React carboxylic acids with reactive metals
A reactive metal displaces hydrogen from a carboxylic acid to form a carboxylate salt and hydrogen gas.
Section 18 · Carboxylic acids and derivatives
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Track the carbon skeleton, name reagents and conditions, and show products and observations. Written practice is self-checked
Before you begin
- Use alcohol oxidation and carbonyl chemistry from Topics 16–17, functional-group names, acid–base equations and balanced reaction formulae.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which gas forms when a reactive metal reacts with a carboxylic acid?
What you will learn
- Recognise the redox reaction
- Predict the carboxylate salt
- Balance hydrogen formation
Hydrogen gas is released
A reactive metal transfers electrons to hydrogen ions from the acid. The metal forms a positive ion and the acid becomes a carboxylate ion; hydrogen gas is produced.
For a monovalent metal: 2RCOOH + 2Na → 2RCOONa + H₂. Use two acid molecules to balance H₂.
Name the salt from the acid and metal
Ethanoic acid with sodium gives sodium ethanoate; propanoic acid with magnesium gives magnesium propanoate.
A carboxylate salt contains RCOO⁻. Do not write hydrogen as a single H atom or as aqueous H⁺ in the collected gas product.
Pause and explain
What is the negative ion in a carboxylate salt?
Worked example
Write a balanced equation for ethanoic acid reacting with sodium.
Show the worked solution
- Sodium forms Na⁺ and ethanoate is CH₃COO⁻.
- Two H atoms combine to form H₂.
- Use two acid and two sodium atoms.
Answer 2CH₃COOH + 2Na → 2CH₃COONa + H₂.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the equation for ethanoic acid and potassium.
Give me a hint
Potassium is +1; pair two acid hydrogens to make H₂.
Compare my reasoning
- The salt is potassium ethanoate.
- Two acid molecules supply two H atoms.
- Balance potassium and H₂.
2CH₃COOH + 2K → 2CH₃COOK + H₂.
Look for these in your work
- Salt formula correct
- Hydrogen gas written
- Equation balanced
Predict the products when propanoic acid reacts with magnesium.
Give me a hint
Magnesium forms Mg²⁺ and needs two carboxylate ions.
Compare my reasoning
- The salt is magnesium propanoate.
- Two acid molecules provide two propanoate ions and two H atoms.
- Hydrogen atoms form H₂.
2CH₃CH₂COOH + Mg → (CH₃CH₂COO)₂Mg + H₂.
Look for these in your work
- Salt stoichiometry correct
- H₂ identified
- Atoms balanced
Why are bubbles observed when sodium reacts with an aqueous carboxylic acid?
Give me a hint
Identify the gaseous product and its source.
Compare my reasoning
- The metal displaces acidic hydrogen.
- Two hydrogen atoms form H₂ molecules.
- The gas escapes as bubbles.
Hydrogen gas forms as sodium produces a carboxylate salt.
Look for these in your work
- Reaction class identified
- Gas named
- Observation explained
Common mistakes
- Writing water instead of hydrogen
- Leaving the equation unbalanced
- Naming the salt as sodium ethanoic acid
What can you explain now?
Which gas forms when a reactive metal reacts with a carboxylic acid?
Compare with the explanation
Hydrogen
Acid hydrogen is reduced to H₂.
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Make it stick
What salt and gas form, and how does the metal charge affect the salt formula?
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