GCSE · Chemistry · AQA · Spec 8462

Carboxylic acids (chem only)

Vinegar is an acid you shake onto chips. It fizzes with a carbonate like any acid, yet it's far weaker than lab hydrochloric acid. The reason? One small group: –COOH.

Carboxylic acids · What happens next?

What did the acid meet?

Start with a carboxylic acid. Choose what it meets, then follow the route to its end.

–COOH is the group that makes these compounds carboxylic acids. Now choose what the acid meets.

5 places the acid can end up.

On A carboxylic acid — it carries the –COOH group. 3 branches to choose from.

Three partners, three different endings — but every route starts from the same –COOH group.

Exam line: Carbonate → salt + carbon dioxide + water (fizzing). Water → weakly acidic solution, universal indicator orange or yellow. Alcohol + acid catalyst → ester + water.

Spot one, then name it

Is it a carboxylic acid — and which one?

Tap an item, then tap the group it belongs in.

Does it have the –COOH group?

Still to sort

Carboxylic acid (0)

Has the –COOH group

Not a carboxylic acid (0)

No –COOH group

Where the line is: Being an acid, or having 'ethano-' or 'methano-' in the name, is not enough — look for –COOH itself.

9 of 9 still to sort.

Sort first by 'carboxylic acid or not', then switch the rule and sort again by which member it is.

Watch out: The carbon in –COOH counts. CH₃COOH has two carbon atoms, so it is ethanoic acid, not methanoic acid.

The fizz

What gas makes the bubbles?

A spatula of sodium carbonate is dropped into some ethanoic acid. The mixture fizzes straight away.

Which gas is making the bubbles? Pick the one closest to what you really think.

Hydrogen — acids always give off hydrogen when they react.Water vapour — water is one of the products, so it bubbles off.Oxygen — a carbonate has oxygen in it, so that must be the gas.Carbon dioxide — acid plus carbonate gives carbon dioxide.

How sure are you?

I'm sureI think soI'm guessing

Hydrogen — acids always give off hydrogen when they react.

This is a common misconception

Hydrogen is the gas you get when an acid reacts with a metal, so it's an easy idea to carry over. But this acid has met a metal carbonate, and that is a different partner with a different gas. A carboxylic acid and a metal carbonate make a salt, carbon dioxide and water — so the bubbles are carbon dioxide.

Water vapour — water is one of the products, so it bubbles off.

This is a common misconception

Good that you remembered water is a product! But it stays in the mixture as a liquid — it doesn't bubble away. The only gas made in this reaction is carbon dioxide, and that's what escapes as bubbles.

Oxygen — a carbonate has oxygen in it, so that must be the gas.

This is a common misconception

It's true that a carbonate has oxygen in it — but oxygen isn't the gas made here. A carboxylic acid and a metal carbonate make a salt, carbon dioxide and water, so the bubbles are carbon dioxide.

Carbon dioxide — acid plus carbonate gives carbon dioxide.

This matches the evidence

Exactly. A carboxylic acid reacts with a metal carbonate to give a salt, carbon dioxide and water. The carbon dioxide escapes as a gas, and the fizzing you see has a name: effervescence.

Carbonate in, carbon dioxide out. When a carboxylic acid meets a metal carbonate, the fizzing (effervescence) is carbon dioxide — not hydrogen.

Exam line: Carboxylic acid + metal carbonate → salt + carbon dioxide + water. The fizzing is the carbon dioxide.

Higher

Why only weakly acidic?

Reason it through

Why does a solution of ethanoic acid have a higher pH than a strong acid solution of the same concentration?

Link 1 of 4

First link · your turn

Ethanoic acid dissolves in water. What do its molecules need to do to make the solution acidic?

Locked — reveal the link above first

Your turn

Describe two reactions in your own words

Ethanoic acid is a carboxylic acid. Describe what happens when ethanoic acid reacts with (a) sodium carbonate and (b) ethanol. [6 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

(a) A salt forms: sodium ethanoate. (1 mark)
(a) Carbon dioxide is given off. (1 mark)
(a) Water forms. (1 mark)
(a) Fizzing / effervescence is seen. (1 mark)
(b) The ester ethyl ethanoate forms, along with water. (1 mark)
(b) An acid catalyst is needed, such as concentrated sulfuric acid. (1 mark)

Model answer

(a) Ethanoic acid reacts with sodium carbonate to make a salt, sodium ethanoate, along with carbon dioxide and water. The carbon dioxide escapes as a gas, so you would see fizzing (effervescence). (b) With ethanol, and an acid catalyst such as concentrated sulfuric acid, ethanoic acid reacts to make the ester ethyl ethanoate and water.

Two routes, two lists. Carbonate: salt + carbon dioxide + water, and you see fizzing. Alcohol: ester + water, and you need an acid catalyst.

Exam line: Name every product, and say what you would see. Words are enough here — no balanced equations needed.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One small group of atoms, –COOH, decides what these acids do — whatever they meet.

What you need to know

Carboxylic acids have the functional group –COOH. That group is how you recognise one from its formula.
The first four carboxylic acids are methanoic acid, ethanoic acid, propanoic acid and butanoic acid — with one, two, three and four carbon atoms. Count every carbon, including the one in –COOH: CH₃COOH is ethanoic acid.
A carboxylic acid reacts with a metal carbonate to give a salt, carbon dioxide and water. The carbon dioxide is seen as fizzing, called effervescence.
The salt's name comes from the metal and the acid: methanoic acid and sodium carbonate make sodium methanoate.
Carboxylic acids dissolve in water to give solutions that are only weakly acidic, because only some of their molecules split up to release H⁺ ions. The solution turns universal indicator orange or yellow, and has a higher pH than a strong acid solution of the same concentration.
A carboxylic acid reacts with an alcohol, when an acid catalyst is present, to make an ester and water. Ethanoic acid and ethanol make ethyl ethanoate and water; concentrated sulfuric acid can be the catalyst.
Higher: carboxylic acids are weak acids because they only partly ionise in water, so they release fewer H⁺ ions than a strong acid of the same concentration and have a higher pH.

The big picture

Carboxylic acids are a family of organic compounds with the functional group –COOH. The first four are methanoic, ethanoic, propanoic and butanoic acid, and you name one by counting every carbon atom, including the one in –COOH (CH₃COOH is ethanoic acid). What a carboxylic acid does depends on what it meets: with a metal carbonate it makes a salt, carbon dioxide and water, and fizzes; in water it forms a weakly acidic solution that turns universal indicator orange or yellow; and with an alcohol and an acid catalyst it makes an ester and water. They are weak acids because only some of their molecules ionise to release H⁺ ions.

Key points

1–COOH is the functional group of every carboxylic acid.

2Meth- 1, eth- 2, prop- 3, but- 4 carbon atoms — and the carbon in –COOH counts.

3Acid + metal carbonate → salt + carbon dioxide + water, with fizzing (effervescence).

4Salt names: metal first, then the acid with '-oic acid' changed to '-oate' (sodium methanoate).

5Acid + water → weakly acidic solution: universal indicator orange or yellow; higher pH than a strong acid of equal concentration.

6Acid + alcohol, with an acid catalyst (e.g. concentrated sulfuric acid) → ester + water. Ethanoic acid + ethanol → ethyl ethanoate + water.

7Weak acid = only some molecules ionise, so fewer H⁺ ions and a higher pH.

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Worked example

Problem

A bottle is labelled C₃H₇COOH. Name the acid, then describe what forms — and what you would see — when it is added to potassium carbonate.

⚠ Watch out

Reading CH₃COOH as methanoic acid because you only notice the CH₃. Count every carbon, including the one in –COOH: CH₃COOH has two carbon atoms, so it is ethanoic acid.

🧠

Memory hook

Monkeys Eat Peanut Butter: meth-, eth-, prop-, but- for 1, 2, 3, 4 carbons — counting the one in –COOH. Then ask 'what did it meet?' Carbonate → carbon dioxide (fizz!), salt, water. Water → weakly acidic, orange or yellow. Alcohol plus acid catalyst → ester and water.

✓

Check yourself

Close the page. List the four acids in order with their carbon counts. Then pick one: what does it make with a carbonate, what happens in water, and what does it make with an alcohol?

Flashcards

(12)

What is the functional group of a carboxylic acid?

–COOH

Name the first four carboxylic acids, in order.

Methanoic acid, ethanoic acid, propanoic acid, butanoic acid.

How do you work out which carboxylic acid a formula shows?

Count every carbon atom, including the one in –COOH: 1 = methanoic, 2 = ethanoic, 3 = propanoic, 4 = butanoic.

Which acid is CH₃COOH?

Ethanoic acid — it has two carbon atoms, because the carbon in –COOH counts.

A carboxylic acid fizzes when it meets a metal carbonate. Which gas causes the fizzing, and what is the fizzing called?

Carbon dioxide causes it, and the fizzing is called effervescence. A salt and water form as well.

Name the salt made from methanoic acid and sodium carbonate.

Sodium methanoate — the second part of the name comes from the acid.

What colour does a carboxylic acid solution turn universal indicator?

Orange or yellow — it is only weakly acidic.

How does a carboxylic acid's pH compare with a strong acid of the same concentration?

It is higher.

What does a carboxylic acid make with an alcohol?

An ester and water — but only when an acid catalyst is present.

What can be used as the catalyst for making an ester?

Concentrated sulfuric acid.

What do ethanoic acid and ethanol make?

Ethyl ethanoate (an ester) and water.

Why is a carboxylic acid a weak acid? (Higher)

Only some of its molecules ionise in water to release H⁺ ions, so there are fewer H⁺ ions than in a strong acid of the same concentration.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What did the acid meet?

Is it a carboxylic acid — and which one?

What gas makes the bubbles?

Describe two reactions in your own words

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