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 displayed formula for 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 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 only partly ionises when it is added to water.

carboxylic acid dissociation

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:

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)
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) → 2NaCl2(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.

sodium carbonate(s) + methanoic acid(aq) → sodium methanoate(aq) + water(l) + carbon dioxide(g)
Na2CO3(s) + 2HCOOH(aq) → 2COONa(aq) + H2O(l) + CO2(g)
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.

comparing reactions of strong and weak acid with chalk

Key Points

Practice questions

Check your understanding - Questions on carboxylic acids