Carboxylic Acids – Structure, Reactions, and Properties

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Carboxylic acids are another example of a homologous series of organic compounds. Carboxylic acids are a family of weak acids which contain the carboxyl group (-COOH) functional group. You may have met some of these carboxylic acids before and not realised it at the time; for example ant bites and wasp stings contain the simplest carboxylic acid; methanoic acid while ethanoic acid is the main ingredient in vinegar. Ethanoic acid is also used to marinated meat and fish and to pickle vegetables.

However as well as common uses mentioned above the carboxylic acids have many industrial uses and they are used in the manufacture of many important industrial and biological chemicals e.g. they are used in the manufacture of solvents, adhesives, soaps, rubber, dyes, perfumes and many types of drugs.

The names and structure of the carboxylic acids

Models of the carboxylic acids

Ball and stick models as well as the structural formula and displayed formula for the first 4 carboxylic acids are shown below

As before when we draw out the formula for these compounds we always draw them with the carboxyl functional group shown e.g. methanoic acid is HCOOH and never CO₂H. Ethanoic acid is CH₃COOH and never C₂H₄0₂.

Reactions and properties of carboxylic acids

Carboxylic acids with less than 6 carbon atoms are soluble in water; they dissolve to form weak acids. If you remember a weak acid forms when a solid or liquid acidic compound only partly ionises when it is added to water.

The dissociation of a carboxylic acid in water is shown below. Here the carboxylic acid molecule ionises to form a carboxylate anion (RCOO^-) and a hydrogen ion (H⁺). It is the presence of the hydrogen ion (H⁺) which gives the molecule its acidic properties. However very few of the carboxylic acid molecules actually ionise, most remain intact. This means that the position of equilibrium lies very much to the left hand side; that is the reactants; hence the reason why carboxylic acids are weak acids with fairly high pH values of between 3-5.

As an example consider the simplest carboxylic acid; methanoic acid (HCOOH). When methanoic acid is added to water only a small proportion of the methanoic acid molecules will ionise to form methanoate ions and hydrogen ions. Most of the methanoic acid molecules stay intact and do not dissociate or break down in water, we can show this as:

similarly with ethanoic acid we have:

In the symbolic equations below the water has been omitted to simplify the equations, the equations show what happens when a carboxylic acid molecule is added to water:

CH₃COOH ⇌ CH₃COO^- + H⁺

C₂H₅COOH ⇌ C₂H₅COO^- + H⁺

Reactions of carboxylic acids with metal carbonates

A reaction you have seen before is when hydrochloric acid reacts with a metal carbonate such as sodium carbonate or calcium carbonate to form a salt, water and carbon dioxide gas. Equations for these reactions are shown below:

metal carbonate_(s) + acid_(aq) → salt_(aq) + water_(l) + carbon dioxide_(g)

calcium carbonate_(s) + hydrochloric acid_(aq) → calcium chloride_(aq) + water_(l) + carbon dioxide_(g)

CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l) + CO_2(g)

sodium carbonate_(s) + hydrochloric acid_(aq) → sodium chloride_(aq) + water_(l) + carbon dioxide_(g)

Na₂CO_3(s) + 2HCl_(aq) → 2NaCl_(aq) + H₂O_(l) + CO_2(g)

swapping the strong acid hydrochloric acid for a weak carboxylic acid will produce a similar reaction but it will be much slower e.g.

sodium carbonate_(s) + methanoic acid_(aq) → sodium methanoate_(aq) + water_(l) + carbon dioxide_(g)

Na₂CO_3(s) + 2HCOOH_(aq) → 2COONa_(aq) + H₂O_(l) + CO_2(g)

calcium carbonate_(s) + ethanoic acid_(aq) → calcium ethanoate_(aq) + water_(l) + carbon dioxide_(g)

CaCO_3(s) + 2CH₃COOH_(aq) → (CH₃COO)₂Ca_(aq) + H₂O_(l) + CO_2(g)

The apparatus to compare the rate of reaction of a weak carboxylic acid and a strong acid with chalk (calcium carbonate) is shown below. Using this simple set-up we can compare the rate of reaction of a strong and a weak acid with calcium carbonate (chalk). All we have to do is count the number of bubbles or modify the apparatus to include a gas syringe and measure the volume of carbon dioxide gas released in a given time.

Self-check

Use the flashcards below to check your understanding on the structure, reactions and properties of the carboxylic acids.

What functional group is found in all carboxylic acids?

All carboxylic acids contain the carboxyl functional group (-COOH).

Which carboxlyic acid is found in vinegar?

Ethanoic acid

Why are carboxylic acids considered weak acids?

Because they only partially dissociate or ionise in water.

What anion is formed when a carboxylic acid dissolves in water?

A carboxylate anion

Name the ion formed when methanoic acid loses a hydrogen ion.

A methanoate ion.

How do the reactions of carboxylic acids compare with those of strong acids?

Weak acids such as the carboxylic acids react slower than strong acids.

What gas is released when metal carbonates react with carboxylic acids?

Carbon dioxide gas

What is the general formula for the carboxylic acids?

CnH2n+1OH

Why do carboxylic acids react slower than strong acids?

Weak acids are only partly ionised in water so have less hydrogen ions present; so reactions are slower.

Describe the test for carbon dioxide gas.

Carbon dioxide gas will turn a limewater solution milky or chalky.

Key Points

• Carboxylic acids are weak acids; this means they are only partly dissociated when dissolved in water.


• All carboxylic acids contain the carboxyl functional group (-COOH).


• Carboxylic acids react with metal carbonates in a similar way to strong acids such as hydrochloric acid but the reactions are much slower.


• When a carboxylic acid reacts with a base, a metal or a metal carbonate it forms a carboxylate salt e.g. methanoic acid will form salts called methanoate, ethanoic acid will form salts called ethanoate.


• A carboxylic acid will react with a metal to form a carboxylate salt and release hydrogen gas.

A carboxylic acid will react with a metal carbonate to form a carboxylate salt, water and carbon dioxide gas.

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