember that metal + acid → salt + H₂. Write the metal as a cation (Na⁺, Ca²⁺, etc.) and the acetate as the conjugate base. Don’t forget the gas bubble! 🚀

(b) Reaction with Bases

Ethanoic acid neutralises a base to give a salt and water. This is the classic acid–base neutralisation.

Sodium hydroxide (NaOH): \$CH3COOH + NaOH \;\longrightarrow\; NaCH3COO + H_2O\$
Salt produced: sodium acetate – \$NaCH_3COO\$

Potassium hydroxide (KOH): \$CH3COOH + KOH \;\longrightarrow\; KCH3COO + H_2O\$
Salt produced: potassium acetate – \$KCH_3COO\$

Calcium hydroxide (Ca(OH)₂): \$2CH3COOH + Ca(OH)2 \;\longrightarrow\; Ca(CH3COO)2 + 2H_2O\$
Salt produced: calcium acetate – \$Ca(CH3COO)2\$

Exam tip: Write the base as a cation (Na⁺, Ca²⁺) and the acetate as the conjugate base. The product is always a salt + water. Keep the stoichiometry balanced! ⚗️

(c) Reaction with Carbonates

Carbonates act like a base but also release carbon dioxide gas when they react with an acid. The reaction is a bit like blowing bubbles in a soda.

Sodium carbonate (Na₂CO₃): \$CH3COOH + Na2CO3 \;\longrightarrow\; 2NaCH3COO + CO2 \uparrow + H2O\$
Salt produced: sodium acetate – \$NaCH_3COO\$

Potassium carbonate (K₂CO₃): \$CH3COOH + K2CO3 \;\longrightarrow\; 2KCH3COO + CO2 \uparrow + H2O\$
Salt produced: potassium acetate – \$KCH_3COO\$

Calcium carbonate (CaCO₃): \$2CH3COOH + CaCO3 \;\longrightarrow\; Ca(CH3COO)2 + CO2 \uparrow + H2O\$
Salt produced: calcium acetate – \$Ca(CH3COO)2\$

Exam tip: Remember the “bubbles” trick: acid + carbonate → salt + CO₂ + H₂O. Write CO₂ as a gas (↑). The salt is always the acetate of the metal present. 🌬️

Summary Table

Reactant	Product(s)	Gas Released
Metal (e.g. Na, Ca)	Acetate salt (NaCH₃COO, Ca(CH₃COO)₂)	\$H_2\$
Base (e.g. NaOH, Ca(OH)₂)	Acetate salt + \$H_2O\$	None
Carbonate (e.g. Na₂CO₃, CaCO₃)	Acetate salt + \$H_2O\$	\$CO_2\$

Final Exam Tip: When you see a reaction involving ethanoic acid, first decide whether the partner is a metal, base, or carbonate. Then remember the general pattern: acid + metal → salt + H₂, acid + base → salt + H₂O, acid + carbonate → salt + CO₂ + H₂O. Write the correct salt name and formula, and don’t forget the gas symbol (↑). Good luck! 🎓