carboxylic acids

Chemistry only

Time to meet another homologous series of organic compounds, the carboxylic acids. Carboxylic acids are a family of weak acids which contain the carboxyl group (-COOH). Their general formula is CnH2n+1COOH. They are named by taking the corresponding alkane and replacing the -e on the end with -oic acid e.g.

alkane carboxylic acid
methane methanoic acid
ethane ethanoic acid
propane propanoic acid
butane butanoic acid

The first 4 carboxylic acids are shown below:

first 4 carboxylic acids

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

Higher tier only

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.

carboxylic acid dissociation

As an example consider the simplest carboxylic acid , methanoic acid (HCOOH). When methanoic acid is added to water a few of 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:

ethanoic acid + water ⇌ hydrogen ion + ethanoate ion
and with propanoic acid we have:
propanoic acid + water ⇌ hydrogen ion + propanoate ion
When the carboxylic acid dissolves or reacts it loses it hydrogen, just as you would expect for an acid. The names of the ions formed in these reactions maybe new to you but simply add -ate for -ic on the end of the acid e.g.

carboxylic acid molecular formula ion formed molecular formula of ion
methanoic HCOOH methanoate HCOO-
ethanoic CH3COOH ethanoate CH3COO-
propanoic C2H5COOH propanoote C2H5COO-
butanoic C3H7COOH butanoate C3H7COO-

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)
For example:
calcium carbonate(s) + hydrochloric acid(aq) → calcium chloride(aq) + water(l) + carbon dioxide(g)
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
sodium carbonate(s) + hydrochloric acid(aq) → sodium chloride(aq) + water(l) + carbon dioxide(g)
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)

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

Example 3:
sodium carbonate(s) + methanoic acid(aq) → sodium methanoate(aq) + water(l) + carbon dioxide(g)
Na2CO3(s) + 2HCOOH(aq) → 2COONa(aq) + H2O(l) + CO2(g)
Example 4:
calcium carbonate(s) + ethanoic acid(aq) → calcium ethanoate(aq) + water(l) + carbon dioxide(g)
CaCO3(s) + 2CH3COOH(aq) → (CH3COO)2Ca(aq) + H2O(l) + CO2(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.

comparing reactions of strong and weak acid with chalk

Key Points

Practice questions

Check your understanding - Questions on carboxylic acids