Propanoic acid reacts with sodium carbonate solution. Explain why propanoic acid reacts with sodium carbonate and describe the observations a student would make during this reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanoic acid (CH₃CH₂COOH) is a carboxylic acid found in some fermented foods. It reacts with sodium carbonate solution to produce a salt, water, and a gas.
Model answer (5 marks)
Propanoic acid is a carboxylic acid and therefore donates a proton (H⁺) from its –COOH group. The carbonate ion (CO₃²⁻) accepts this proton and is converted to bicarbonate (HCO₃⁻) and then to carbon dioxide (CO₂) and water:
CH₃CH₂COOH + Na₂CO₃ → CH₃CH₂COONa + CO₂↑ + H₂O
The proton transfer makes the reaction an acid–base reaction, producing the sodium salt of propanoic acid (sodium propanoate). The CO₂ that is released is a gas.
During the experiment a student will see vigorous effervescence – bubbles of CO₂ rising from the mixture. The solid sodium carbonate dissolves, giving a clear solution, and the CO₂ may turn a test of limewater milky.
CH₃CH₂COOH + Na₂CO₃ → CH₃CH₂COONa + CO₂↑ + H₂O
The proton transfer makes the reaction an acid–base reaction, producing the sodium salt of propanoic acid (sodium propanoate). The CO₂ that is released is a gas.
During the experiment a student will see vigorous effervescence – bubbles of CO₂ rising from the mixture. The solid sodium carbonate dissolves, giving a clear solution, and the CO₂ may turn a test of limewater milky.
Examiner tips
- Use the term ‘donates a proton’ to show acid behaviour; mention the carbonate accepting the proton and forming CO₂; state the product salt explicitly; describe effervescence and solution clarity; link the gas to the milky limewater test if mentioned.
Common mistakes
- Saying the reaction produces only water, ignoring the salt; confusing CO₂ with CO; not mentioning the proton transfer step; describing the solution as cloudy instead of clear.
Mark scheme (5 marks)
- Propanoic acid is an acid / it donates hydrogen ions (H⁺) / protons to the carbonate ions
- The carboxyl / –COOH group reacts with the carbonate / the reaction produces carbon dioxide gas
- A salt (sodium propanoate) is formed as a product of the reaction
- Observation: effervescence / bubbling is seen
- Observation: the solid sodium carbonate dissolves / the solution becomes clear OR the gas produced turns limewater milky
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain and Describe" question →
- Decode the mark scheme abbreviations →
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