11.03.2 Carboxylic Acids

Syllabus

    • Carboxylic acids have the functional group –COOH.

    • The first four members of a homologous series of Carboxylic Acids are Methanoic acid, Ethanoic acid, Propanoic acid and Butanoic acid.

    • The structures of Carboxylic acids can be represented in the following forms: CH3COOH or

  • Students should be able to:

  1. describe what happens when any of the first four Carboxylic acids react with Carbonates, dissolve in water, react with Alcohols

  2. (HT only) explain why Carboxylic acids are weak acids in terms of ionisation and pH

  3. recognise Carboxylic acids from their names or from given formulae.

  • Students do not need to

  1. know the names of individual carboxylic acids other than Methanoic acid, Ethanoic acid, Propanoic acid and Butanoic acid.

  2. write balanced chemical equations for the reactions of carboxylic acids.

  3. know the names of esters other than ethyl ethanoate.

What does this mean?

Homologous Series and Functional Group.

As we've seen, an Homologous Series is family of similar substances which share the same general formula and the same functional group (reactive part of the molecules), so they react in similar ways.

With Alkanes the first four members were called Methane, Ethane, Propane and Butane.

With Alcohols the first four members were called Methanol, Ethanol, Propanol and Butanol.

With Carboxylic Acids the first four members were called Methanoic Acid, Ethanoic Acid, Propanoic Acid and Butanoic Acid.

The functional group in a Carboxylic Acid comes at the end of the chain and is often written as COOH

In the picture above the R could represent a Hydrogen atom, or a chain of Carbons.

To name an acid we:

  1. Count the number of Carbon atoms in the chain

  2. Remember that 1 Carbon is Meth, 2 Carbons is Eth, 3 Carbons is Prop, 4 Carbons is But

  3. Add -anoic acid on the end

Why are they acidic?

In Year 9 we learned that acids are substances that ionise when they dissolve in water and produce Hydrogen ions (H+)

eg HCl --> H+ (aq) + Cl-(aq)

Hydrochloric acid is a strong acid because it ionises 100% - every HCl molecule breaks up.

Carboxylic acids are weak because only a few of the molecules ionise, most molecules dissolve and stay un-ionised.

How do they react?

With Carbonates

Only extremely weak acids don't react with Carbonates.

Carboxylic acids do and follow the pattern we learned in Year 9:

Acid + Carbonate --> Salt + Water + Carbon Dioxide

So,

Methanoic Acid + Sodium Carbonate --> Sodium Methanoate + Water + Carbon Dioxide

Ethanoic Acid + Potassium Carbonate --> Potassium Ethanoate + Water + Carbon Dioxide

Propanoic Acid + Calcium Carbonate --> Calcium Propanoate + Water + Carbon Dioxide

Butanoic Acid + Magnesium Carbonate --> Magnesium Butanoate + Water + Carbon Dioxide

We would see bubbles being made, but we wouldn't know for sure that they were Carbon Dioxide unless we tested with Limewater.

The vinegar volcano you may have made in Year 8 involved a reaction between a Carboxylic Acid in vinegar and a Carbonate in Baking Powder (with some red food dye.)

With Alcohols


Alcohols and Acids react together to make sweet smelling compounds called Esters.

The only one you need to learn is:

Ethanoic Acid + Ethanol --> Ethyl Ethanoate + Water

A little concentrated Sulphuric acid and some heat helps.

Esters are used in food and as perfumes, as well as solvents like nail varnish remover.

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