GCSE · Chemistry · Edexcel · Spec 1CH0

Carboxylic acid -COOH and acidic properties

Lemons taste sharp thanks to citric acid. Hidden in its tangle of atoms is one small group, –COOH, that marks it as a carboxylic acid. Learn to spot it.

Chemistry · The –COOH group

Spot the difference: ethanol vs ethanoic acid

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Same two carbons, and both have an OH. Compare the two end carbons: what does the acid's have that ethanol's doesn't? Then tap each part to check.

Both molecules have two carbons and an OH. Only one of them is an acid.

Chemistry · Carboxylic acids

Acid, alcohol or neither?

Pick a molecule, then the group it belongs to. If it's an acid, name it in your head before you check.

Still to sort

Carboxylic acid (0)

C=O and OH on the same carbon: –COOH

Where the line is: An OH alone isn't enough. The carbon it's on must also carry a C=O.

Alcohol (0)

an OH, with no C=O on its carbon

Where the line is: It has the same OH as an acid, but no C=O on that carbon, so there's no –COOH.

Neither (0)

no –COOH and no OH

Where the line is: A C=O by itself doesn't make an acid, and with no OH it can't be an alcohol.

8 of 8 still to sort.

Use the rule from the diagram: check the carbon the OH sits on.

Into water: only some molecules split

Add to water: 0. State: Pure ethanoic acid. static

Add to water0

No water yet. Every particle is a whole CH₃COOH molecule: nothing has split. Now slide it into water.

Each dot is one ethanoic acid molecule. Blue: still whole. Orange: split into a CH₃COO⁻ ion and an H⁺ ion. The share shown is illustrative, not measured.

Watch out: The ⇌ doesn't mean 'half and half'. It means the change goes both ways and only part of the acid is split at any moment.

Chemistry · Weak acids

Why orange, not red?

Two solutions are made at the same concentration: one of ethanoic acid, one of a strong acid. Universal indicator goes red in the strong acid, but only orange or yellow in the ethanoic acid.

Which explanation is closest to what you think right now?

The ethanoic acid must be more dilute: there's less acid in it.Ethanoic acid doesn't release any H⁺ ions, so it hardly counts as an acid.Only some of its molecules split up, so there are fewer H⁺ ions and the pH is higher.

How sure are you?

I'm sureI think soI'm guessing

The ethanoic acid must be more dilute: there's less acid in it.

This is a common misconception

Look back at the set-up: both solutions have the same concentration, so the same amount of acid is dissolved in each. 'Dilute' and 'concentrated' tell you how much acid is dissolved. 'Weak' and 'strong' tell you how much of that acid splits into ions. They're different questions, and a weak acid can be made up concentrated.

Ethanoic acid doesn't release any H⁺ ions, so it hardly counts as an acid.

This is a common misconception

It does release H⁺ ions: that's why the indicator isn't green. Orange or yellow still means acidic. It just releases fewer, because only some of its molecules split up.

Only some of its molecules split up, so there are fewer H⁺ ions and the pH is higher.

This matches the evidence

Yes. Ethanoic acid only partially dissociates, so at the same concentration it gives fewer H⁺ ions than a strong acid. Fewer H⁺ ions means a higher pH: still below 7, but not as low. That's why you see orange or yellow rather than red.

Weak isn't the same as dilute. At the same concentration, a weak acid splits less, makes fewer H⁺ ions and has a higher pH: orange or yellow with universal indicator.

Chemistry · Reactions of carboxylic acids

Carbonate + carboxylic acid: predict the products

Worked for you: sodium carbonate + methanoic acid → sodium methanoate + carbon dioxide + water, and it fizzes. Your turn: calcium carbonate is added to propanoic acid. What forms, and what would you see?

Start from the pattern: metal carbonate + carboxylic acid → salt + carbon dioxide + water.It's the same three products every time. Only the salt's name changes.
missing step

Which line is step 2?

The salt's first name comes from the metal: calcium.The salt's first name comes from the acid: propanoic.The salt's first name comes from the carbonate: calcium carbonate.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Then find out why this kind of acid is only a weak one.

What you need to know

–COOH = a carbon with a C=O and an OH both attached to it. An OH on its own makes an alcohol, not an acid.
Name a carboxylic acid by counting ALL its carbons (including the –COOH one), then adding '-oic acid'.
Carboxylic acids are weak acids: in water only some molecules split into H⁺ ions, so the equation uses ⇌.
Weak doesn't mean dilute. At the same concentration a weak acid has a higher pH than a strong acid: orange or yellow with universal indicator.
Metal carbonate + carboxylic acid → salt + carbon dioxide + water, with fizzing. The salt is named metal + acid stem + '-oate'.

The big picture

Carboxylic acids are a homologous series with the –COOH functional group: one carbon carrying both a C=O and an OH. An OH on its own makes an alcohol instead. Their names are the carbon count plus '-oic acid': methanoic, ethanoic, propanoic and butanoic acid. In water they are weak acids: only some molecules split to release H⁺ ions (CH₃COOH ⇌ CH₃COO⁻ + H⁺). So at the same concentration they have a higher pH than a strong acid, and universal indicator turns orange or yellow. With a metal carbonate they give a salt, carbon dioxide and water, with fizzing; the salt is named metal + acid stem + '-oate'.

Key points

1Carboxylic acids are a homologous series. Every member has the –COOH functional group.

2The first four are methanoic (1 C, HCOOH), ethanoic (2 C, CH₃COOH), propanoic (3 C, CH₃CH₂COOH or C₂H₅COOH) and butanoic (4 C, C₃H₇COOH).

3Partial dissociation of ethanoic acid: CH₃COOH ⇌ CH₃COO⁻ + H⁺.

4Their solutions are weakly acidic: pH below 7 but higher than a strong acid of the same concentration.

5Effervescence shows the carbon dioxide made when a carboxylic acid reacts with a metal carbonate.

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Worked example

Problem

Butanoic acid has the formula C₃H₇COOH. (a) Show how its name follows from its formula. (b) Write an equation for what happens when it dissolves in water, and explain the symbol you use.

⚠ Watch out

Calling a weak acid 'dilute'. Weak is about how much of the acid splits into ions; dilute is about how much acid is dissolved in the water. Ethanoic acid is weak whether you make it up concentrated or dilute.

🧠

Memory hook

Look for the carbon with two oxygens: one double-bonded (C=O), one holding an H (OH). That's –COOH. The acid ends in '-oic', its salt in '-oate'.

✓

Check yourself

Cover the page. Where exactly is the OH in a –COOH group? And why does ethanoic acid have a higher pH than a strong acid of the same concentration?

Flashcards

(15)

What is a carboxylic acid?

A member of the homologous series of compounds that contain the –COOH functional group.

What exactly is in the –COOH group?

One carbon atom with a double bond to an oxygen (C=O) and an OH group, both attached to that same carbon.

A molecule has an OH group but no C=O on that carbon. What is it?

An alcohol, not a carboxylic acid. You need the C=O and the OH together on one carbon.

How do you build the name of a carboxylic acid?

Count every carbon, including the –COOH one, for the stem (meth-, eth-, prop-, but-), then add '-oic acid'.

Name the first four carboxylic acids, in order.

Methanoic acid, ethanoic acid, propanoic acid, butanoic acid (1, 2, 3 and 4 carbons).

Give the structural formula of ethanoic acid.

CH₃COOH: a CH₃ group joined to the –COOH group.

What does 'weak acid' mean?

It only partially dissociates in solution: just some of its molecules split to release H⁺ ions.

Which ions form when ethanoic acid dissociates?

The ethanoate ion, CH₃COO⁻, and the hydrogen ion, H⁺.

Why is ⇌ used in CH₃COOH ⇌ CH₃COO⁻ + H⁺?

The change is reversible and only part of the acid is split at any moment. That's partial dissociation.

Weak acid vs dilute acid: what's the difference?

Weak = only some of it splits into ions. Dilute = not much acid dissolved in the water. They're separate ideas.

Carboxylic acid vs strong acid at the same concentration: which has the higher pH?

The carboxylic acid. It makes fewer H⁺ ions because it only partially dissociates.

What colour does universal indicator go in a carboxylic acid solution?

Orange or yellow.

Metal carbonate + carboxylic acid → ?

A salt + carbon dioxide + water.

What do you see when a carboxylic acid reacts with a metal carbonate, and why?

Fizzing (effervescence): carbon dioxide gas is being given off.

How is the salt from a carboxylic acid named?

Metal name first, then the acid's stem with '-oate'. For example, magnesium carbonate + ethanoic acid gives magnesium ethanoate.

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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Acid, alcohol or neither?

Into water: only some molecules split

Why orange, not red?

Carbonate + carboxylic acid: predict the products

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