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Carboxylic acids are weak acids that only partially ionise, and react with alcohols to make esters. Free GCSE AQA notes with key terms and 9 questions answered.
Chemical Properties of Carboxylic Acids is part of Reactions of alkenes and alcohols (chemistry only) in GCSE AQA Higher Chemistry.
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Carboxylic acids are weak acids that only partially ionise, and react with alcohols to make esters. Free GCSE AQA notes with key terms and 9 questions answered.
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Use this lesson to stay inside Reactions of alkenes and alcohols (chemistry only) while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Chemical Properties of Carboxylic Acids before testing yourself from memory.
You've probably encountered carboxylic acids without realising it. The sharp tang of vinegar? That's ethanoic acid. The sting of ant bites? Methanoic acid. These compounds are everywhere in nature and industry.
What makes carboxylic acids unique is their functional group: -COOH. This group gives them their acidic properties and determines how they react with other substances.
The first four carboxylic acids you need to know are methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (C₂H₅COOH), and butanoic acid (C₃H₇COOH). Notice how the carbon chain gets longer, but they all have that crucial -COOH group.
Here's where many students get confused. Carboxylic acids are acids, but they're weak acids. This doesn't mean they're not important - it's about how they behave in water.
When ethanoic acid dissolves in water, only a small fraction of the molecules actually release their hydrogen ions. Most stay as complete CH₃COOH molecules. This partial ionisation is what makes them weak.
Compare this to hydrochloric acid (a strong acid) where virtually every HCl molecule splits into H⁺ and Cl⁻ ions. The pH of carboxylic acids is therefore higher (less acidic) than strong acids of the same concentration.
This is one of the most dramatic reactions of carboxylic acids. When any carboxylic acid meets a carbonate, you get an instant fizzing reaction that produces carbon dioxide gas.
The general pattern is: carboxylic acid + carbonate → salt + water + carbon dioxide. You'll see bubbles of CO₂ gas escaping, which is your visual clue that the reaction is happening.
Worked example: Observing Ethanoic Acid with Sodium Carbonate
Describe what happens when ethanoic acid is added to sodium carbonate powder.
Answer: Vigorous fizzing produces carbon dioxide gas, which can be identified by the limewater test.
This reaction works with all carbonates - sodium carbonate, calcium carbonate (limestone), even the carbonate in indigestion tablets. It's a reliable test for identifying carboxylic acids.
When carboxylic acids dissolve in water, they create acidic solutions. But remember - they're weak acids, so the pH won't be as low as you might expect from the same concentration of a strong acid.
The -COOH group can release its hydrogen ion to form H⁺ (or H₃O⁺ if you want to be precise about hydrated ions). This is what makes the solution acidic and allows it to turn blue litmus paper red.
This is where carboxylic acid chemistry gets really interesting. When a carboxylic acid reacts with an alcohol, they form an ester - a completely new type of compound with its own distinctive properties.
The reaction removes a water molecule (H₂O) from the acid and alcohol, joining them together. This type of reaction is called condensation because a small molecule (water) is eliminated.
Worked example: Formation of Ethyl Ethanoate
Describe how ethyl ethanoate is formed from ethanoic acid and ethanol.
Answer: Ethyl ethanoate (CH₃COOC₂H₅) and water are produced through condensation.
Ethyl ethanoate is the only ester name you need to remember for GCSE. It smells like nail varnish remover and is used as a solvent. Many other esters have fruity smells and are used in food flavourings and perfumes.
Exam questions on carboxylic acids often test your understanding of their reactions and properties. The key is recognising the patterns and using the correct scientific language.
Remember that you don't need to write balanced chemical equations for these reactions at GCSE level. Focus on describing what happens and identifying the products formed.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Ethyl ethanoate is an ester formed from ethanoic acid and ethanol. It smells like nail varnish remover and is used as a solvent; many other esters have fruity smells and are used in food flavourings and perfumes.
Carboxylic acids are weak acids because they only partially ionise in water: only a small fraction of the molecules release their hydrogen ions. Their pH is therefore higher than that of a strong acid of the same concentration.
Esters are used in food flavourings and perfumes because many have fruity smells. Ethyl ethanoate, which smells like nail varnish remover, is used as a solvent.
An ester is made when a carboxylic acid reacts with an alcohol, removing a water molecule in a condensation reaction. For example, ethanoic acid and ethanol react to form ethyl ethanoate (CH₃COOC₂H₅) and water.
A carboxylic acid reacting with a carbonate produces a salt, water and carbon dioxide, so there is vigorous fizzing. For example, ethanoic acid with sodium carbonate fizzes, and the gas turns limewater milky, showing it is carbon dioxide.
Carboxylic acids are weak acids that only partially ionise in water. They react with carbonates to give a salt, water and carbon dioxide, dissolve in water to form acidic solutions, and react with alcohols to make esters.
When carboxylic acids dissolve in water, the -COOH group releases hydrogen ions (H⁺), making an acidic solution that turns blue litmus paper red. Because they are weak acids, the pH is not as low as that of a strong acid of the same concentration.
Carboxylic acids turn blue litmus paper red. When they dissolve in water, the -COOH group can release its hydrogen ion to form H⁺, making the solution acidic, though the pH is higher than a strong acid of the same concentration.
Making an ester is a condensation reaction because a small molecule, water, is removed when the carboxylic acid and alcohol join. The H from the alcohol and the OH from the acid form H₂O, and the remaining parts join to form the ester.