React carboxylic acids with carbonates
Acids react with carbonates to form a carboxylate salt, water and carbon dioxide.
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 carboxylic acid reacts with a carbonate?
What you will learn
- Recognise acid–carbonate products
- Balance the mole ratio
- Identify carbon dioxide evidence
Carbonate consumes acid and releases CO₂
A carboxylic acid reacts with a carbonate to form a carboxylate salt, water and carbon dioxide: 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂.
Two monoprotic acid molecules are required for each carbonate ion because CO₃²⁻ accepts two protons.
Use the gas test carefully
Effervescence indicates a gas is released; carbon dioxide turns limewater milky.
Do not confuse this with the hydrogen from a reactive metal or the absence of gas in alkali neutralisation.
Pause and explain
How many ethanoic acid molecules react with one Na₂CO₃ formula unit?
Worked example
Write a balanced equation for ethanoic acid reacting with sodium carbonate.
Show the worked solution
- Carbonate requires two H⁺ ions.
- Two ethanoic-acid molecules form two ethanoate ions.
- Balance water and carbon dioxide.
Answer 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.
Try a topic test
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Test this topic →From guided practice to a new situation
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.
Balance the reaction between propanoic acid and sodium carbonate.
Give me a hint
Use two acid molecules for each carbonate.
Compare my reasoning
- The salt is sodium propanoate.
- One carbonate gives one CO₂ and one H₂O.
- Balance sodium with two carboxylate ions.
2CH₃CH₂COOH + Na₂CO₃ → 2CH₃CH₂COONa + H₂O + CO₂.
Look for these in your work
- Salt correct
- 1:1 carbonate-to-CO₂
- Equation balanced
A solid carbonate is added to ethanoic acid and the gas turns limewater milky. Identify the gas and reaction products.
Give me a hint
The limewater test identifies the gas.
Compare my reasoning
- Milky limewater supports CO₂.
- The acid forms an ethanoate salt.
- Water is also a product.
Carbon dioxide forms with sodium ethanoate (if the carbonate is sodium carbonate) and water.
Look for these in your work
- Gas identified
- Salt class stated
- Water included
Explain why two moles of a monoprotic acid are needed per mole of carbonate ion.
Give me a hint
Use the charges and proton count.
Compare my reasoning
- Carbonate is CO₃²⁻.
- It can accept two protons.
- Each carboxylic-acid molecule supplies one proton.
Two acid molecules supply the two protons needed per carbonate ion.
Look for these in your work
- Carbonate charge used
- Acid proton count used
- Ratio justified
Common mistakes
- Using a 1:1 acid-to-carbonate ratio
- Writing H₂ instead of CO₂
- Omitting water
What can you explain now?
Which gas forms when a carboxylic acid reacts with a carbonate?
Compare with the explanation
Carbon dioxide
Carbonate plus acid forms CO₂ and water.
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Make it stick
How do gas identity and stoichiometry distinguish metal, alkali and carbonate reactions?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.